man > GCC(1)

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⚡ TLDR: GCC (tldr-pages)

Preprocess and compile C and C++ source files, then assemble and link them together.

Compile multiple source files into an executable
gcc {{path/to/source1.c path/to/source2.c ...}} {{-o|--output}} {{path/to/output_executable}}
Activate output of all errors and warnings
gcc {{path/to/source.c}} -Wall {{-o|--output}} {{output_executable}}
Show common warnings, debug symbols in output, and optimize without affecting debugging
gcc {{path/to/source.c}} -Wall {{-g|--debug}} -Og {{-o|--output}} {{path/to/output_executable}}
Include libraries from a different path
gcc {{path/to/source.c}} {{-o|--output}} {{path/to/output_executable}} -I{{path/to/header}} -L{{path/to/library}} -l{{library_name}}
Compile source code into Assembler instructions
gcc {{-S|--assemble}} {{path/to/source.c}}
Compile source code into an object file without linking
gcc {{-c|--compile}} {{path/to/source.c}}
Optimize the compiled program for performance
gcc {{path/to/source.c}} -O{{1|2|3|fast}} {{-o|--output}} {{path/to/output_executable}}
Display version
gcc --version

GCC(1) GNU GCC(1)

NAME
gcc - GNU project C and C++ compiler

SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...

Only the most useful options are listed here; see below for the remainder. g++ accepts
mostly the same options as gcc.

DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The
"overall options" allow you to stop this process at an intermediate stage. For example, the
-c option says not to run the linker. Then the output consists of object files output by the
assembler.

Other options are passed on to one or more stages of processing. Some options control the
preprocessor and others the compiler itself. Yet other options control the assembler and
linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command-line options that you can use with GCC are useful for C programs; when an
option is only useful with another language (usually C++), the explanation says so
explicitly. If the description for a particular option does not mention a source language,
you can use that option with all supported languages.

The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-
compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++
programs, you should invoke GCC as g++ instead.

The gcc program accepts options and file names as operands. Many options have multi-letter
names; therefore multiple single-letter options may not be grouped: -dv is very different
from -d -v.

You can mix options and other arguments. For the most part, the order you use doesn't
matter. Order does matter when you use several options of the same kind; for example, if you
specify -L more than once, the directories are searched in the order specified. Also, the
placement of the -l option is significant.

Many options have long names starting with -f or with -W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative
forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two
forms, whichever one is not the default.

Some options take one or more arguments typically separated either by a space or by the
equals sign (=) from the option name. Unless documented otherwise, an argument can be either
numeric or a string. Numeric arguments must typically be small unsigned decimal or
hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to
options that specify a size threshold of some sort may be arbitrarily large decimal or
hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as
"kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and
mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated
by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and
international standards for the full listing and explanation of the binary and decimal byte
size prefixes.

OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following
sections.

Overall Options
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf -dumpdir dumppfx -x
language -v -### --help[=class[,...]] --target-help --version -pass-exit-codes -pipe
-specs=file -wrapper @file -ffile-prefix-map=old=new -fcanon-prefix-map -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file

C Language Options
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function
-fcond-mismatch -ffreestanding -fgimple -fgnu-tm -fgnu89-inline -fhosted
-flax-vector-conversions -fms-extensions -foffload=arg -foffload-options=arg -fopenacc
-fopenacc-dim=geom -fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type]
-fpermitted-flt-eval-methods=standard -fplan9-extensions -fsigned-bitfields
-funsigned-bitfields -fsigned-char -funsigned-char -fstrict-flex-arrays[=n]
-fsso-struct=endianness

C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n -fargs-in-order=n -fchar8_t
-fcheck-new -fconstexpr-depth=n -fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules-ts -fmodule-implicit-inline
-fno-module-lazy -fmodule-mapper=specification -fmodule-version-ignore -fms-extensions
-fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins -fnothrow-opt
-fno-operator-names -fno-optional-diags -fpermissive -fno-pretty-templates -fno-rtti
-fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals
-flang-info-include-translate[=header] -flang-info-include-translate-not
-flang-info-module-cmi[=module] -stdlib=libstdc++,libc++ -Wabi-tag -Wcatch-value
-Wcatch-value=n -Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null -Wctad-maybe-unsupported
-Wctor-dtor-privacy -Wdangling-reference -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wno-deprecated-array-compare -Wdeprecated-copy
-Wdeprecated-copy-dtor -Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++ -Wno-exceptions -Wextra-semi
-Wno-inaccessible-base -Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof -Wno-literal-suffix
-Wmismatched-new-delete -Wmismatched-tags -Wmultiple-inheritance -Wnamespaces
-Wnarrowing -Wnoexcept -Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new -Wplacement-new=n -Wrange-loop-construct -Wredundant-move
-Wredundant-tags -Wreorder -Wregister -Wstrict-null-sentinel -Wno-subobject-linkage
-Wtemplates -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override -Wno-terminate
-Wuseless-cast -Wno-vexing-parse -Wvirtual-inheritance -Wno-virtual-move-assign
-Wvolatile -Wzero-as-null-pointer-constant

Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers
-fobjc-abi-version=n -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions
-fobjc-gc -fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package] -freplace-objc-classes -fzero-link
-gen-decls -Wassign-intercept -Wno-property-assign-default -Wno-protocol
-Wobjc-root-class -Wselector -Wstrict-selector-match -Wundeclared-selector

Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-plain-output -fdiagnostics-show-location=[once|every-
line] -fdiagnostics-color=[auto|never|always] -fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-stderr|json-file]
-fno-diagnostics-show-option -fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe -fno-diagnostics-show-rule
-fdiagnostics-minimum-margin-width=width -fdiagnostics-parseable-fixits
-fdiagnostics-generate-patch -fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column -fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin -fdiagnostics-escape-format=[unicode|bytes]

Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Wabi=n
-Waddress -Wno-address-of-packed-member -Waggregate-return
-Walloc-size-larger-than=byte-size -Walloc-zero -Walloca -Walloca-larger-than=byte-size
-Wno-aggressive-loop-optimizations -Warith-conversion -Warray-bounds -Warray-bounds=n
-Warray-compare -Wno-attributes -Wattribute-alias=n -Wno-attribute-alias
-Wno-attribute-warning -Wbidi-chars=[none|unpaired|any|ucn] -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined
-Wc90-c99-compat -Wc99-c11-compat -Wc11-c2x-compat -Wc++-compat -Wc++11-compat
-Wc++14-compat -Wc++17-compat -Wc++20-compat -Wno-c++11-extensions
-Wno-c++14-extensions -Wno-c++17-extensions -Wno-c++20-extensions -Wno-c++23-extensions
-Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual -Wchar-subscripts
-Wclobbered -Wcomment -Wno-complain-wrong-lang -Wconversion -Wno-coverage-mismatch
-Wno-cpp -Wdangling-else -Wdangling-pointer -Wdangling-pointer=n -Wdate-time
-Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-array-qualifiers -Wno-discarded-qualifiers
-Wno-div-by-zero -Wdouble-promotion -Wduplicated-branches -Wduplicated-cond
-Wempty-body -Wno-endif-labels -Wenum-compare -Wenum-conversion -Wenum-int-mismatch
-Werror -Werror=* -Wexpansion-to-defined -Wfatal-errors -Wfloat-conversion
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args
-Wformat-nonliteral -Wformat-overflow=n -Wformat-security -Wformat-signedness
-Wformat-truncation=n -Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wno-if-not-aligned -Wno-ignored-attributes
-Wignored-qualifiers -Wno-incompatible-pointer-types -Wimplicit -Wimplicit-fallthrough
-Wimplicit-fallthrough=n -Wno-implicit-function-declaration -Wno-implicit-int
-Winfinite-recursion -Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context
-Wno-int-to-pointer-cast -Wno-invalid-memory-model -Winvalid-pch -Winvalid-utf8
-Wno-unicode -Wjump-misses-init -Wlarger-than=byte-size -Wlogical-not-parentheses
-Wlogical-op -Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn
-Wno-missing-profile -Wno-multichar -Wmultistatement-macros -Wnonnull
-Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr
-Wopenacc-parallelism -Wopenmp-simd -Wno-overflow -Woverlength-strings
-Wno-override-init-side-effects -Wpacked -Wno-packed-bitfield-compat
-Wpacked-not-aligned -Wpadded -Wparentheses -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas -Wno-prio-ctor-dtor
-Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type
-Wno-scalar-storage-order -Wsequence-point -Wshadow -Wshadow=global -Wshadow=local
-Wshadow=compatible-local -Wno-shadow-ivar -Wno-shift-count-negative
-Wno-shift-count-overflow -Wshift-negative-value -Wno-shift-overflow -Wshift-overflow=n
-Wsign-compare -Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector -Wstack-usage=byte-
size -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n
-Wstring-compare -Wno-stringop-overflow -Wno-stringop-overread -Wno-stringop-truncation
-Wstrict-flex-arrays -Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wswitch
-Wno-switch-bool -Wswitch-default -Wswitch-enum -Wno-switch-outside-range
-Wno-switch-unreachable -Wsync-nand -Wsystem-headers -Wtautological-compare
-Wtrampolines -Wtrigraphs -Wtrivial-auto-var-init -Wtsan -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants -Wunused
-Wunused-but-set-parameter -Wunused-but-set-variable -Wunused-const-variable
-Wunused-const-variable=n -Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-macros -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable
-Wno-varargs -Wvariadic-macros -Wvector-operation-performance -Wvla
-Wvla-larger-than=byte-size -Wno-vla-larger-than -Wvolatile-register-var
-Wwrite-strings -Wxor-used-as-pow -Wzero-length-bounds

Static Analyzer Options
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name -fno-analyzer-feasibility
-fanalyzer-fine-grained -fno-analyzer-state-merge -fno-analyzer-state-purge
-fno-analyzer-suppress-followups -fanalyzer-transitivity -fno-analyzer-undo-inlining
-fanalyzer-verbose-edges -fanalyzer-verbose-state-changes -fanalyzer-verbosity=level
-fdump-analyzer -fdump-analyzer-callgraph -fdump-analyzer-exploded-graph
-fdump-analyzer-exploded-nodes -fdump-analyzer-exploded-nodes-2
-fdump-analyzer-exploded-nodes-3 -fdump-analyzer-exploded-paths
-fdump-analyzer-feasibility -fdump-analyzer-json -fdump-analyzer-state-purge
-fdump-analyzer-stderr -fdump-analyzer-supergraph -fdump-analyzer-untracked
-Wno-analyzer-double-fclose -Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file -Wno-analyzer-exposure-through-uninit-copy
-Wno-analyzer-fd-access-mode-mismatch -Wno-analyzer-fd-double-close -Wno-analyzer-fd-leak
-Wno-analyzer-fd-phase-mismatch -Wno-analyzer-fd-type-mismatch
-Wno-analyzer-fd-use-after-close -Wno-analyzer-fd-use-without-check
-Wno-analyzer-file-leak -Wno-analyzer-free-of-non-heap
-Wno-analyzer-imprecise-fp-arithmetic -Wno-analyzer-infinite-recursion
-Wno-analyzer-jump-through-null -Wno-analyzer-malloc-leak
-Wno-analyzer-mismatching-deallocation -Wno-analyzer-null-argument
-Wno-analyzer-null-dereference -Wno-analyzer-out-of-bounds
-Wno-analyzer-possible-null-argument -Wno-analyzer-possible-null-dereference
-Wno-analyzer-putenv-of-auto-var -Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size -Wno-analyzer-tainted-assertion
-Wno-analyzer-tainted-array-index -Wno-analyzer-tainted-divisor
-Wno-analyzer-tainted-offset -Wno-analyzer-tainted-size -Wanalyzer-too-complex
-Wno-analyzer-unsafe-call-within-signal-handler -Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value -Wno-analyzer-va-arg-type-mismatch
-Wno-analyzer-va-list-exhausted -Wno-analyzer-va-list-leak
-Wno-analyzer-va-list-use-after-va-end -Wno-analyzer-write-to-const
-Wno-analyzer-write-to-string-literal

C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type
-Wmissing-prototypes -Wnested-externs -Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign

Debugging Options
-g -glevel -gdwarf -gdwarf-version -gbtf -gctf -gctflevel -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstrict-dwarf -gno-strict-dwarf
-gas-loc-support -gno-as-loc-support -gas-locview-support -gno-as-locview-support
-gcolumn-info -gno-column-info -gdwarf32 -gdwarf64 -gstatement-frontiers
-gno-statement-frontiers -gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views -ginline-points
-gno-inline-points -gvms -gz[=type] -gsplit-dwarf -gdescribe-dies -gno-describe-dies
-fdebug-prefix-map=old=new -fdebug-types-section -fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list] -fno-eliminate-unused-debug-symbols
-femit-class-debug-always -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments

Optimization Options
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]]
-falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]]
-falign-loops[=n[:m:[n2[:m2]]]] -fno-allocation-dce -fallow-store-data-races
-fassociative-math -fauto-profile -fauto-profile[=path] -fauto-inc-dec
-fbranch-probabilities -fcaller-saves -fcombine-stack-adjustments -fconserve-stack
-fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively
-fdevirtualize-at-ltrans -fdse -fearly-inlining -fipa-sra -fexpensive-optimizations
-ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -ffinite-loops -fforward-propagate -ffp-contract=style
-ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm
-fgraphite-identity -fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2
-findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=n
-finline-small-functions -fipa-modref -fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-reference-addressable
-fipa-stack-alignment -fipa-icf -fira-algorithm=algorithm -flive-patching=level
-fira-region=region -fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions
-fkeep-static-functions -fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange -floop-strip-mine
-floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all -flra-remat -flto
-flto-compression-level -flto-partition=alg -fmerge-all-constants -fmerge-constants
-fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fmove-loop-stores
-fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction -fprofile-use
-fprofile-use=path -fprofile-partial-training -fprofile-values
-fprofile-reorder-functions -freciprocal-math -free -frename-registers
-freorder-blocks -freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops
-frounding-math -fsave-optimization-record -fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n]
-fsched-stalled-insns[=n] -fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic
-fsched-dep-count-heuristic -fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining
-fsel-sched-pipelining-outer-loops -fsemantic-interposition -fshrink-wrap
-fshrink-wrap-separate -fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types
-fsplit-wide-types-early -fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging
-fstrict-aliasing -fipa-strict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-fcode-hoisting -ftree-loop-if-convert -ftree-loop-im -ftree-phiprop
-ftree-loop-distribution -ftree-loop-distribute-patterns -ftree-loop-ivcanon
-ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vectorize -ftree-vrp -ftrivial-auto-var-init -funconstrained-commons
-funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-math-optimizations
-funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt
-fweb -fwhole-program -fwpa -fuse-linker-plugin -fzero-call-used-regs --param
name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og -Oz

Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage -fprofile-abs-path
-fprofile-dir=path -fprofile-generate -fprofile-generate=path -fprofile-info-section
-fprofile-info-section=name -fprofile-note=path -fprofile-prefix-path=path
-fprofile-update=method -fprofile-filter-files=regex -fprofile-exclude-files=regex
-fprofile-reproducible=[multithreaded|parallel-runs|serial] -fsanitize=style
-fsanitize-recover -fsanitize-recover=style -fsanitize-trap -fsanitize-trap=style
-fasan-shadow-offset=number -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcf-protection=[full|branch|return|none|check] -fharden-compares
-fharden-conditional-branches -fstack-protector -fstack-protector-all
-fstack-protector-strong -fstack-protector-explicit -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug -finstrument-functions
-finstrument-functions-once -finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,... -fprofile-prefix-map=old=new

Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU
-fdebug-cpp -fdirectives-only -fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset -flarge-source-files
-fmacro-prefix-map=old=new -fmax-include-depth=depth -fno-canonical-system-headers
-fpch-deps -fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros file -include file -M -MD
-MF -MG -MM -MMD -MP -MQ -MT -Mno-modules -no-integrated-cpp -P -pthread -remap
-traditional -traditional-cpp -trigraphs -Umacro -undef -Wp,option -Xpreprocessor
option

Assembler Options
-Wa,option -Xassembler option

Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs -nolibc
-nostdlib -nostdlib++ -e entry --entry=entry -pie -pthread -r -rdynamic -s -static
-static-pie -static-libgcc -static-libstdc++ -static-libasan -static-libtsan
-static-liblsan -static-libubsan -shared -shared-libgcc -symbolic -T script
-Wl,option -Xlinker option -u symbol -z keyword

Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -iplugindir=dir
-iprefix file -iquote dir -isysroot dir -isystem dir -iwithprefix dir
-iwithprefixbefore dir -Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc
-nostdinc++ --sysroot=dir

Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions
-fdelete-dead-exceptions -funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fcommon -fno-ident -fpcc-struct-return -fpic -fPIC -fpie
-fPIE -fno-plt -fno-jump-tables -fno-bit-tests -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n] -fleading-underscore
-ftls-model=model -fstack-reuse=reuse_level -ftrampolines -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields
-fsync-libcalls

Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion
-fcallgraph-info[=su,da] -fchecking -fchecking=n -fdbg-cnt-list -fdbg-cnt=counter-
value-list -fdisable-ipa-pass_name -fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-
list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-debug
-fdump-earlydebug -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-final-insns[=file] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline
-fdump-lang-all -fdump-lang-switch -fdump-lang-switch-options
-fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass
-fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch
-fdump-tree-switch-options -fdump-tree-switch-options=filename -fcompare-debug[=opts]
-fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-list -fira-verbose=n
-flto-report -flto-report-wpa -fmem-report-wpa -fmem-report -fpre-ipa-mem-report
-fpost-ipa-mem-report -fopt-info -fopt-info-options[=file] -fmultiflags
-fprofile-report -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose
-fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats -fstack-usage
-ftime-report -ftime-report-details -fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-multi-os-directory -print-prog-name=program -print-search-dirs
-Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd
-save-temps=obj -time[=file]

Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only
-mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align -mno-strict-align
-momit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size
-mfix-cortex-a53-835769 -mfix-cortex-a53-843419 -mlow-precision-recip-sqrt
-mlow-precision-sqrt -mlow-precision-div -mpc-relative-literal-loads
-msign-return-address=scope -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti
-mharden-sls=opts -march=name -mcpu=name -mtune=name -moverride=string
-mverbose-cost-dump -mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation -moutline-atomics

Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num
-mcmove -mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify
-mstack-offset=num -mround-nearest -mlong-calls -mshort-calls -msmall16 -mfp-mode=mode
-mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg

AMD GCN Options -march=gpu -mtune=gpu -mstack-size=bytes

ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700
-mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64
-matomic -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc
-mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape -mtelephony -mxy
-misize -mannotate-align -marclinux -marclinux_prof -mlong-calls -mmedium-calls
-msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache
-mtp-regno=regno -malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc
-mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi
-mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact
-mlra-priority-noncompact -mmillicode -mmixed-code -mq-class -mRcq -mRcw
-msize-level=level -mtune=cpu -mmultcost=num -mcode-density-frame
-munalign-prob-threshold=probability -mmpy-option=multo -mdiv-rem -mcode-density
-mll64 -mfpu=fpu -mrf16 -mbranch-index

ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check
-mno-apcs-stack-check -mapcs-reentrant -mno-apcs-reentrant -mgeneral-regs-only
-msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32
-mfloat-abi=name -mfp16-format=name -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -mtune=name -mprint-tune-info -mstructure-size-boundary=n
-mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base
-mpic-register=reg -mnop-fun-dllimport -mpoke-function-name -mthumb -marm -mflip-thumb
-mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd
-mfix-cortex-a57-aes-1742098 -mfix-cortex-a72-aes-1655431 -munaligned-access
-mneon-for-64bits -mslow-flash-data -masm-syntax-unified -mrestrict-it
-mverbose-cost-dump -mpure-code -mcmse -mfix-cmse-cve-2021-35465
-mstack-protector-guard=guard -mstack-protector-guard-offset=offset -mfdpic
-mbranch-protection=none|standard|pac-ret[+leaf] [+bti]|bti[+pac-ret[+leaf]]

AVR Options -mmcu=mcu -mabsdata -maccumulate-args -mbranch-cost=cost -mcall-prologues
-mgas-isr-prologues -mint8 -mdouble=bits -mlong-double=bits -mn_flash=size
-mno-interrupts -mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack
-mfract-convert-truncate -mshort-calls -mskip-bug -nodevicelib -nodevicespecs
-Waddr-space-convert -Wmisspelled-isr

Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library
-mno-id-shared-library -mshared-library-id=n -mleaf-id-shared-library
-mno-leaf-id-shared-library -msep-data -mno-sep-data -mlong-calls -mno-long-calls
-mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb

C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type

CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -metrax4 -metrax100
-mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -melf -maout -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround

C-SKY Options -march=arch -mcpu=cpu -mbig-endian -EB -mlittle-endian -EL -mhard-float
-msoft-float -mfpu=fpu -mdouble-float -mfdivdu -mfloat-abi=name -melrw -mistack -mmp
-mcp -mcache -msecurity -mtrust -mdsp -medsp -mvdsp -mdiv -msmart -mhigh-registers
-manchor -mpushpop -mmultiple-stld -mconstpool -mstack-size -mccrt -mbranch-cost=n
-mcse-cc -msched-prolog -msim

Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only
-bind_at_load -bundle -bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file -dylinker_install_name
-dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -iframework
-image_base -init -install_name -keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding
-nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign
-sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table
-seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr
-segs_read_write_addr -single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=version
-mkernel -mone-byte-bool

DEC Alpha Options -mno-fp-regs -msoft-float -mieee -mieee-with-inexact
-mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode
-mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax
-mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text
-mmemory-latency=time

eBPF Options -mbig-endian -mlittle-endian -mkernel=version -mframe-limit=bytes -mxbpf
-mco-re -mno-co-re -mjmpext -mjmp32 -malu32 -mcpu=version

FR30 Options -msmall-model -mno-lsim

FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm

FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc
-mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd
-mno-muladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack
-mno-eflags -mcond-move -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc
-mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS
-mtls -mcpu=cpu

GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc
-tno-android-ld

H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300

HPPA Options -march=architecture-type -matomic-libcalls -mbig-switch -mcaller-copies
-mdisable-fpregs -mdisable-indexing -mordered -mfast-indirect-calls -mgas -mgnu-ld
-mhp-ld -mfixed-range=register-range -mcoherent-ldcw -mjump-in-delay -mlinker-opt
-mlong-calls -mlong-load-store -mno-atomic-libcalls -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay
-mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type
-mspace-regs -msoft-mult -msio -mwsio -munix=unix-std -nolibdld -static -threads

IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mregister-names -msdata -mno-sdata -mconstant-gp -mauto-pic
-mfused-madd -minline-float-divide-min-latency -minline-float-divide-max-throughput
-mno-inline-float-divide -minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-inline-int-divide -minline-sqrt-min-latency
-minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64
-msched-br-data-spec -msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns

LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled
-msign-extend-enabled -muser-enabled

LoongArch Options -march=cpu-type -mtune=cpu-type -mabi=base-abi-type -mfpu=fpu-type
-msoft-float -msingle-float -mdouble-float -mbranch-cost=n -mcheck-zero-division
-mno-check-zero-division -mcond-move-int -mno-cond-move-int -mcond-move-float
-mno-cond-move-float -memcpy -mno-memcpy -mstrict-align -mno-strict-align
-mmax-inline-memcpy-size=n -mexplicit-relocs -mno-explicit-relocs -mdirect-extern-access
-mno-direct-extern-access -mcmodel=code-model -mrelax -mpass-mrelax-to-as

M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops
-missue-rate=number -mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type
-mno-flush-func -mflush-func=name -mno-flush-trap -mflush-trap=number -G num

M32C Options -mcpu=cpu -msim -memregs=number

M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40
-m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407
-mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd
-mdiv -mno-div -mshort -mno-short -mhard-float -m68881 -msoft-float -mpcrel
-malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n
-mid-shared-library -mno-id-shared-library -mxgot -mno-xgot -mlong-jump-table-offsets

MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions
-mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment

MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy
-mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check
-mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model
-mpic-data-is-text-relative

MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32
-mips32r2 -mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5
-mips64r6 -mips16 -mno-mips16 -mflip-mips16 -minterlink-compressed
-mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16 -mabi=abi -mabicalls
-mno-abicalls -mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64
-mfp32 -mfpxx -mfp64 -mhard-float -msoft-float -mno-float -msingle-float
-mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp
-mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa
-mcrc -mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa
-mloongson-mmi -mno-loongson-mmi -mloongson-ext -mno-loongson-ext -mloongson-ext2
-mno-loongson-ext2 -mfpu=fpu-type -msmartmips -mno-smartmips -mpaired-single
-mno-paired-single -mdmx -mno-mdmx -mips3d -mno-mips3d -mmt -mno-mt -mllsc
-mno-llsc -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs -munaligned-access -mno-unaligned-access
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mimadd -mno-imadd
-mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400 -mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000
-mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130
-mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num
-mbranch-likely -mno-branch-likely -mcompact-branches=policy -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci -mlxc1-sxc1
-mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls
-mmcount-ra-address -mframe-header-opt -mno-frame-header-opt

MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware
-mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict
-mno-branch-predict -mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit

MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-
type -mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb

Moxie Options -meb -mel -mmul.x -mno-crt0

MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mwarn-mcu
-mcode-region= -mdata-region= -msilicon-errata= -msilicon-errata-warn= -mhwmult=
-minrt -mtiny-printf -mmax-inline-shift=

NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov
-mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string
-mv3push -mno-v3push -m16bit -mno-16bit -misr-vector-size=num -mcache-block-size=num
-march=arch -mcmodel=code-model -mctor-dtor -mrelax

Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt -mgprel-sec=regexp
-mr0rel-sec=regexp -mel -meb -mno-bypass-cache -mbypass-cache -mno-cache-volatile
-mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx
-mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N -mno-custom-insn
-mcustom-fpu-cfg=name -mhal -msmallc -msys-crt0=name -msys-lib=name -march=arch -mbmx
-mno-bmx -mcdx -mno-cdx

Nvidia PTX Options -m64 -mmainkernel -moptimize

OpenRISC Options -mboard=name -mnewlib -mhard-mul -mhard-div -msoft-mul -msoft-div
-msoft-float -mhard-float -mdouble-float -munordered-float -mcmov -mror -mrori
-msext -msfimm -mshftimm -mcmodel=code-model

PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16
-mint16 -mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra

PowerPC Options See RS/6000 and PowerPC Options.

PRU Options -mmcu=mcu -minrt -mno-relax -mloop -mabi=variant

RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv
-mno-fdiv -mdiv -mno-div -misa-spec=ISA-spec-string -march=ISA-string -mtune=processor-
string -mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore
-mno-save-restore -mshorten-memrefs -mno-shorten-memrefs -mstrict-align
-mno-strict-align -mcmodel=medlow -mcmodel=medany -mexplicit-relocs
-mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute -mno-riscv-attribute
-malign-data=type -mbig-endian -mlittle-endian -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mcsr-check
-mno-csr-check -minline-atomics -mno-inline-atomics

RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13
-mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles
-msave-mduc-in-interrupts

RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mpowerpc64 -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd
-mfprnd -mno-fprnd -mcmpb -mno-cmpb -mhard-dfp -mno-hard-dfp -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat
-mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple
-mupdate -mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align -mstrict-align
-mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mdynamic-no-pic -mswdiv -msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi
-mcall-freebsd -mcall-linux -mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return -msvr4-struct-return
-mabi=abi-type -msecure-plt -mbss-plt -mlongcall -mno-longcall -mpltseq -mno-pltseq
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mno-block-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mno-isel -mvrsave -mno-vrsave -mmulhw
-mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme -mads
-myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type -mfriz
-mno-friz -mpointers-to-nested-functions -mno-pointers-to-nested-functions
-msave-toc-indirect -mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion
-mpower8-vector -mno-power8-vector -mcrypto -mno-crypto -mhtm -mno-htm -mquad-memory
-mno-quad-memory -mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mfloat128 -mno-float128 -mfloat128-hardware
-mno-float128-hardware -mgnu-attribute -mno-gnu-attribute -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mprefixed
-mno-prefixed -mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect
-mprivileged -mno-privileged

RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data
-mlittle-endian-data -msmall-data -msim -mno-sim -mas100-syntax -mno-as100-syntax
-mrelax -mmax-constant-size= -mint-register= -mpid -mallow-string-insns
-mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts

S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float
-mhard-dfp -mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64
-m31 -mdebug -mno-debug -mesa -mzarch -mhtm -mvx -mzvector -mtpf-trace
-mno-tpf-trace -mtpf-trace-skip -mno-tpf-trace-skip -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords

SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3 -m3e
-m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single
-m4a -m4al -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct
-mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args -matomic-model=atomic-model
-mbranch-cost=num -mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas

Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text
-pthreads

SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model
-m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat
-mno-flat -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias -mno-stack-bias -mstd-struct-return
-mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles -muser-mode
-mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3
-mno-vis3 -mvis4 -mno-vis4 -mvis4b -mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf
-mfsmuld -mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699
-mfix-ut700 -mfix-gr712rc -mlra -mno-lra

System V Options -Qy -Qn -YP,paths -Ym,dir

V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function
-mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs
-mdisable-callt -mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float -mhard-float -mgcc-abi
-mrh850-abi -mbig-switch

VAX Options -mg -mgnu -munix -mlra

Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float -mcpu=cpu-type
-mtune=cpu-type -msv-mode -muser-mode

VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size

VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now

x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list
-mdump-tune-features -mno-default -mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -m80387 -mhard-float -msoft-float -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld -mcx16
-msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128
-mprefer-vector-width=opt -mmove-max=bits -mstore-max=bits -mmmx -msse -msse2 -msse3
-mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er
-mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes
-mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite
-mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves -msse4a -m3dnow -m3dnowa
-mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave
-mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes
-mwaitpkg -mshstk -mmanual-endbr -mcet-switch -mforce-indirect-call -mavx512vbmi2
-mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b
-mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw -mprfchw -mrdpid
-mrdseed -msgx -mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mavx512fp16 -mavxifma -mavxvnniint8 -mavxneconvert
-mcmpccxadd -mamx-fp16 -mprefetchi -mraoint -mamx-complex -mcldemote -mms-bitfields
-mno-align-stringops -minline-all-stringops -minline-stringops-dynamically
-mstringop-strategy=alg -mkl -mwidekl -mmemcpy-strategy=strategy
-mmemset-strategy=strategy -mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128 -mregparm=num
-msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mdaz-ftz
-mstackrealign -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -mabi=name -maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu
-mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount -mnop-mcount
-m8bit-idiv -minstrument-return=type -mfentry-name=name -mfentry-section=name
-mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mstack-protector-guard-symbol=symbol
-mgeneral-regs-only -mcall-ms2sysv-xlogues -mrelax-cmpxchg-loop -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register -mharden-sls=choice
-mindirect-branch-cs-prefix -mneeded -mno-direct-extern-access -munroll-only-small-loops
-mlam=choice

x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport -mthread
-municode -mwin32 -mwindows -fno-set-stack-executable

Xstormy16 Options -msim

Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic
-mserialize-volatile -mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mauto-litpools -mno-auto-litpools -mtarget-align
-mno-target-align -mlongcalls -mno-longcalls -mabi=abi-type -mextra-l32r-costs=cycles

zSeries Options See S/390 and zSeries Options.

Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and
linking, always in that order. GCC is capable of preprocessing and compiling several files
either into several assembler input files, or into one assembler input file; then each
assembler input file produces an object file, and linking combines all the object files
(those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

file.c
C source code that must be preprocessed.

file.i
C source code that should not be preprocessed.

file.ii
C++ source code that should not be preprocessed.

file.m
Objective-C source code. Note that you must link with the libobjc library to make an
Objective-C program work.

file.mi
Objective-C source code that should not be preprocessed.

file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an
Objective-C++ program work. Note that .M refers to a literal capital M.

file.mii
Objective-C++ source code that should not be preprocessed.

file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header
(default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec
switch).

file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the last two letters must
both be literally x. Likewise, .C refers to a literal capital C.

file.mm
file.M
Objective-C++ source code that must be preprocessed.

file.mii
Objective-C++ source code that should not be preprocessed.

file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.

file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.

file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional
preprocessor).

file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.

file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the traditional
preprocessor).

file.go
Go source code.

file.d
D source code.

file.di
D interface file.

file.dd
D documentation code (Ddoc).

file.ads
Ada source code file that contains a library unit declaration (a declaration of a
package, subprogram, or generic, or a generic instantiation), or a library unit renaming
declaration (a package, generic, or subprogram renaming declaration). Such files are
also called specs.

file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such
files are also called bodies.

file.s
Assembler code.

file.S
file.sx
Assembler code that must be preprocessed.

other
An object file to be fed straight into linking. Any file name with no recognized suffix
is treated this way.

You can specify the input language explicitly with the -x option:

-x language
Specify explicitly the language for the following input files (rather than letting the
compiler choose a default based on the file name suffix). This option applies to all
following input files until the next -x option. Possible values for language are:

c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go

-x none
Turn off any specification of a language, so that subsequent files are handled according
to their file name suffixes (as they are if -x has not been used at all).

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to
tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop.
Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at
all.

-c Compile or assemble the source files, but do not link. The linking stage simply is not
done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix .c,
.i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

-S Stop after the stage of compilation proper; do not assemble. The output is in the form
of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix .c,
.i, etc., with .s.

Input files that don't require compilation are ignored.

-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the
form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

-o file
Place the primary output in file file. This applies to whatever sort of output is being
produced, whether it be an executable file, an object file, an assembler file or
preprocessed C code.

If -o is not specified, the default is to put an executable file in a.out, the object
file for source.suffix in source.o, its assembler file in source.s, a precompiled header
file in source.suffix.gch, and all preprocessed C source on standard output.

Though -o names only the primary output, it also affects the naming of auxiliary and dump
outputs. See the examples below. Unless overridden, both auxiliary outputs and dump
outputs are placed in the same directory as the primary output. In auxiliary outputs,
the suffix of the input file is replaced with that of the auxiliary output file type; in
dump outputs, the suffix of the dump file is appended to the input file suffix. In
compilation commands, the base name of both auxiliary and dump outputs is that of the
primary output; in compile and link commands, the primary output name, minus the
executable suffix, is combined with the input file name. If both share the same base
name, disregarding the suffix, the result of the combination is that base name,
otherwise, they are concatenated, separated by a dash.

gcc -c foo.c ...

will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g.,
aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.

If a non-linker output file is explicitly specified, aux and dump files by default take
the same base name:

gcc -c foo.c -o dir/foobar.o ...

will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.

A linker output will instead prefix aux and dump outputs:

gcc foo.c bar.c -o dir/foobar ...

will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs
dir/foobar-foo.c.* and dir/foobar-bar.c.*.

The one exception to the above is when the executable shares the base name with the
single input:

gcc foo.c -o dir/foo ...

in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.

The location and the names of auxiliary and dump outputs can be adjusted by the options
-dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.

-dumpbase dumpbase
This option sets the base name for auxiliary and dump output files. It does not affect
the name of the primary output file. Intermediate outputs, when preserved, are not
regarded as primary outputs, but as auxiliary outputs:

gcc -save-temps -S foo.c

saves the (no longer) temporary preprocessed file in foo.i, and then compiles to the
(implied) output file foo.s, whereas:

gcc -save-temps -dumpbase save-foo -c foo.c

preprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus
auxiliary output), and then assembles to the (implied) output file foo.o.

Absent this option, dump and aux files take their names from the input file, or from the
(non-linker) output file, if one is explicitly specified: dump output files (e.g. those
requested by -fdump-* options) with the input name suffix, and aux output files (those
requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf",
"-fcallgraph-info") without it.

Similar suffix differentiation of dump and aux outputs can be attained for explicitly-
given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.

If dumpbase is explicitly specified with any directory component, any dumppfx
specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to
it, dumpbase fully overrides it:

gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...

creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./
prefix implied by -save-temps=cwd, and pfx- in -dumpdir.

When -dumpbase is specified in a command that compiles multiple inputs, or that compiles
and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each
input file is compiled using the combined dumppfx, and default values for dumpbase and
auxdropsuf are computed for each input file:

gcc foo.c bar.c -c -dumpbase main ...

creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump
outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named
main-foo.* and main-bar.*.

An empty string specified as dumpbase avoids the influence of the output basename in the
naming of auxiliary and dump outputs during compilation, computing default values :

gcc -c foo.c -o dir/foobar.o -dumpbase " ...

will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames
are taken from the input name, but the directory still defaults to that of the output.

The empty-string dumpbase does not prevent the use of the output basename for outputs
during linking:

gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...

The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*,
and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use
dir/foobar. as the prefix for dumps and auxiliary files.

-dumpbase-ext auxdropsuf
When forming the name of an auxiliary (but not a dump) output file, drop trailing
auxdropsuf from dumpbase before appending any suffixes. If not specified, this option
defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when
-dumpbase is not present in the command line, or dumpbase is combined with dumppfx.

gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...

creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*,
taking the location of the primary output, and dropping the .c suffix from the dumpbase.
Dump outputs retain the suffix: dir/x-foo.c.*.

This option is disregarded if it does not match the suffix of a specified dumpbase,
except as an alternative to the executable suffix when appending the linker output base
name to dumppfx, as specified below:

gcc foo.c bar.c -o main.out -dumpbase-ext .out ...

creates main.out as the primary output, and avoids overwriting the auxiliary and dump
outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary
outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and
main-bar.c.*.

-dumpdir dumppfx
When forming the name of an auxiliary or dump output file, use dumppfx as a prefix:

gcc -dumpdir pfx- -c foo.c ...

creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the
given dumppfx with the default dumpbase derived from the default primary output, derived
in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.

If dumppfx is to be used as a directory name, it must end with a directory separator:

gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...

creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining
the given dumppfx with the default dumpbase derived from the primary output name. Dump
outputs also take the input name suffix: dir/bar.c.*.

It defaults to the location of the output file, unless the output file is a special file
like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just
like an explicit -dumpdir option. In case multiple such options are given, the last one
prevails:

gcc -dumpdir pfx- -c foo.c -save-temps=obj ...

outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the
dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the
default for -save-temps, nor that the output directory is implicitly the current
directory. Dump outputs are named foo.c.*.

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a
auxdropsuf suffix, and a dash are appended to (or override, if containing any directory
components) an explicit or defaulted dumppfx, so that each of the multiple compilations
gets differently-named aux and dump outputs.

gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...

outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase-
to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and
dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:

gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...

that, applying -dumpbase to a single source, does not compute and append a separate
dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.

When compiling and then linking from multiple input files, a defaulted or explicitly
specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation
of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given,
the linker output base name, minus auxdropsuf, if specified, or the executable suffix
otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that
unlike earlier cases of linking:

gcc foo.c bar.c -dumpdir dir/pfx- -o main ...

does not append the output name main to dumppfx, because -dumpdir is explicitly
specified. The goal is that the explicitly-specified dumppfx may contain the specified
output name as part of the prefix, if desired; only an explicitly-specified -dumpbase
would be combined with it, in order to avoid simply discarding a meaningful option.

When compiling and then linking from a single input file, the linker output base name
will only be appended to the default dumppfx as above if it does not share the base name
with the single input file name. This has been covered in single-input linking cases
above, but not with an explicit -dumpdir that inhibits the combination, even if
overridden by -save-temps=*:

gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...

Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working
directory as ultimately requested by -save-temps=cwd.

Summing it all up for an intuitive though slightly imprecise data flow: the primary
output name is broken into a directory part and a basename part; dumppfx is set to the
former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the
latter, unless overriden by -dumpbase. If there are multiple inputs or linking, this
dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output
names for each input are formed by combining dumppfx, dumpbase minus suffix, and the
auxiliary output suffix; dump output names are only different in that the suffix from
dumpbase is retained.

When it comes to auxiliary and dump outputs created during LTO recompilation, a
combination of dumppfx and dumpbase, as given or as derived from the linker output name
but not from inputs, even in cases in which this combination would not otherwise be used
as such, is passed down with a trailing period replacing the compiler-added dash, if any,
as a -dumpdir option to lto-wrapper; being involved in linking, this program does not
normally get any -dumpbase and -dumpbase-ext, and it ignores them.

When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx,
ensures it contains a directory component so that it overrides any -dumpdir, and passes
that as -dumpbase to sub-compilers.

-v Print (on standard error output) the commands executed to run the stages of compilation.
Also print the version number of the compiler driver program and of the preprocessor and
the compiler proper.

-###
Like -v except the commands are not executed and arguments are quoted unless they contain
only alphanumeric characters or "./-_". This is useful for shell scripts to capture the
driver-generated command lines.

--help
Print (on the standard output) a description of the command-line options understood by
gcc. If the -v option is also specified then --help is also passed on to the various
processes invoked by gcc, so that they can display the command-line options they accept.
If the -Wextra option has also been specified (prior to the --help option), then command-
line options that have no documentation associated with them are also displayed.

--target-help
Print (on the standard output) a description of target-specific command-line options for
each tool. For some targets extra target-specific information may also be printed.

--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line options understood by
the compiler that fit into all specified classes and qualifiers. These are the supported
classes:

optimizers
Display all of the optimization options supported by the compiler.

warnings
Display all of the options controlling warning messages produced by the compiler.

target
Display target-specific options. Unlike the --target-help option however, target-
specific options of the linker and assembler are not displayed. This is because
those tools do not currently support the extended --help= syntax.

params
Display the values recognized by the --param option.

language
Display the options supported for language, where language is the name of one of the
languages supported in this version of GCC. If an option is supported by all
languages, one needs to select common class.

common
Display the options that are common to all languages.

These are the supported qualifiers:

undocumented
Display only those options that are undocumented.

joined
Display options taking an argument that appears after an equal sign in the same
continuous piece of text, such as: --help=target.

separate
Display options taking an argument that appears as a separate word following the
original option, such as: -o output-file.

Thus for example to display all the undocumented target-specific switches supported by
the compiler, use:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for
example to display all binary warning options (i.e., ones that are either on or off and
that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers.

Combining several classes is possible, although this usually restricts the output so much
that there is nothing to display. One case where it does work, however, is when one of
the classes is target. For example, to display all the target-specific optimization
options, use:

--help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its
requested class of options, skipping those that have already been displayed. If --help
is also specified anywhere on the command line then this takes precedence over any
--help= option.

If the -Q option appears on the command line before the --help= option, then the
descriptive text displayed by --help= is changed. Instead of describing the displayed
options, an indication is given as to whether the option is enabled, disabled or set to a
specific value (assuming that the compiler knows this at the point where the --help=
option is used).

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]

The output is sensitive to the effects of previous command-line options, so for example
it is possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled

--version
Display the version number and copyrights of the invoked GCC.

-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of the compiler returns a
non-success return code. If you specify -pass-exit-codes, the gcc program instead
returns with the numerically highest error produced by any phase returning an error
indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is
encountered.

-pipe
Use pipes rather than temporary files for communication between the various stages of
compilation. This fails to work on some systems where the assembler is unable to read
from a pipe; but the GNU assembler has no trouble.

-specs=file
Process file after the compiler reads in the standard specs file, in order to override
the defaults which the gcc driver program uses when determining what switches to pass to
cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command
line, and they are processed in order, from left to right.

-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper program and its
parameters are passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb
--args cc1 ....

-ffile-prefix-map=old=new
When compiling files residing in directory old, record any references to them in the
result of the compilation as if the files resided in directory new instead. Specifying
this option is equivalent to specifying all the individual -f*-prefix-map options. This
can be used to make reproducible builds that are location independent. Directories
referenced by directives are not affected by these options. See also -fmacro-prefix-map,
-fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.

-fcanon-prefix-map
For the -f*-prefix-map options normally comparison of old prefix against the filename
that would be normally referenced in the result of the compilation is done using textual
comparison of the prefixes, or ignoring character case for case insensitive filesystems
and considering slashes and backslashes as equal on DOS based filesystems. The
-fcanon-prefix-map causes such comparisons to be done on canonicalized paths of old and
the referenced filename.

-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the
compiler. The base name of the shared object file is used to identify the plugin for the
purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should
define the callback functions specified in the Plugins API.

-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.

-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.

-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of
parent unit.

-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in file. This
generates Go "const", "type", "var", and "func" declarations which may be a useful way to
start writing a Go interface to code written in some other language.

@file
Read command-line options from file. The options read are inserted in place of the
original @file option. If file does not exist, or cannot be read, then the option will
be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in
an option by surrounding the entire option in either single or double quotes. Any
character (including a backslash) may be included by prefixing the character to be
included with a backslash. The file may itself contain additional @file options; any
such options will be processed recursively.

Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or
.cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and
preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and
compiles them as C++ programs even if you call the compiler the same way as for compiling C
programs (usually with the name gcc).

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and
automatically specifies linking against the C++ library. It treats .c, .h and .i files as
C++ source files instead of C source files unless -x is used. This program is also useful
when precompiling a C header file with a .h extension for use in C++ compilations. On many
systems, g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-line options that you
use for compiling programs in any language; or command-line options meaningful for C and
related languages; or options that are meaningful only for C++ programs.

Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++,
Objective-C and Objective-C++) that the compiler accepts:

-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling
C code), or of standard C++ (when compiling C++ code), such as the "asm" and "typeof"
keywords, and predefined macros such as "unix" and "vax" that identify the type of system
you are using. It also enables the undesirable and rarely used ISO trigraph feature.
For the C compiler, it disables recognition of C++ style // comments as well as the
"inline" keyword.

The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue
to work despite -ansi. You would not want to use them in an ISO C program, of course,
but it is useful to put them in header files that might be included in compilations done
with -ansi. Alternate predefined macros such as "__unix__" and "__vax__" are also
available, with or without -ansi.

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that,
-Wpedantic is required in addition to -ansi.

The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header
files may notice this macro and refrain from declaring certain functions or defining
certain macros that the ISO standard doesn't call for; this is to avoid interfering with
any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as
"alloca" and "ffs") are not built-in functions when -ansi is used.

-std=
Determine the language standard. This option is currently only supported when compiling
C or C++.

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of
those standards, such as gnu90 or gnu++98. When a base standard is specified, the
compiler accepts all programs following that standard plus those using GNU extensions
that do not contradict it. For example, -std=c90 turns off certain features of GCC that
are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such as omitting the middle term of a
"?:" expression. On the other hand, when a GNU dialect of a standard is specified, all
features supported by the compiler are enabled, even when those features change the
meaning of the base standard. As a result, some strict-conforming programs may be
rejected. The particular standard is used by -Wpedantic to identify which features are
GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic
warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

A value for this option must be provided; possible values are

c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are
disabled). Same as -ansi for C code.

iso9899:199409
ISO C90 as modified in amendment 1.

c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported, modulo bugs and
floating-point issues (mainly but not entirely relating to optional C99 features from
Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more information.
The names c9x and iso9899:199x are deprecated.

c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This standard is substantially
completely supported, modulo bugs, floating-point issues (mainly but not entirely
relating to optional C11 features from Annexes F and G) and the optional Annexes K
(Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.

c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard
is same as C11 except for corrections of defects (all of which are also applied with
-std=c11) and a new value of "__STDC_VERSION__", and so is supported to the same
extent as C11.

c2x The next version of the ISO C standard, still under development. The support for
this version is experimental and incomplete.

gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).

gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.

gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.

gnu17
gnu18
GNU dialect of ISO C17. This is the default for C code.

gnu2x
The next version of the ISO C standard, still under development, plus GNU extensions.
The support for this version is experimental and incomplete.

c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional
defect reports. Same as -ansi for C++ code.

gnu++98
gnu++03
GNU dialect of -std=c++98.

c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.

gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.

c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.

gnu++14
gnu++1y
GNU dialect of -std=c++14. The name gnu++1y is deprecated.

c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.

gnu++17
gnu++1z
GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is
deprecated.

c++20
c++2a
The 2020 ISO C++ standard plus amendments. Support is experimental, and could change
in incompatible ways in future releases. The name c++2a is deprecated.

gnu++20
gnu++2a
GNU dialect of -std=c++20. Support is experimental, and could change in incompatible
ways in future releases. The name gnu++2a is deprecated.

c++2b
c++23
The next revision of the ISO C++ standard, planned for 2023. Support is highly
experimental, and will almost certainly change in incompatible ways in future
releases.

gnu++2b
gnu++23
GNU dialect of -std=c++2b. Support is highly experimental, and will almost certainly
change in incompatible ways in future releases.

-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or
defined in a translation unit, including those in header files. This option is silently
ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration
(source file and line), whether the declaration was implicit, prototyped or unprototyped
(I, N for new or O for old, respectively, in the first character after the line number
and the colon), and whether it came from a declaration or a definition (C or F,
respectively, in the following character). In the case of function definitions, a
K&R-style list of arguments followed by their declarations is also provided, inside
comments, after the declaration.

-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these
words as identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__"
instead. In C, -ansi implies -fno-asm.

In C++, "inline" is a standard keyword and is not affected by this switch. You may want
to use the -fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and
"inline". In C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and
"typeof" keywords, since "inline" is a standard keyword in ISO C99. In C2X mode
(-std=c2x or -std=gnu2x), this switch only affects the "asm" keyword, since "typeof" is a
standard keyword in ISO C2X.

-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.

GCC normally generates special code to handle certain built-in functions more
efficiently; for instance, calls to "alloca" may become single instructions which adjust
the stack directly, and calls to "memcpy" may become inline copy loops. The resulting
code is often both smaller and faster, but since the function calls no longer appear as
such, you cannot set a breakpoint on those calls, nor can you change the behavior of the
functions by linking with a different library. In addition, when a function is
recognized as a built-in function, GCC may use information about that function to warn
about problems with calls to that function, or to generate more efficient code, even if
the resulting code still contains calls to that function. For example, warnings are
given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is
known not to modify global memory.

With the -fno-builtin-function option only the built-in function function is disabled.
function must not begin with __builtin_. If a function is named that is not built-in in
this version of GCC, this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in functions selectively when
using -fno-builtin or -ffreestanding, you may define macros such as:

#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))

-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments.
The value of such an expression is void. This option is not supported for C++.

-ffreestanding
Assert that compilation targets a freestanding environment. This implies -fno-builtin.
A freestanding environment is one in which the standard library may not exist, and
program startup may not necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.

-fgimple
Enable parsing of function definitions marked with "__GIMPLE". This is an experimental
feature that allows unit testing of GIMPLE passes.

-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates code for the Linux variant
of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6
2009). This is an experimental feature whose interface may change in future versions of
GCC, as the official specification changes. Please note that not all architectures are
supported for this feature.

For more information on GCC's support for transactional memory,

Note that the transactional memory feature is not supported with non-call exceptions
(-fnon-call-exceptions).

-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline"
functions when in C99 mode.

Using this option is roughly equivalent to adding the "gnu_inline" function attribute to
all inline functions.

The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline"
when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not
supported in -std=c90 or -std=gnu90 mode.

The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to
check which semantics are in effect for "inline" functions.

-fhosted
Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted
environment is one in which the entire standard library is available, and in which "main"
has a return type of "int". Examples are nearly everything except a kernel. This is
equivalent to -fno-freestanding.

-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or
incompatible element types. This option should not be used for new code.

-fms-extensions
Accept some non-standard constructs used in Microsoft header files.

In C++ code, this allows member names in structures to be similar to previous types
declarations.

typedef int UOW;
struct ABC {
UOW UOW;
};

Some cases of unnamed fields in structures and unions are only accepted with this option.

Note that this option is off for all targets except for x86 targets using ms-abi.

-foffload=disable
-foffload=default
-foffload=target-list
Specify for which OpenMP and OpenACC offload targets code should be generated. The
default behavior, equivalent to -foffload=default, is to generate code for all supported
offload targets. The -foffload=disable form generates code only for the host fallback,
while -foffload=target-list generates code only for the specified comma-separated list of
offload targets.

Offload targets are specified in GCC's internal target-triplet format. You can run the
compiler with -v to show the list of configured offload targets under
"OFFLOAD_TARGET_NAMES".

-foffload-options=options
-foffload-options=target-triplet-list=options
With -foffload-options=options, GCC passes the specified options to the compilers for all
enabled offloading targets. You can specify options that apply only to a specific target
or targets by using the -foffload-options=target-list=options form. The target-list is a
comma-separated list in the same format as for the -foffload= option.

Typical command lines are

-foffload-options=-lgfortran -foffload-options=-lm
-foffload-options="-lgfortran -lm" -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm

-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++ and "!$acc" in Fortran.
When -fopenacc is specified, the compiler generates accelerated code according to the
OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. This option
implies -pthread, and thus is only supported on targets that have support for -pthread.

-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions that do not explicitly
specify. The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker'
and, 'vector'. A size can be omitted, to use a target-specific default value.

-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++, "[[omp::directive(...)]]"
and "[[omp::sequence(...)]]" in C++ and "!$omp" in Fortran. When -fopenmp is specified,
the compiler generates parallel code according to the OpenMP Application Program
Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only
supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.

-fopenmp-simd
Enable handling of OpenMP's "simd", "declare simd", "declare reduction", "assume",
"ordered", "scan", "loop" directives and combined or composite directives with "simd" as
constituent with "#pragma omp" in C/C++, "[[omp::directive(...)]]" and
"[[omp::sequence(...)]]" in C++ and "!$omp" in Fortran. Other OpenMP directives are
ignored.

-fopenmp-target-simd-clone
-fopenmp-target-simd-clone=device-type
In addition to generating SIMD clones for functions marked with the "declare simd"
directive, GCC also generates clones for functions marked with the OpenMP "declare
target" directive that are suitable for vectorization when this option is in effect. The
device-type may be one of "none", "host", "nohost", and "any", which correspond to
keywords for the "device_type" clause of the "declare target" directive; clones are
generated for the intersection of devices specified. -fopenmp-target-simd-clone is
equivalent to -fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is
equivalent to -fopenmp-target-simd-clone=none.

At -O2 and higher (but not -Os or -Og) this optimization defaults to
-fopenmp-target-simd-clone=nohost; otherwise it is disabled by default.

-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate
that operations and constants with a semantic type that is an interchange or extended
format should be evaluated to the precision and range of that type. These new values are
a superset of those permitted under C99/C11, which does not specify the meaning of other
positive values of "FLT_EVAL_METHOD". As such, code conforming to C11 may not have been
written expecting the possibility of the new values.

-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values
of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in
ISO/IEC TS 18661-3.

style is either "c11" or "ts-18661-3" as appropriate.

The default when in a standards compliant mode (-std=c11 or similar) is
-fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or
similar) is -fpermitted-flt-eval-methods=ts-18661-3.

-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.

This enables -fms-extensions, permits passing pointers to structures with anonymous
fields to functions that expect pointers to elements of the type of the field, and
permits referring to anonymous fields declared using a typedef. This is only supported
for C, not C++.

-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration
does not use either "signed" or "unsigned". By default, such a bit-field is signed,
because this is consistent: the basic integer types such as "int" are signed types.

-fsigned-char
Let the type "char" be signed, like "signed char".

Note that this is equivalent to -fno-unsigned-char, which is the negative form of
-funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.

-funsigned-char
Let the type "char" be unsigned, like "unsigned char".

Each kind of machine has a default for what "char" should be. It is either like
"unsigned char" by default or like "signed char" by default.

Ideally, a portable program should always use "signed char" or "unsigned char" when it
depends on the signedness of an object. But many programs have been written to use plain
"char" and expect it to be signed, or expect it to be unsigned, depending on the machines
they were written for. This option, and its inverse, let you make such a program work
with the opposite default.

The type "char" is always a distinct type from each of "signed char" or "unsigned char",
even though its behavior is always just like one of those two.

-fstrict-flex-arrays
Control when to treat the trailing array of a structure as a flexible array member for
the purpose of accessing the elements of such an array. The positive form is equivalent
to -fstrict-flex-arrays=3, which is the strictest. A trailing array is treated as a
flexible array member only when it is declared as a flexible array member per C99
standard onwards. The negative form is equivalent to -fstrict-flex-arrays=0, which is
the least strict. All trailing arrays of structures are treated as flexible array
members.

-fstrict-flex-arrays=level
Control when to treat the trailing array of a structure as a flexible array member for
the purpose of accessing the elements of such an array. The value of level controls the
level of strictness.

The possible values of level are the same as for the "strict_flex_array" attribute.

You can control this behavior for a specific trailing array field of a structure by using
the variable attribute "strict_flex_array" attribute.

-fsso-struct=endianness
Set the default scalar storage order of structures and unions to the specified
endianness. The accepted values are big-endian, little-endian and native for the native
endianness of the target (the default). This option is not supported for C++.

Warning: the -fsso-struct switch causes GCC to generate code that is not binary
compatible with code generated without it if the specified endianness is not the native
endianness of the target.

Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs.
You can also use most of the GNU compiler options regardless of what language your program is
in. For example, you might compile a file firstClass.C like this:

g++ -g -fstrict-enums -O -c firstClass.C

In this example, only -fstrict-enums is an option meant only for C++ programs; you can use
the other options with any language supported by GCC.

Some options for compiling C programs, such as -std, are also relevant for C++ programs.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=n
Use version n of the C++ ABI. The default is version 0.

Version 0 refers to the version conforming most closely to the C++ ABI specification.
Therefore, the ABI obtained using version 0 will change in different versions of G++ as
ABI bugs are fixed.

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the
default through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector
types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute
const/volatile on function pointer types, decltype of a plain decl, and use of a function
parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11
scoped enums and the mangling of template argument packs, const/static_cast, prefix ++
and --, and a class scope function used as a template argument.

Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and
corrects the mangling of lambdas in default argument scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of
function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of "nullptr_t".

Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type
identity, such as ia32 calling convention attributes (e.g. stdcall).

Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions
and operator names. For multiple entities with the same name within a function, that are
declared in different scopes, the mangling now changes starting with the twelfth
occurrence. It also implies -fnew-inheriting-ctors.

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty
classes on the x86_64 target and for classes with only deleted copy/move constructors.
It accidentally changes the calling convention for classes with a deleted copy
constructor and a trivial move constructor.

Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.

Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr
expression.

Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.

Version 16, which first appeared in G++ 11, changes the mangling of "__alignof__" to be
distinct from that of "alignof", and dependent operator names.

Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from
aggregate classes with default member initializers in C++14 and up.

Version 18, which first appeard in G++ 13, fixes manglings of lambdas that have
additional context.

See also -Wvla-larger-than=byte-size.

-Wno-alloca-larger-than
Disable -Walloca-larger-than= warnings. The option is equivalent to
-Walloca-larger-than=SIZE_MAX or larger.

-Warith-conversion
Do warn about implicit conversions from arithmetic operations even when conversion of the
operands to the same type cannot change their values. This affects warnings from
-Wconversion, -Wfloat-conversion, and -Wsign-conversion.

void f (char c, int i)
{
c = c + i; // warns with B<-Wconversion>
c = c + 1; // only warns with B<-Warith-conversion>
}

-Warray-bounds
-Warray-bounds=n
Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by
-Wall. It is more effective when -ftree-vrp is active (the default for -O2 and above)
but a subset of instances are issued even without optimization.

By default, the trailing array of a structure will be treated as a flexible array member
by -Warray-bounds or -Warray-bounds=n if it is declared as either a flexible array member
per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an one-element
array ([1]). As a result, out of bounds subscripts or offsets into zero-length arrays or
one-element arrays are not warned by default.

You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how
this option treat trailing array of a structure as a flexible array member:

when level<=1, no change to the default behavior.

when level=2, additional warnings will be issued for out of bounds subscripts or offsets
into one-element arrays;

when level=3, in addition to level=2, additional warnings will be issued for out of
bounds subscripts or offsets into zero-length arrays.

-Warray-bounds=1
This is the default warning level of -Warray-bounds and is enabled by -Wall; higher
levels are not, and must be explicitly requested.

-Warray-bounds=2
This warning level also warns about the intermediate results of pointer arithmetic
that may yield out of bounds values. This warning level may give a larger number of
false positives and is deactivated by default.

-Warray-compare
Warn about equality and relational comparisons between two operands of array type. This
comparison was deprecated in C++20. For example:

int arr1[5];
int arr2[5];
bool same = arr1 == arr2;

-Warray-compare is enabled by -Wall.

-Warray-parameter
-Warray-parameter=n
Warn about redeclarations of functions involving arguments of array or pointer types of
inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such
parameters by warnings such as -Warray-bounds.

If the first function declaration uses the array form the bound specified in the array is
assumed to be the minimum number of elements expected to be provided in calls to the
function and the maximum number of elements accessed by it. Failing to provide arguments
of sufficient size or accessing more than the maximum number of elements may be diagnosed
by warnings such as -Warray-bounds. At level 1 the warning diagnoses inconsistencies
involving array parameters declared using the "T[static N]" form.

For example, the warning triggers for the following redeclarations because the first one
allows an array of any size to be passed to "f" while the second one with the keyword
"static" specifies that the array argument must have at least four elements.

void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)

void g (void)
{
int *p = (int *)malloc (4);
f (p); // warning (array too small)
...
}

At level 2 the warning also triggers for redeclarations involving any other inconsistency
in array or pointer argument forms denoting array sizes. Pointers and arrays of
unspecified bound are considered equivalent and do not trigger a warning.

void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)

-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings
for similar inconsistencies involving Variable Length Array arguments.

-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the "alias" and similar attributes whose target is
incompatible with the type of the alias.

-Wattribute-alias=1
The default warning level of the -Wattribute-alias option diagnoses incompatibilities
between the type of the alias declaration and that of its target. Such
incompatibilities are typically indicative of bugs.

-Wattribute-alias=2
At this level -Wattribute-alias also diagnoses cases where the attributes of the
alias declaration are more restrictive than the attributes applied to its target.
These mismatches can potentially result in incorrect code generation. In other cases
they may be benign and could be resolved simply by adding the missing attribute to
the target. For comparison, see the -Wmissing-attributes option, which controls
diagnostics when the alias declaration is less restrictive than the target, rather
than more restrictive.

Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot",
"leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and
"returns_twice".

-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can
disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.

-Wbidi-chars=[none|unpaired|any|ucn]
Warn about possibly misleading UTF-8 bidirectional control characters in comments, string
literals, character constants, and identifiers. Such characters can change left-to-right
writing direction into right-to-left (and vice versa), which can cause confusion between
the logical order and visual order. This may be dangerous; for instance, it may seem
that a piece of code is not commented out, whereas it in fact is.

There are three levels of warning supported by GCC. The default is
-Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts.
-Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of
bidirectional control characters.

By default, this warning does not warn about UCNs. It is, however, possible to turn on
such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using
-Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous
-Wbidi-chars=any was specified.

-Wbool-compare
Warn about boolean expression compared with an integer value different from
"true"/"false". For instance, the following comparison is always false:

int n = 5;
...
if ((n > 1) == 2) { ... }

This warning is enabled by -Wall.

-Wbool-operation
Warn about suspicious operations on expressions of a boolean type. For instance, bitwise
negation of a boolean is very likely a bug in the program. For C, this warning also
warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++,
decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17,
and deprecated otherwise.)

This warning is enabled by -Wall.

-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects cases like

if (p != NULL)
return 0;
else
return 0;

It doesn't warn when both branches contain just a null statement. This warning also warn
for conditional operators:

int i = x ? *p : *p;

-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For instance, warn for the
following code:

if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }

-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address is called with an
argument greater than 0. Such calls may return indeterminate values or crash the
program. The warning is included in -Wall.

-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler
warns if a "const char *" variable is passed to a function that takes a "char *"
parameter. This option can be used to suppress such a warning.

-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets are being discarded.
Typically, the compiler warns if a "const int (*)[]" variable is passed to a function
that takes a "int (*)[]" parameter. This option can be used to suppress such a warning.

-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have incompatible types.
This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer
argument passing or assignment with different signedness.

-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to integer conversions.
This warning is about implicit conversions; for explicit conversions the warnings
-Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.

-Wzero-length-bounds
Warn about accesses to elements of zero-length array members that might overlap other
members of the same object. Declaring interior zero-length arrays is discouraged because
accesses to them are undefined.

For example, the first two stores in function "bad" are diagnosed because the array
elements overlap the subsequent members "b" and "c". The third store is diagnosed by
-Warray-bounds because it is beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; };
struct X x;

void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}

Option -Wzero-length-bounds is enabled by -Warray-bounds.

-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point division by zero
is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system
headers are normally suppressed, on the assumption that they usually do not indicate real
problems and would only make the compiler output harder to read. Using this command-line
option tells GCC to emit warnings from system headers as if they occurred in user code.
However, note that using -Wall in conjunction with this option does not warn about
unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.

-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This warning detects
various mistakes such as:

int i = 1;
...
if (i > i) { ... }

This warning also warns about bitwise comparisons that always evaluate to true or false,
for instance:

if ((a & 16) == 10) { ... }

will always be false.

This warning is enabled by -Wall.

-Wtrampolines
Warn about trampolines generated for pointers to nested functions. A trampoline is a
small piece of data or code that is created at run time on the stack when the address of
a nested function is taken, and is used to call the nested function indirectly. For some
targets, it is made up of data only and thus requires no special treatment. But, for
most targets, it is made up of code and thus requires the stack to be made executable in
order for the program to work properly.

-Wfloat-equal
Warn if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the programmer) to consider
floating-point values as approximations to infinitely precise real numbers. If you are
doing this, then you need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces, and allow for it when
performing comparisons (and when producing output, but that's a different problem). In
particular, instead of testing for equality, you should check to see whether the two
values have ranges that overlap; and this is done with the relational operators, so
equality comparisons are probably mistaken.

-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also
warn about ISO C constructs that have no traditional C equivalent, and/or problematic
constructs that should be avoided.

* Macro parameters that appear within string literals in the macro body. In
traditional C macro replacement takes place within string literals, but in ISO C it
does not.

* In traditional C, some preprocessor directives did not exist. Traditional
preprocessors only considered a line to be a directive if the # appeared in column 1
on the line. Therefore -Wtraditional warns about directives that traditional C
understands but ignores because the # does not appear as the first character on the
line. It also suggests you hide directives like "#pragma" not understood by
traditional C by indenting them. Some traditional implementations do not recognize
"#elif", so this option suggests avoiding it altogether.

* A function-like macro that appears without arguments.

* The unary plus operator.

* The U integer constant suffix, or the F or L floating-point constant suffixes.
(Traditional C does support the L suffix on integer constants.) Note, these suffixes
appear in macros defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user code might normally
lead to spurious warnings, however GCC's integrated preprocessor has enough context
to avoid warning in these cases.

* A function declared external in one block and then used after the end of the block.

* A "switch" statement has an operand of type "long".

* A non-"static" function declaration follows a "static" one. This construct is not
accepted by some traditional C compilers.

* The ISO type of an integer constant has a different width or signedness from its
traditional type. This warning is only issued if the base of the constant is ten.
I.e. hexadecimal or octal values, which typically represent bit patterns, are not
warned about.

* Usage of ISO string concatenation is detected.

* Initialization of automatic aggregates.

* Identifier conflicts with labels. Traditional C lacks a separate namespace for
labels.

* Initialization of unions. If the initializer is zero, the warning is omitted. This
is done under the assumption that the zero initializer in user code appears
conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.

* Conversions by prototypes between fixed/floating-point values and vice versa. The
absence of these prototypes when compiling with traditional C causes serious
problems. This is a subset of the possible conversion warnings; for the full set use
-Wtraditional-conversion.

* Use of ISO C style function definitions. This warning intentionally is not issued
for prototype declarations or variadic functions because these ISO C features appear
in your code when using libiberty's traditional C compatibility macros, "PARAMS" and
"VPARAMS". This warning is also bypassed for nested functions because that feature
is already a GCC extension and thus not relevant to traditional C compatibility.

-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would happen to
the same argument in the absence of a prototype. This includes conversions of fixed
point to floating and vice versa, and conversions changing the width or signedness of a
fixed-point argument except when the same as the default promotion.

-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct, known
from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not
supported by ISO C90.

-Wshadow
Warn whenever a local variable or type declaration shadows another variable, parameter,
type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in
function is shadowed. Note that in C++, the compiler warns if a local variable shadows
an explicit typedef, but not if it shadows a struct/class/enum. If this warning is
enabled, it includes also all instances of local shadowing. This means that
-Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used.
Same as -Wshadow=global.

-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable in an Objective-C
method.

-Wshadow=global
Warn for any shadowing. Same as -Wshadow.

-Wshadow=local
Warn when a local variable shadows another local variable or parameter.

-Wshadow=compatible-local
Warn when a local variable shadows another local variable or parameter whose type is
compatible with that of the shadowing variable. In C++, type compatibility here means
the type of the shadowing variable can be converted to that of the shadowed variable.
The creation of this flag (in addition to -Wshadow=local) is based on the idea that when
a local variable shadows another one of incompatible type, it is most likely intentional,
not a bug or typo, as shown in the following example:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}

Since the two variable "i" in the example above have incompatible types, enabling only
-Wshadow=compatible-local does not emit a warning. Because their types are incompatible,
if a programmer accidentally uses one in place of the other, type checking is expected to
catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can
possibly reduce the number of warnings triggered by intentional shadowing. Note that
this also means that shadowing "const char *i" by "char *i" does not emit a warning.

This warning is also enabled by -Wshadow=local.

-Wlarger-than=byte-size
Warn whenever an object is defined whose size exceeds byte-size.
-Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can
be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-larger-than.

Also warn for calls to bounded functions such as "memchr" or "strnlen" that specify a
bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default.
These warnings can only be disabled by -Wno-larger-than.

-Wno-larger-than
Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or
larger.

-Wframe-larger-than=byte-size
Warn if the size of a function frame exceeds byte-size. The computation done to
determine the stack frame size is approximate and not conservative. The actual
requirements may be somewhat greater than byte-size even if you do not get a warning. In
addition, any space allocated via "alloca", variable-length arrays, or related constructs
is not included by the compiler when determining whether or not to issue a warning.
-Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option
can be disabled either by specifying byte-size of SIZE_MAX or more or by
-Wno-frame-larger-than.

-Wno-frame-larger-than
Disable -Wframe-larger-than= warnings. The option is equivalent to
-Wframe-larger-than=SIZE_MAX or larger.

-Wfree-nonheap-object
Warn when attempting to deallocate an object that was either not allocated on the heap,
or by using a pointer that was not returned from a prior call to the corresponding
allocation function. For example, because the call to "stpcpy" returns a pointer to the
terminating nul character and not to the beginning of the object, the call to "free"
below is diagnosed.

void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}

-Wfree-nonheap-object is included in -Wall.

-Wstack-usage=byte-size
Warn if the stack usage of a function might exceed byte-size. The computation done to
determine the stack usage is conservative. Any space allocated via "alloca", variable-
length arrays, or related constructs is included by the compiler when determining whether
or not to issue a warning.

The message is in keeping with the output of -fstack-usage.

* If the stack usage is fully static but exceeds the specified amount, it's:

warning: stack usage is 1120 bytes

* If the stack usage is (partly) dynamic but bounded, it's:

warning: stack usage might be 1648 bytes

* If the stack usage is (partly) dynamic and not bounded, it's:

warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can
be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-stack-usage.

-Wno-stack-usage
Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or
larger.

-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot assume anything on the
bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler makes
such assumptions.

-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU extensions, this option
disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32",
"I64", and "I" used on Windows targets, which depend on the MS runtime.

-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C
assigns these types a size of 1, for convenience in calculations with "void *" pointers
and pointers to functions. In C++, warn also when an arithmetic operation involves
"NULL". This warning is also enabled by -Wpedantic.

-Wno-pointer-compare
Do not warn if a pointer is compared with a zero character constant. This usually means
that the pointer was meant to be dereferenced. For example:

const char *p = foo ();
if (p == '\0')
return 42;

Note that the code above is invalid in C++11.

This warning is enabled by default.

-Wtsan
Warn about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support "std::atomic_thread_fence" and can report false
positives.

This warning is enabled by default.

-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data
type, but do not warn for constant expressions. For example, warn if an unsigned
variable is compared against zero with "<" or ">=". This warning is also enabled by
-Wextra.

-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the absolute value of an argument when
a more appropriate standard function is available. For example, calling abs(3.14)
triggers the warning because the appropriate function to call to compute the absolute
value of a double argument is "fabs". The option also triggers warnings when the
argument in a call to such a function has an unsigned type. This warning can be
suppressed with an explicit type cast and it is also enabled by -Wextra.

-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a
backslash-newline appears in a // comment. This warning is enabled by -Wall.

-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program.
Trigraphs within comments are not warned about, except those that would form escaped
newlines.

This option is implied by -Wall. If -Wall is not given, this option is still enabled
unless trigraphs are enabled. To get trigraph conversion without warnings, but get the
other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive. Such identifiers are
replaced with zero.

-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro (including the case
where the macro is expanded by an #if directive). Such usage is not portable. This
warning is also enabled by -Wpedantic and -Wextra.

-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is
expanded or tested for existence at least once. The preprocessor also warns if the macro
has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files
are not warned about.

Note: If a macro is actually used, but only used in skipped conditional blocks, then the
preprocessor reports it as unused. To avoid the warning in such a case, you might
improve the scope of the macro's definition by, for example, moving it into the first
skipped block. Alternatively, you could provide a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by text. This sometimes
happens in older programs with code of the form

#if FOO
...
#else FOO
...
#endif FOO

The second and third "FOO" should be in comments. This warning is on by default.

-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For example, warn if a call to
a function returning an integer type is cast to a pointer type.

-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99. For instance, warn
about use of variable length arrays, "long long" type, "bool" type, compound literals,
designated initializers, and so on. This option is independent of the standards mode.
Warnings are disabled in the expression that follows "__extension__".

-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11. For instance, warn
about use of anonymous structures and unions, "_Atomic" type qualifier, "_Thread_local"
storage-class specifier, "_Alignas" specifier, "Alignof" operator, "_Generic" keyword,
and so on. This option is independent of the standards mode. Warnings are disabled in
the expression that follows "__extension__".

-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C2X. For instance, warn
about omitting the string in "_Static_assert", use of [[]] syntax for attributes, use of
decimal floating-point types, and so on. This option is independent of the standards
mode. Warnings are disabled in the expression that follows "__extension__".

-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++,
e.g. request for implicit conversion from "void *" to a pointer to non-"void" type.

-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011,
e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns
on -Wnarrowing and is enabled by -Wall.

-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014.
This warning is enabled by -Wall.

-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017.
This warning is enabled by -Wall.

-Wc++20-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020.
This warning is enabled by -Wall.

-Wno-c++11-extensions (C++ and Objective-C++ only)
Do not warn about C++11 constructs in code being compiled using an older C++ standard.
Even without this option, some C++11 constructs will only be diagnosed if -Wpedantic is
used.

-Wno-c++14-extensions (C++ and Objective-C++ only)
Do not warn about C++14 constructs in code being compiled using an older C++ standard.
Even without this option, some C++14 constructs will only be diagnosed if -Wpedantic is
used.

-Wno-c++17-extensions (C++ and Objective-C++ only)
Do not warn about C++17 constructs in code being compiled using an older C++ standard.
Even without this option, some C++17 constructs will only be diagnosed if -Wpedantic is
used.

-Wno-c++20-extensions (C++ and Objective-C++ only)
Do not warn about C++20 constructs in code being compiled using an older C++ standard.
Even without this option, some C++20 constructs will only be diagnosed if -Wpedantic is
used.

-Wno-c++23-extensions (C++ and Objective-C++ only)
Do not warn about C++23 constructs in code being compiled using an older C++ standard.
Even without this option, some C++23 constructs will only be diagnosed if -Wpedantic is
used.

-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type.
For example, warn if a "const char *" is cast to an ordinary "char *".

Also warn when making a cast that introduces a type qualifier in an unsafe way. For
example, casting "char **" to "const char **" is unsafe, as in this example:

/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';

-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is
increased. For example, warn if a "char *" is cast to an "int *" on machines where
integers can only be accessed at two- or four-byte boundaries.

-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of the target is
increased. For example, warn if a "char *" is cast to an "int *" regardless of the
target machine.

-Wcast-function-type
Warn when a function pointer is cast to an incompatible function pointer. In a cast
involving function types with a variable argument list only the types of initial
arguments that are provided are considered. Any parameter of pointer-type matches any
other pointer-type. Any benign differences in integral types are ignored, like "int" vs.
"long" on ILP32 targets. Likewise type qualifiers are ignored. The function type "void
(*) (void)" is special and matches everything, which can be used to suppress this
warning. In a cast involving pointer to member types this warning warns whenever the
type cast is changing the pointer to member type. This warning is enabled by -Wextra.

-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the
address of one into a non-"const" "char *" pointer produces a warning. These warnings
help you find at compile time code that can try to write into a string constant, but only
if you have been very careful about using "const" in declarations and prototypes.
Otherwise, it is just a nuisance. This is why we did not make -Wall request these
warnings.

When compiling C++, warn about the deprecated conversion from string literals to "char
*". This warning is enabled by default for C++ programs.

-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork". This warning is also
enabled by -Wextra.

-Wno-complain-wrong-lang
By default, language front ends complain when a command-line option is valid, but not
applicable to that front end. This may be disabled with -Wno-complain-wrong-lang, which
is mostly useful when invoking a single compiler driver for multiple source files written
in different languages, for example:

$ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to
compile b.f90. The latter front end diagnoses f951: Warning: command-line option
'-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with
-Wno-complain-wrong-lang.

-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between
real and integer, like "abs (x)" when "x" is "double"; conversions between signed and
unsigned, like "unsigned ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)".
Do not warn for explicit casts like "abs ((int) x)" and "ui = (unsigned) -1", or if the
value is not changed by the conversion like in "abs (2.0)". Warnings about conversions
between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

For C++, also warn for confusing overload resolution for user-defined conversions; and
conversions that never use a type conversion operator: conversions to "void", the same
type, a base class or a reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly
enabled.

Warnings about conversion from arithmetic on a small type back to that type are only
given with -Warith-conversion.

-Wdangling-else
Warn about constructions where there may be confusion to which "if" statement an "else"
branch belongs. Here is an example of such a case:

{
if (a)
if (b)
foo ();
else
bar ();
}

In C/C++, every "else" branch belongs to the innermost possible "if" statement, which in
this example is "if (b)". This is often not what the programmer expected, as illustrated
in the above example by indentation the programmer chose. When there is the potential
for this confusion, GCC issues a warning when this flag is specified. To eliminate the
warning, add explicit braces around the innermost "if" statement so there is no way the
"else" can belong to the enclosing "if". The resulting code looks like this:

{
if (a)
{
if (b)
foo ();
else
bar ();
}
}

This warning is enabled by -Wparentheses.

-Wdangling-pointer
-Wdangling-pointer=n
Warn about uses of pointers (or C++ references) to objects with automatic storage
duration after their lifetime has ended. This includes local variables declared in
nested blocks, compound literals and other unnamed temporary objects. In addition, warn
about storing the address of such objects in escaped pointers. The warning is enabled at
all optimization levels but may yield different results with optimization than without.

-Wdangling-pointer=1
At level 1 the warning diagnoses only unconditional uses of dangling pointers. For
example

int f (int c1, int c2, x)
{
char *p = strchr ((char[]){ c1, c2 }, c3);
// warning: dangling pointer to a compound literal
return p ? *p : 'x';
}

In the following function the store of the address of the local variable "x" in the
escaped pointer *p also triggers the warning.

void g (int **p)
{
int x = 7;
// warning: storing the address of a local variable in *p
*p = &x;
}

-Wdangling-pointer=2
At level 2, in addition to unconditional uses the warning also diagnoses conditional
uses of dangling pointers.

For example, because the array a in the following function is out of scope when the
pointer s that was set to point is used, the warning triggers at this level.

void f (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer to a may be used
strcat (s, ".tmp");
...
}

-Wdangling-pointer=2 is included in -Wall.

-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as they might
prevent bit-wise-identical reproducible compilations.

-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while" statement. This warning is
also enabled by -Wextra.

-Wno-endif-labels
Do not warn about stray tokens after "#else" and "#endif".

-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++ enumerated
type mismatches in conditional expressions are also diagnosed and the warning is enabled
by default. In C this warning is enabled by -Wall.

-Wenum-conversion
Warn when a value of enumerated type is implicitly converted to a different enumerated
type. This warning is enabled by -Wextra in C.

-Wenum-int-mismatch (C and Objective-C only)
Warn about mismatches between an enumerated type and an integer type in declarations.
For example:

enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);

In C, an enumerated type is compatible with "char", a signed integer type, or an unsigned
integer type. However, since the choice of the underlying type of an enumerated type is
implementation-defined, such mismatches may cause portability issues. In C++, such
mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat.

-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the
initialization of a variable, or jumps backward to a label after the variable has been
initialized. This only warns about variables that are initialized when they are
declared. This warning is only supported for C and Objective-C; in C++ this sort of
branch is an error in any case.

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the
-Wno-jump-misses-init option.

-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect
result when the signed value is converted to unsigned. In C++, this warning is also
enabled by -Wall. In C, it is also enabled by -Wextra.

-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value, like
assigning a signed integer expression to an unsigned integer variable. An explicit cast
silences the warning. In C, this option is enabled also by -Wconversion.

-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real value. This includes
conversions from real to integer, and from higher precision real to lower precision real
values. This option is also enabled by -Wconversion.

-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage order.

-Wsizeof-array-div
Warn about divisions of two sizeof operators when the first one is applied to an array
and the divisor does not equal the size of the array element. In such a case, the
computation will not yield the number of elements in the array, which is likely what the
user intended. This warning warns e.g. about

int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}

This warning is enabled by -Wall.

-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide the pointer size by
the element size, which is the usual way to compute the array size but won't work out
correctly with pointers. This warning warns e.g. about "sizeof (ptr) / sizeof (ptr[0])"
if "ptr" is not an array, but a pointer. This warning is enabled by -Wall.

-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in functions if
the argument uses "sizeof". This warning triggers for example for "memset (ptr, 0,
sizeof (ptr));" if "ptr" is not an array, but a pointer, and suggests a possible fix, or
about "memcpy (&foo, ptr, sizeof (&foo));". -Wsizeof-pointer-memaccess also warns about
calls to bounded string copy functions like "strncat" or "strncpy" that specify as the
bound a "sizeof" expression of the source array. For example, in the following function
the call to "strncat" specifies the size of the source string as the bound. That is
almost certainly a mistake and so the call is diagnosed.

void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}

The -Wsizeof-pointer-memaccess option is enabled by -Wall.

-Wno-sizeof-array-argument
Do not warn when the "sizeof" operator is applied to a parameter that is declared as an
array in a function definition. This warning is enabled by default for C and C++
programs.

-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if the first argument
references an array, and the third argument is a number equal to the number of elements,
but not equal to the size of the array in memory. This indicates that the user has
omitted a multiplication by the element size. This warning is enabled by -Wall.

-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function where the second argument is
not zero and the third argument is zero. For example, the call "memset (buf, sizeof buf,
0)" is diagnosed because "memset (buf, 0, sizeof buf)" was meant instead. The diagnostic
is only emitted if the third argument is a literal zero. Otherwise, if it is an
expression that is folded to zero, or a cast of zero to some type, it is far less likely
that the arguments have been mistakenly transposed and no warning is emitted. This
warning is enabled by -Wall.

-Waddress
Warn about suspicious uses of address expressions. These include comparing the address of
a function or a declared object to the null pointer constant such as in

void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}

comparisons of a pointer to a string literal, such as in

void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}

and tests of the results of pointer addition or subtraction for equality to null, such as
in

void f (const int *p, int i)
{
return p + i == NULL;
}

Such uses typically indicate a programmer error: the address of most functions and
objects necessarily evaluates to true (the exception are weak symbols), so their use in a
conditional might indicate missing parentheses in a function call or a missing
dereference in an array expression. The subset of the warning for object pointers can be
suppressed by casting the pointer operand to an integer type such as "intptr_t" or
"uintptr_t". Comparisons against string literals result in unspecified behavior and are
not portable, and suggest the intent was to call "strcmp". The warning is suppressed if
the suspicious expression is the result of macro expansion. -Waddress warning is enabled
by -Wall.

-Wno-address-of-packed-member
Do not warn when the address of packed member of struct or union is taken, which usually
results in an unaligned pointer value. This is enabled by default.

-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using
logical operators in contexts where a bit-wise operator is likely to be expected. Also
warns when the operands of a logical operator are the same:

extern int a;
if (a < 0 && a < 0) { ... }

-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison. This option
does not warn if the right operand is considered to be a boolean expression. Its purpose
is to detect suspicious code like the following:

int a;
...
if (!a > 1) { ... }

It is possible to suppress the warning by wrapping the LHS into parentheses:

if ((!a) > 1) { ... }

This warning is enabled by -Wall.

-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In
languages where you can return an array, this also elicits a warning.)

-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler detects undefined
behavior in some statement during one or more of the iterations.

-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes,
function attributes applied to variables, etc. This does not stop errors for incorrect
use of supported attributes.

Additionally, using -Wno-attributes=, it is possible to suppress warnings about unknown
scoped attributes (in C++11 and C2X). For example, -Wno-attributes=vendor::attr disables
warning about the following declaration:

[[vendor::attr]] void f();

It is also possible to disable warning about all attributes in a namespace using
-Wno-attributes=vendor:: which prevents warning about both of these declarations:

[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();

Note that -Wno-attributes= does not imply -Wno-attributes.

-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible signature or as a non-
function, or when a built-in function declared with a type that does not include a
prototype is called with arguments whose promoted types do not match those expected by
the function. When -Wextra is specified, also warn when a built-in function that takes
arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is
enabled by default. To avoid the warning include the appropriate header to bring the
prototypes of built-in functions into scope.

For example, the call to "memset" below is diagnosed by the warning because the function
expects a value of type "size_t" as its argument but the type of 32 is "int". With
-Wextra, the declaration of the function is diagnosed as well.

extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}

-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for
redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".

-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument types. (An
old-style function definition is permitted without a warning if preceded by a declaration
that specifies the argument types.)

-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For example,
warn if storage-class specifiers like "static" are not the first things in a declaration.
This warning is also enabled by -Wextra.

-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if there is a
previous prototype. A definition using () is not considered an old-style definition in
C2X mode, because it is equivalent to (void) in that case, but is considered an old-style
definition for older standards.

-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by -Wextra.

-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype declaration. This
warning is issued even if the definition itself provides a prototype. Use this option to
detect global functions that do not have a matching prototype declaration in a header
file. This option is not valid for C++ because all function declarations provide
prototypes and a non-matching declaration declares an overload rather than conflict with
an earlier declaration. Use -Wmissing-declarations to detect missing declarations in
C++.

-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so even if the
definition itself provides a prototype. Use this option to detect global functions that
are not declared in header files. In C, no warnings are issued for functions with
previous non-prototype declarations; use -Wmissing-prototypes to detect missing
prototypes. In C++, no warnings are issued for function templates, or for inline
functions, or for functions in anonymous namespaces.

-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the following
code causes such a warning, because "x.h" is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

This option does not warn about designated initializers, so the following modification
does not trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C this option does not warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Likewise, in C++ this option does not warn about the empty { } initializer, for example:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. To get other -Wextra warnings without this one, use
-Wextra -Wno-missing-field-initializers.

-Wno-missing-requires
By default, the compiler warns about a concept-id appearing as a C++20 simple-
requirement:

bool satisfied = requires { C<T> };

Here satisfied will be true if C<T> is a valid expression, which it is for all T.
Presumably the user meant to write

bool satisfied = requires { requires C<T> };

so satisfied is only true if concept C is satisfied for type T.

This warning can be disabled with -Wno-missing-requires.

-Wno-missing-template-keyword
The member access tokens ., -> and :: must be followed by the "template" keyword if the
parent object is dependent and the member being named is a template.

template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}

In rare cases it is possible to get false positives. To silence this, wrap the expression
in parentheses. For example, the following is treated as a template, even where m and N
are integers:

void NotATemplate (my_class t)
{
int N = 5;

bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}

This warning can be disabled with -Wno-missing-template-keyword.

-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo
in the user's code, as they have implementation-defined values, and should not be used in
portable code.

-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are different sequences of
characters. However, sometimes when characters outside the basic ASCII character set are
used, you can have two different character sequences that look the same. To avoid
confusion, the ISO 10646 standard sets out some normalization rules which when applied
ensure that two sequences that look the same are turned into the same sequence. GCC can
warn you if you are using identifiers that have not been normalized; this option controls
that warning.

There are four levels of warning supported by GCC. The default is -Wnormalized=nfc,
which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC.
NFC is the recommended form for most uses. It is equivalent to -Wnormalized.

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++
that, when turned into NFC, are not allowed in identifiers. That is, there's no way to
use these symbols in portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters. It is hoped that future
versions of the standards involved will correct this, which is why this option is not the
default.

You can switch the warning off for all characters by writing -Wnormalized=none or
-Wno-normalized. You should only do this if you are using some other normalization
scheme (like "D"), because otherwise you can easily create bugs that are literally
impossible to see.

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or
display methodologies, especially once formatting has been applied. For instance
"\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that has
been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert
all these into a standard form as well, and GCC warns if your code is not in NFKC if you
use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that
contains the letter O because it might be confused with the digit 0, and so is not the
default, but may be useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.

-Wno-attribute-warning
Do not warn about usage of functions declared with "warning" attribute. By default, this
warning is enabled. -Wno-attribute-warning can be used to disable the warning or
-Wno-error=attribute-warning can be used to disable the error when compiled with -Werror
flag.

-Wno-deprecated
Do not warn about usage of deprecated features.

-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using
the "deprecated" attribute.

-Wno-overflow
Do not warn about compile-time overflow in constant expressions.

-Wno-odr
Warn about One Definition Rule violations during link-time optimization. Enabled by
default.

-Wopenacc-parallelism
Warn about potentially suboptimal choices related to OpenACC parallelism.

-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The
-fsimd-cost-model=unlimited option can be used to relax the cost model.

-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated
initializers.

This warning is included in -Wextra. To get other -Wextra warnings without this one, use
-Wextra -Wno-override-init.

-Wno-override-init-side-effects (C and Objective-C only)
Do not warn if an initialized field with side effects is overridden when using designated
initializers. This warning is enabled by default.

-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect
on the layout or size of the structure. Such structures may be mis-aligned for little
benefit. For instance, in this code, the variable "f.x" in "struct bar" is misaligned
even though "struct bar" does not itself have the packed attribute:

struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};

-Wnopacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type
"char". This was fixed in GCC 4.4 but the change can lead to differences in the
structure layout. GCC informs you when the offset of such a field has changed in GCC
4.4. For example there is no longer a 4-bit padding between field "a" and "b" in this
structure:

struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this
warning.

-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment in a packed struct or union
is misaligned. For example, a warning will be issued on "struct S", like, "warning:
alignment 1 of 'struct S' is less than 8", in this code:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};

This warning is enabled by -Wall.

-Wpadded
Warn if padding is included in a structure, either to align an element of the structure
or to align the whole structure. Sometimes when this happens it is possible to rearrange
the fields of the structure to reduce the padding and so make the structure smaller.

-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where
multiple declaration is valid and changes nothing.

-Wrestrict
Warn when an object referenced by a "restrict"-qualified parameter (or, in C++, a
"__restrict"-qualified parameter) is aliased by another argument, or when copies between
such objects overlap. For example, the call to the "strcpy" function below attempts to
truncate the string by replacing its initial characters with the last four. However,
because the call writes the terminating NUL into "a[4]", the copies overlap and the call
is diagnosed.

void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}

The -Wrestrict option detects some instances of simple overlap even without optimization
but works best at -O2 and above. It is included in -Wall.

-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.

-Winline
Warn if a function that is declared as inline cannot be inlined. Even with this option,
the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a
function. For example, the compiler takes into account the size of the function being
inlined and the amount of inlining that has already been done in the current function.
Therefore, seemingly insignificant changes in the source program can cause the warnings
produced by -Winline to appear or disappear.

-Winterference-size
Warn about use of C++17 "std::hardware_destructive_interference_size" without specifying
its value with --param destructive-interference-size. Also warn about questionable
values for that option.

This variable is intended to be used for controlling class layout, to avoid false sharing
in concurrent code:

struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};

Here one and two are intended to be far enough apart that stores to one won't require
accesses to the other to reload the cache line.

By default, --param destructive-interference-size and --param constructive-interference-
size are set based on the current -mtune option, typically to the L1 cache line size for
the particular target CPU, sometimes to a range if tuning for a generic target. So all
translation units that depend on ABI compatibility for the use of these variables must be
compiled with the same -mtune (or -mcpu).

If ABI stability is important, such as if the use is in a header for a library, you
should probably not use the hardware interference size variables at all. Alternatively,
you can force a particular value with --param.

If you are confident that your use of the variable does not affect ABI outside a single
build of your project, you can turn off the warning with -Wno-interference-size.

-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are expected, such as
conditional expressions (?:) using non-boolean integer constants in boolean context, like
"if (a <= b ? 2 : 3)". Or left shifting of signed integers in boolean context, like "for
(a = 0; 1 << a; a++);". Likewise for all kinds of multiplications regardless of the data
type. This warning is enabled by -Wall.

-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size. In C++,
casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by
default.

-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.

-Winvalid-pch
Warn if a precompiled header is found in the search path but cannot be used.

-Winvalid-utf8
Warn if an invalid UTF-8 character is found. This warning is on by default for C++23 if
-finput-charset=UTF-8 is used and turned into error with -pedantic-errors.

-Wno-unicode
Don't diagnose invalid forms of delimited or named escape sequences which are treated as
separate tokens. Wunicode is enabled by default.

-Wlong-long
Warn if "long long" type is used. This is enabled by either -Wpedantic or -Wtraditional
in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.

-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used
in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the
warning messages, use -Wno-variadic-macros.

-Wno-varargs
Do not warn upon questionable usage of the macros used to handle variable arguments like
"va_start". These warnings are enabled by default.

-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture.
Mainly useful for the performance tuning. Vector operation can be implemented
"piecewise", which means that the scalar operation is performed on every vector element;
"in parallel", which means that the vector operation is implemented using scalars of
wider type, which normally is more performance efficient; and "as a single scalar", which
means that vector fits into a scalar type.

-Wvla
Warn if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic
warning of the variable-length array.

-Wvla-larger-than=byte-size
If this option is used, the compiler warns for declarations of variable-length arrays
whose size is either unbounded, or bounded by an argument that allows the array size to
exceed byte-size bytes. This is similar to how -Walloca-larger-than=byte-size works, but
with variable-length arrays.

Note that GCC may optimize small variable-length arrays of a known value into plain
arrays, so this warning may not get triggered for such arrays.

-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when
-ftree-vrp is active (default for -O2 and above).

See also -Walloca-larger-than=byte-size.

-Wno-vla-larger-than
Disable -Wvla-larger-than= warnings. The option is equivalent to
-Wvla-larger-than=SIZE_MAX or larger.

-Wvla-parameter
Warn about redeclarations of functions involving arguments of Variable Length Array types
of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to
such parameters by warnings such as -Warray-bounds.

If the first function declaration uses the VLA form the bound specified in the array is
assumed to be the minimum number of elements expected to be provided in calls to the
function and the maximum number of elements accessed by it. Failing to provide arguments
of sufficient size or accessing more than the maximum number of elements may be
diagnosed.

For example, the warning triggers for the following redeclarations because the first one
allows an array of any size to be passed to "f" while the second one specifies that the
array argument must have at least "n" elements. In addition, calling "f" with the
associated VLA bound parameter in excess of the actual VLA bound triggers a warning as
well.

void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);

void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}

-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for
similar problems involving ordinary array arguments.

-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit
all optimizations that may eliminate reads and/or writes to register variables. This
warning is enabled by -Wall.

-Wxor-used-as-pow (C, C++, Objective-C and Objective-C++ only)
Warn about uses of "^", the exclusive or operator, where it appears the user meant
exponentiation. Specifically, the warning occurs when the left-hand side is the decimal
constant 2 or 10 and the right-hand side is also a decimal constant.

In C and C++, "^" means exclusive or, whereas in some other languages (e.g. TeX and some
versions of BASIC) it means exponentiation.

This warning is enabled by default. It can be silenced by converting one of the operands
to hexadecimal.

-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally
indicate that there is anything wrong with your code; it merely indicates that GCC's
optimizers are unable to handle the code effectively. Often, the problem is that your
code is too big or too complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.

-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness. This option
is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which
can be disabled with -Wno-pointer-sign.

-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions
that are not protected against stack smashing.

-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length specified
in the C standard. Modern compilers generally allow string constants that are much
longer than the standard's minimum limit, but very portable programs should avoid using
longer strings.

The limit applies after string constant concatenation, and does not count the trailing
NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does
not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a suffix. When used
together with -Wsystem-headers it warns about such constants in system header files.
This can be useful when preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma
from the decimal floating-point extension to C99.

-Wno-lto-type-mismatch
During the link-time optimization, do not warn about type mismatches in global
declarations from different compilation units. Requires -flto to be enabled. Enabled by
default.

-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to initialize a structure that
has been marked with the "designated_init" attribute.

Options That Control Static Analysis
-fanalyzer
This option enables an static analysis of program flow which looks for "interesting"
interprocedural paths through the code, and issues warnings for problems found on them.

This analysis is much more expensive than other GCC warnings.

In technical terms, it performs coverage-guided symbolic execution of the code being
compiled. It is neither sound nor complete: it can have false positives and false
negatives. It is a bug-finding tool, rather than a tool for proving program correctness.

The analyzer is only suitable for use on C code in this release.

Enabling this option effectively enables the following warnings:

-Wanalyzer-allocation-size -Wanalyzer-deref-before-check -Wanalyzer-double-fclose
-Wanalyzer-double-free -Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy -Wanalyzer-fd-access-mode-mismatch
-Wanalyzer-fd-double-close -Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close -Wanalyzer-fd-use-without-check
-Wanalyzer-file-leak -Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-recursion -Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument -Wanalyzer-null-dereference
-Wanalyzer-out-of-bounds -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free -Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value -Wanalyzer-va-arg-type-mismatch
-Wanalyzer-va-list-exhausted -Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal

This option is only available if GCC was configured with analyzer support enabled.

-Wanalyzer-too-complex
If -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the
control flow and data flow in the program, but these can be defeated by sufficiently
complicated code.

By default, the analysis silently stops if the code is too complicated for the analyzer
to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option
warns if this occurs.

-Wno-analyzer-allocation-size
This warning requires -fanalyzer, which enables it; to disable it, use
-Wno-analyzer-allocation-size.

This diagnostic warns for paths through the code in which a pointer to a buffer is
assigned to point at a buffer with a size that is not a multiple of "sizeof (*pointer)".

See CWE-131: Incorrect Calculation of Buffer Size
("https://cwe.mitre.org/data/definitions/131.html").

-Wno-analyzer-deref-before-check
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check
to disable it.

This diagnostic warns for paths through the code in which a pointer is checked for "NULL"
*after* it has already been dereferenced, suggesting that the pointer could have been
NULL. Such cases suggest that the check for NULL is either redundant, or that it needs
to be moved to before the pointer is dereferenced.

This diagnostic also considers values passed to a function argument marked with
"__attribute__((nonnull))" as requiring a non-NULL value, and thus will complain if such
values are checked for "NULL" after returning from such a function call.

This diagnostic is unlikely to be reported when any level of optimization is enabled, as
GCC's optimization logic will typically consider such checks for NULL as being redundant,
and optimize them away before the analyzer "sees" them. Hence optimization should be
disabled when attempting to trigger this diagnostic.

-Wno-analyzer-double-fclose
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to
disable it.

This diagnostic warns for paths through the code in which a "FILE *" can have "fclose"
called on it more than once.

See CWE-1341: Multiple Releases of Same Resource or Handle
("https://cwe.mitre.org/data/definitions/1341.html").

-Wno-analyzer-double-free
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-free to
disable it.

This diagnostic warns for paths through the code in which a pointer can have a
deallocator called on it more than once, either "free", or a deallocator referenced by
attribute "malloc".

See CWE-415: Double Free ("https://cwe.mitre.org/data/definitions/415.html").

-Wno-analyzer-exposure-through-output-file
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is
written to an output file (such as writing a password to a log file).

See CWE-532: Information Exposure Through Log Files
("https://cwe.mitre.org/data/definitions/532.html").

-Wanalyzer-exposure-through-uninit-copy
This warning requires both -fanalyzer and the use of a plugin to specify a function that
copies across a "trust boundary". Use -Wno-analyzer-exposure-through-uninit-copy to
disable it.

This diagnostic warns for "infoleaks" - paths through the code in which uninitialized
values are copied across a security boundary (such as code within an OS kernel that
copies a partially-initialized struct on the stack to user space).

See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor
("https://cwe.mitre.org/data/definitions/200.html").

-Wno-analyzer-fd-access-mode-mismatch
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-access-mode-mismatch to disable it.

This diagnostic warns for paths through code in which a "read" on a write-only file
descriptor is attempted, or vice versa.

This diagnostic also warns for code paths in a which a function with attribute
"fd_arg_read (N)" is called with a file descriptor opened with "O_WRONLY" at referenced
argument "N" or a function with attribute "fd_arg_write (N)" is called with a file
descriptor opened with "O_RDONLY" at referenced argument N.

-Wno-analyzer-fd-double-close
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-double-close to
disable it.

This diagnostic warns for paths through code in which a file descriptor can be closed
more than once.

See CWE-1341: Multiple Releases of Same Resource or Handle
("https://cwe.mitre.org/data/definitions/1341.html").

-Wno-analyzer-fd-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-leak to disable
it.

This diagnostic warns for paths through code in which an open file descriptor is leaked.

See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime
("https://cwe.mitre.org/data/definitions/775.html").

-Wno-analyzer-fd-phase-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch
to disable it.

This diagnostic warns for paths through code in which an operation is attempted in the
wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to
call "accept" on a stream socket that has not yet had "listen" successfully called on it.

See CWE-666: Operation on Resource in Wrong Phase of Lifetime
("https://cwe.mitre.org/data/definitions/666.html").

-Wno-analyzer-fd-type-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-type-mismatch to
disable it.

This diagnostic warns for paths through code in which an operation is attempted on the
wrong type of file descriptor. For example, it will warn on attempts to use socket
operations on a file descriptor obtained via "open", or when attempting to use a stream
socket operation on a datagram socket.

-Wno-analyzer-fd-use-after-close
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-after-close
to disable it.

This diagnostic warns for paths through code in which a read or write is called on a
closed file descriptor.

This diagnostic also warns for paths through code in which a function with attribute
"fd_arg (N)" or "fd_arg_read (N)" or "fd_arg_write (N)" is called with a closed file
descriptor at referenced argument "N".

-Wno-analyzer-fd-use-without-check
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-use-without-check to disable it.

This diagnostic warns for paths through code in which a file descriptor is used without
being checked for validity.

This diagnostic also warns for paths through code in which a function with attribute
"fd_arg (N)" or "fd_arg_read (N)" or "fd_arg_write (N)" is called with a file descriptor,
at referenced argument "N", without being checked for validity.

-Wno-analyzer-file-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-file-leak to
disable it.

This diagnostic warns for paths through the code in which a "<stdio.h>" "FILE *" stream
object is leaked.

See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime
("https://cwe.mitre.org/data/definitions/775.html").

-Wno-analyzer-free-of-non-heap
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to
disable it.

This diagnostic warns for paths through the code in which "free" is called on a non-heap
pointer (e.g. an on-stack buffer, or a global).

See CWE-590: Free of Memory not on the Heap
("https://cwe.mitre.org/data/definitions/590.html").

-Wno-analyzer-imprecise-fp-arithmetic
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-imprecise-fp-arithmetic to disable it.

This diagnostic warns for paths through the code in which floating-point arithmetic is
used in locations where precise computation is needed. This diagnostic only warns on use
of floating-point operands inside the calculation of an allocation size at the moment.

-Wno-analyzer-infinite-recursion
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-infinite-recursion
to disable it.

This diagnostics warns for paths through the code which appear to lead to infinite
recursion.

Specifically, when the analyzer "sees" a recursive call, it will compare the state of
memory at the entry to the new frame with that at the entry to the previous frame of that
function on the stack. The warning is issued if nothing in memory appears to be
changing; any changes observed to parameters or globals are assumed to lead to
termination of the recursion and thus suppress the warning.

This diagnostic is likely to miss cases of infinite recursion that are convered to
iteration by the optimizer before the analyzer "sees" them. Hence optimization should be
disabled when attempting to trigger this diagnostic.

Compare with -Winfinite-recursion, which provides a similar diagnostic, but is
implemented in a different way.

-Wno-analyzer-jump-through-null
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-jump-through-null
to disable it.

This diagnostic warns for paths through the code in which a "NULL" function pointer is
called.

-Wno-analyzer-malloc-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to
disable it.

This diagnostic warns for paths through the code in which a pointer allocated via an
allocator is leaked: either "malloc", or a function marked with attribute "malloc".

See CWE-401: Missing Release of Memory after Effective Lifetime
("https://cwe.mitre.org/data/definitions/401.html").

-Wno-analyzer-mismatching-deallocation
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-mismatching-deallocation to disable it.

This diagnostic warns for paths through the code in which the wrong deallocation function
is called on a pointer value, based on which function was used to allocate the pointer
value. The diagnostic will warn about mismatches between "free", scalar "delete" and
vector "delete[]", and those marked as allocator/deallocator pairs using attribute
"malloc".

See CWE-762: Mismatched Memory Management Routines
("https://cwe.mitre.org/data/definitions/762.html").

-Wno-analyzer-out-of-bounds
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-out-of-bounds to
disable it.

This diagnostic warns for paths through the code in which a buffer is definitely read or
written out-of-bounds. The diagnostic applies for cases where the analyzer is able to
determine a constant offset and for accesses past the end of a buffer, also a constant
capacity. Further, the diagnostic does limited checking for accesses past the end when
the offset as well as the capacity is symbolic.

See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
("https://cwe.mitre.org/data/definitions/119.html").

-Wno-analyzer-possible-null-argument
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-possible-null-argument to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is passed
to a function argument marked with "__attribute__((nonnull))" as requiring a non-NULL
value.

See CWE-690: Unchecked Return Value to NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/690.html").

-Wno-analyzer-possible-null-dereference
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-possible-null-dereference to disable it.

This diagnostic warns for paths through the code in which a possibly-NULL value is
dereferenced.

See CWE-690: Unchecked Return Value to NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/690.html").

-Wno-analyzer-null-argument
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-argument to
disable it.

This diagnostic warns for paths through the code in which a value known to be NULL is
passed to a function argument marked with "__attribute__((nonnull))" as requiring a non-
NULL value.

See CWE-476: NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/476.html").

-Wno-analyzer-null-dereference
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-dereference to
disable it.

This diagnostic warns for paths through the code in which a value known to be NULL is
dereferenced.

See CWE-476: NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/476.html").

-Wno-analyzer-putenv-of-auto-var
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-putenv-of-auto-var
to disable it.

This diagnostic warns for paths through the code in which a call to "putenv" is passed a
pointer to an automatic variable or an on-stack buffer.

See POS34-C. Do not call putenv() with a pointer to an automatic variable as the
argument ("https://wiki.sei.cmu.edu/confluence/x/6NYxBQ").

-Wno-analyzer-shift-count-negative
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-shift-count-negative to disable it.

This diagnostic warns for paths through the code in which a shift is attempted with a
negative count. It is analogous to the -Wshift-count-negative diagnostic implemented in
the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather
than merely parsing the syntax tree. However, the analyzer does not prioritize detection
of such paths, so false negatives are more likely relative to other warnings.

-Wno-analyzer-shift-count-overflow
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-shift-count-overflow to disable it.

This diagnostic warns for paths through the code in which a shift is attempted with a
count greater than or equal to the precision of the operand's type. It is analogous to
the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but is
implemented based on analyzing interprocedural paths, rather than merely parsing the
syntax tree. However, the analyzer does not prioritize detection of such paths, so false
negatives are more likely relative to other warnings.

-Wno-analyzer-stale-setjmp-buffer
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-stale-setjmp-buffer
to disable it.

This diagnostic warns for paths through the code in which "longjmp" is called to rewind
to a "jmp_buf" relating to a "setjmp" call in a function that has returned.

When "setjmp" is called on a "jmp_buf" to record a rewind location, it records the stack
frame. The stack frame becomes invalid when the function containing the "setjmp" call
returns. Attempting to rewind to it via "longjmp" would reference a stack frame that no
longer exists, and likely lead to a crash (or worse).

-Wno-analyzer-tainted-allocation-size
This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use
-Wno-analyzer-tainted-allocation-size to disable it.

This diagnostic warns for paths through the code in which a value that could be under an
attacker's control is used as the size of an allocation without being sanitized, so that
an attacker could inject an excessively large allocation and potentially cause a denial
of service attack.

See CWE-789: Memory Allocation with Excessive Size Value
("https://cwe.mitre.org/data/definitions/789.html").

-Wno-analyzer-tainted-assertion
This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use
-Wno-analyzer-tainted-assertion to disable it.

This diagnostic warns for paths through the code in which a value that could be under an
attacker's control is used as part of a condition without being first sanitized, and that
condition guards a call to a function marked with attribute "noreturn" (such as the
function "__builtin_unreachable"). Such functions typically indicate abnormal
termination of the program, such as for assertion failure handlers. For example:

assert (some_tainted_value < SOME_LIMIT);

In such cases:

* when assertion-checking is enabled: an attacker could trigger a denial of service by
injecting an assertion failure

* when assertion-checking is disabled, such as by defining "NDEBUG", an attacker could
inject data that subverts the process, since it presumably violates a precondition
that is being assumed by the code.

Note that when assertion-checking is disabled, the assertions are typically removed by
the preprocessor before the analyzer has a chance to "see" them, so this diagnostic can
only generate warnings on builds in which assertion-checking is enabled.

For the purpose of this warning, any function marked with attribute "noreturn" is
considered as a possible assertion failure handler, including "__builtin_unreachable".
Note that these functions are sometimes removed by the optimizer before the analyzer
"sees" them. Hence optimization should be disabled when attempting to trigger this
diagnostic.

See CWE-617: Reachable Assertion ("https://cwe.mitre.org/data/definitions/617.html").

The warning can also report problematic constructions such as

switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;

default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}

despite the above not being an assertion failure, strictly speaking.

-Wno-analyzer-tainted-array-index
This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use
-Wno-analyzer-tainted-array-index to disable it.

This diagnostic warns for paths through the code in which a value that could be under an
attacker's control is used as the index of an array access without being sanitized, so
that an attacker could inject an out-of-bounds access.

See CWE-129: Improper Validation of Array Index
("https://cwe.mitre.org/data/definitions/129.html").

-Wno-analyzer-tainted-divisor
This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use
-Wno-analyzer-tainted-divisor to disable it.

This diagnostic warns for paths through the code in which a value that could be under an
attacker's control is used as the divisor in a division or modulus operation without
being sanitized, so that an attacker could inject a division-by-zero.

See CWE-369: Divide By Zero ("https://cwe.mitre.org/data/definitions/369.html").

-Wno-analyzer-tainted-offset
This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use
-Wno-analyzer-tainted-offset to disable it.

This diagnostic warns for paths through the code in which a value that could be under an
attacker's control is used as a pointer offset without being sanitized, so that an
attacker could inject an out-of-bounds access.

See CWE-823: Use of Out-of-range Pointer Offset
("https://cwe.mitre.org/data/definitions/823.html").

-Wno-analyzer-tainted-size
This warning requires both -fanalyzer and -fanalyzer-checker=taint to enable it; use
-Wno-analyzer-tainted-size to disable it.

This diagnostic warns for paths through the code in which a value that could be under an
attacker's control is used as the size of an operation such as "memset" without being
sanitized, so that an attacker could inject an out-of-bounds access.

See CWE-129: Improper Validation of Array Index
("https://cwe.mitre.org/data/definitions/129.html").

-Wno-analyzer-unsafe-call-within-signal-handler
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-unsafe-call-within-signal-handler to disable it.

This diagnostic warns for paths through the code in which a function known to be async-
signal-unsafe (such as "fprintf") is called from a signal handler.

See CWE-479: Signal Handler Use of a Non-reentrant Function
("https://cwe.mitre.org/data/definitions/479.html").

-Wno-analyzer-use-after-free
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to
disable it.

This diagnostic warns for paths through the code in which a pointer is used after a
deallocator is called on it: either "free", or a deallocator referenced by attribute
"malloc".

See CWE-416: Use After Free ("https://cwe.mitre.org/data/definitions/416.html").

-Wno-analyzer-use-of-pointer-in-stale-stack-frame
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.

This diagnostic warns for paths through the code in which a pointer is dereferenced that
points to a variable in a stale stack frame.

-Wno-analyzer-va-arg-type-mismatch
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-va-arg-type-mismatch to disable it.

This diagnostic warns for interprocedural paths through the code for which the analyzer
detects an attempt to use "va_arg" to extract a value passed to a variadic call, but uses
a type that does not match that of the expression passed to the call.

See CWE-686: Function Call With Incorrect Argument Type
("https://cwe.mitre.org/data/definitions/686.html").

-Wno-analyzer-va-list-exhausted
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-exhausted
to disable it.

This diagnostic warns for interprocedural paths through the code for which the analyzer
detects an attempt to use "va_arg" to access the next value passed to a variadic call,
but all of the values in the "va_list" have already been consumed.

See CWE-685: Function Call With Incorrect Number of Arguments
("https://cwe.mitre.org/data/definitions/685.html").

-Wno-analyzer-va-list-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-leak to
disable it.

This diagnostic warns for interprocedural paths through the code for which the analyzer
detects that "va_start" or "va_copy" has been called on a "va_list" without a
corresponding call to "va_end".

-Wno-analyzer-va-list-use-after-va-end
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-va-list-use-after-va-end to disable it.

This diagnostic warns for interprocedural paths through the code for which the analyzer
detects an attempt to use a "va_list" after "va_end" has been called on it. "va_list".

-Wno-analyzer-write-to-const
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-const to
disable it.

This diagnostic warns for paths through the code in which the analyzer detects an attempt
to write through a pointer to a "const" object. However, the analyzer does not
prioritize detection of such paths, so false negatives are more likely relative to other
warnings.

-Wno-analyzer-write-to-string-literal
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-write-to-string-literal to disable it.

This diagnostic warns for paths through the code in which the analyzer detects an attempt
to write through a pointer to a string literal. However, the analyzer does not
prioritize detection of such paths, so false negatives are more likely relative to other
warnings.

-Wno-analyzer-use-of-uninitialized-value
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-of-uninitialized-value to disable it.

This diagnostic warns for paths through the code in which an uninitialized value is used.

See CWE-457: Use of Uninitialized Variable
("https://cwe.mitre.org/data/definitions/457.html").

The analyzer has hardcoded knowledge about the behavior of the following memory-management
functions:

*<"alloca">
*<The built-in functions "__builtin_alloc",>
"__builtin_alloc_with_align", @item "__builtin_calloc", "__builtin_free",
"__builtin_malloc", "__builtin_memcpy", "__builtin_memcpy_chk", "__builtin_memset",
"__builtin_memset_chk", "__builtin_realloc", "__builtin_stack_restore", and
"__builtin_stack_save"

*<"calloc">
*<"free">
*<"malloc">
*<"memset">
*<"operator delete">
*<"operator delete []">
*<"operator new">
*<"operator new []">
*<"realloc">
*<"strdup">
*<"strndup">

of the following functions for working with file descriptors:

*<"open">
*<"close">
*<"creat">
*<"dup", "dup2" and "dup3">
*<"isatty">
*<"pipe", and "pipe2">
*<"read">
*<"write">
*<"socket", "bind", "listen", "accept", and "connect">

of the following functions for working with "<stdio.h>" streams:

*<The built-in functions "__builtin_fprintf",>
"__builtin_fprintf_unlocked", "__builtin_fputc", "__builtin_fputc_unlocked",
"__builtin_fputs", "__builtin_fputs_unlocked", "__builtin_fwrite",
"__builtin_fwrite_unlocked", "__builtin_printf", "__builtin_printf_unlocked",
"__builtin_putc", "__builtin_putchar", "__builtin_putchar_unlocked",
"__builtin_putc_unlocked", "__builtin_puts", "__builtin_puts_unlocked",
"__builtin_vfprintf", and "__builtin_vprintf"

*<"fopen">
*<"fclose">
*<"ferror">
*<"fgets">
*<"fgets_unlocked">
*<"fileno">
*<"fread">
*<"getc">
*<"getchar">
*<"fprintf">
*<"printf">
*<"fwrite">

and of the following functions:

*<The built-in functions "__builtin_expect",>
"__builtin_expect_with_probability", "__builtin_strchr", "__builtin_strcpy",
"__builtin_strcpy_chk", "__builtin_strlen", "__builtin_va_copy", and "__builtin_va_start"

*<The GNU extensions "error" and "error_at_line">
*<"getpass">
*<"longjmp">
*<"putenv">
*<"setjmp">
*<"siglongjmp">
*<"signal">
*<"sigsetjmp">
*<"strchr">
*<"strlen">

In addition, various functions with an "__analyzer_" prefix have special meaning to the
analyzer, described in the GCC Internals manual.

Pertinent parameters for controlling the exploration are:

*<--param analyzer-bb-explosion-factor=value>
*<--param analyzer-max-enodes-per-program-point=value>
*<--param analyzer-max-recursion-depth=value>
*<--param analyzer-min-snodes-for-call-summary=value>

The following options control the analyzer.

-fanalyzer-call-summaries
Simplify interprocedural analysis by computing the effect of certain calls, rather than
exploring all paths through the function from callsite to each possible return.

If enabled, call summaries are only used for functions with more than one call site, and
that are sufficiently complicated (as per --param
analyzer-min-snodes-for-call-summary=value).

-fanalyzer-checker=name
Restrict the analyzer to run just the named checker, and enable it.

Some checkers are disabled by default (even with -fanalyzer), such as the "taint" checker
that implements -Wanalyzer-tainted-array-index, and this option is required to enable
them.

Note: currently, -fanalyzer-checker=taint disables the following warnings from
-fanalyzer:

-Wanalyzer-deref-before-check -Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-fd-access-mode-mismatch
-Wanalyzer-fd-double-close -Wanalyzer-fd-leak -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak -Wanalyzer-free-of-non-heap
-Wanalyzer-malloc-leak -Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free -Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end

-fno-analyzer-feasibility
This option is intended for analyzer developers.

By default the analyzer verifies that there is a feasible control flow path for each
diagnostic it emits: that the conditions that hold are not mutually exclusive.
Diagnostics for which no feasible path can be found are rejected. This filtering can be
suppressed with -fno-analyzer-feasibility, for debugging issues in this code.

-fanalyzer-fine-grained
This option is intended for analyzer developers.

Internally the analyzer builds an "exploded graph" that combines control flow graphs with
data flow information.

By default, an edge in this graph can contain the effects of a run of multiple statements
within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge.

-fanalyzer-show-duplicate-count
This option is intended for analyzer developers: if multiple diagnostics have been
detected as being duplicates of each other, it emits a note when reporting the best
diagnostic, giving the number of additional diagnostics that were suppressed by the
deduplication logic.

-fno-analyzer-state-merge
This option is intended for analyzer developers.

By default the analyzer attempts to simplify analysis by merging sufficiently similar
states at each program point as it builds its "exploded graph". With
-fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling
issues.

-fno-analyzer-state-purge
This option is intended for analyzer developers.

By default the analyzer attempts to simplify analysis by purging aspects of state at a
program point that appear to no longer be relevant e.g. the values of locals that aren't
accessed later in the function and which aren't relevant to leak analysis.

With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging
state-handling issues.

-fno-analyzer-suppress-followups
This option is intended for analyzer developers.

By default the analyzer will stop exploring an execution path after encountering certain
diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics.

The diagnostics that terminate analysis along a path are:

*<-Wanalyzer-null-argument>
*<-Wanalyzer-null-dereference>
*<-Wanalyzer-use-after-free>
*<-Wanalyzer-use-of-pointer-in-stale-stack-frame>
*<-Wanalyzer-use-of-uninitialized-value>

With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths
even after such diagnostics, which may be helpful for debugging issues in the analyzer,
or for microbenchmarks for detecting undefined behavior.

-fanalyzer-transitivity
This option enables transitivity of constraints within the analyzer.

-fno-analyzer-undo-inlining
This option is intended for analyzer developers.

-fanalyzer runs relatively late compared to other code analysis tools, and some
optimizations have already been applied to the code. In particular function inlining may
have occurred, leading to the interprocedural execution paths emitted by the analyzer
containing function frames that don't correspond to those in the original source code.

By default the analyzer attempts to reconstruct the original function frames, and to emit
events showing the inlined calls.

With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame
information can be be disabled, which may be of help when debugging issues in the
analyzer.

-fanalyzer-verbose-edges
This option is intended for analyzer developers. It enables more verbose, lower-level
detail in the descriptions of control flow within diagnostic paths.

-fanalyzer-verbose-state-changes
This option is intended for analyzer developers. It enables more verbose, lower-level
detail in the descriptions of events relating to state machines within diagnostic paths.

-fanalyzer-verbosity=level
This option controls the complexity of the control flow paths that are emitted for
analyzer diagnostics.

The level can be one of:

0 At this level, interprocedural call and return events are displayed, along with the
most pertinent state-change events relating to a diagnostic. For example, for a
double-"free" diagnostic, both calls to "free" will be shown.

1 As per the previous level, but also show events for the entry to each function.

2 As per the previous level, but also show events relating to control flow that are
significant to triggering the issue (e.g. "true path taken" at a conditional).

This level is the default.

3 As per the previous level, but show all control flow events, not just significant
ones.

4 This level is intended for analyzer developers; it adds various other events intended
for debugging the analyzer.

-fdump-analyzer
Dump internal details about what the analyzer is doing to file.analyzer.txt.
-fdump-analyzer-stderr overrides this option.

-fdump-analyzer-stderr
Dump internal details about what the analyzer is doing to stderr. This option overrides
-fdump-analyzer.

-fdump-analyzer-callgraph
Dump a representation of the call graph suitable for viewing with GraphViz to
file.callgraph.dot.

-fdump-analyzer-exploded-graph
Dump a representation of the "exploded graph" suitable for viewing with GraphViz to
file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state
changes.

-fdump-analyzer-exploded-nodes
Emit diagnostics showing where nodes in the "exploded graph" are in relation to the
program source.

-fdump-analyzer-exploded-nodes-2
Dump a textual representation of the "exploded graph" to file.eg.txt.

-fdump-analyzer-exploded-nodes-3
Dump a textual representation of the "exploded graph" to one dump file per node, to
file.eg-id.txt. This is typically a large number of dump files.

-fdump-analyzer-exploded-paths
Dump a textual representation of the "exploded path" for each diagnostic to
file.idx.kind.epath.txt.

-fdump-analyzer-feasibility
Dump internal details about the analyzer's search for feasible paths. The details are
written in a form suitable for viewing with GraphViz to filenames of the form
file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt.

-fdump-analyzer-json
Dump a compressed JSON representation of analyzer internals to file.analyzer.json.gz.
The precise format is subject to change.

-fdump-analyzer-state-purge
As per -fdump-analyzer-supergraph, dump a representation of the "supergraph" suitable for
viewing with GraphViz, but annotate the graph with information on what state will be
purged at each node. The graph is written to file.state-purge.dot.

-fdump-analyzer-supergraph
Dump representations of the "supergraph" suitable for viewing with GraphViz to
file.supergraph.dot and to file.supergraph-eg.dot. These show all of the control flow
graphs in the program, with interprocedural edges for calls and returns. The second dump
contains annotations showing nodes in the "exploded graph" and diagnostics associated
with them.

-fdump-analyzer-untracked
Emit custom warnings with internal details intended for analyzer developers.

Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost all cases you need
only to add -g to your other options. Some debug formats can co-exist (like DWARF with CTF)
when each of them is enabled explicitly by adding the respective command line option to your
other options.

GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be
surprising: some variables you declared may not exist at all; flow of control may briefly
move where you did not expect it; some statements may not be executed because they compute
constant results or their values are already at hand; some statements may execute in
different places because they have been moved out of loops. Nevertheless it is possible to
debug optimized output. This makes it reasonable to use the optimizer for programs that
might have bugs.

If you are not using some other optimization option, consider using -Og with -g. With no -O
option at all, some compiler passes that collect information useful for debugging do not run
at all, so that -Og may result in a better debugging experience.

-g Produce debugging information in the operating system's native format (stabs, COFF,
XCOFF, or DWARF). GDB can work with this debugging information.

On most systems that use stabs format, -g enables use of extra debugging information that
only GDB can use; this extra information makes debugging work better in GDB but probably
makes other debuggers crash or refuse to read the program. If you want to control for
certain whether to generate the extra information, use -gvms (see below).

-ggdb
Produce debugging information for use by GDB. This means to use the most expressive
format available (DWARF, stabs, or the native format if neither of those are supported),
including GDB extensions if at all possible.

-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). The value of
version may be either 2, 3, 4 or 5; the default version for most targets is 5 (with the
exception of VxWorks, TPF and Darwin/Mac OS X, which default to version 2, and AIX, which
defaults to version 4).

Note that with DWARF Version 2, some ports require and always use some non-conflicting
DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version
5 requires GDB 8.0 or higher.

GCC no longer supports DWARF Version 1, which is substantially different than Version 2
and later. For historical reasons, some other DWARF-related options such as
-fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their names, but apply to
all currently-supported versions of DWARF.

-gbtf
Request BTF debug information. BTF is the default debugging format for the eBPF target.
On other targets, like x86, BTF debug information can be generated along with DWARF debug
information when both of the debug formats are enabled explicitly via their respective
command line options.

-gctf
-gctflevel
Request CTF debug information and use level to specify how much CTF debug information
should be produced. If -gctf is specified without a value for level, the default level
of CTF debug information is 2.

CTF debug information can be generated along with DWARF debug information when both of
the debug formats are enabled explicitly via their respective command line options.

Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf.

Level 1 produces CTF information for tracebacks only. This includes callsite
information, but does not include type information.

Level 2 produces type information for entities (functions, data objects etc.) at file-
scope or global-scope only.

-gvms
Produce debugging information in Alpha/VMS debug format (if that is supported). This is
the format used by DEBUG on Alpha/VMS systems.

-glevel
-ggdblevel
-gvmslevel
Request debugging information and also use level to specify how much information. The
default level is 2.

Level 0 produces no debug information at all. Thus, -g0 negates -g.

Level 1 produces minimal information, enough for making backtraces in parts of the
program that you don't plan to debug. This includes descriptions of functions and
external variables, and line number tables, but no information about local variables.

Level 3 includes extra information, such as all the macro definitions present in the
program. Some debuggers support macro expansion when you use -g3.

If you use multiple -g options, with or without level numbers, the last such option is
the one that is effective.

-gdwarf does not accept a concatenated debug level, to avoid confusion with
-gdwarf-level. Instead use an additional -glevel option to change the debug level for
DWARF.

-fno-eliminate-unused-debug-symbols
By default, no debug information is produced for symbols that are not actually used. Use
this option if you want debug information for all symbols.

-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one object file, emit
it in all object files using the class. This option should be used only with debuggers
that are unable to handle the way GCC normally emits debugging information for classes
because using this option increases the size of debugging information by as much as a
factor of two.

-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information that are
identical in different object files. Merging is not supported by all assemblers or
linkers. Merging decreases the size of the debug information in the output file at the
cost of increasing link processing time. Merging is enabled by default.

-fdebug-prefix-map=old=new
When compiling files residing in directory old, record debugging information describing
them as if the files resided in directory new instead. This can be used to replace a
build-time path with an install-time path in the debug info. It can also be used to
change an absolute path to a relative path by using . for new. This can give more
reproducible builds, which are location independent, but may require an extra command to
tell GDB where to find the source files. See also -ffile-prefix-map and
-fcanon-prefix-map.

-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in
code. Better debugging information is then generated (if the debugging information
format supports this information).

It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging
information (-g) and the debug info format supports it.

-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the
annotations over throughout the compilation all the way to the end, in an attempt to
improve debug information while optimizing. Use of -gdwarf-4 is recommended along with
it.

It can be enabled even if var-tracking is disabled, in which case annotations are created
and maintained, but discarded at the end. By default, this flag is enabled together with
-fvar-tracking, except when selective scheduling is enabled.

-gsplit-dwarf
If DWARF debugging information is enabled, separate as much debugging information as
possible into a separate output file with the extension .dwo. This option allows the
build system to avoid linking files with debug information. To be useful, this option
requires a debugger capable of reading .dwo files.

-gdwarf32
-gdwarf64
If DWARF debugging information is enabled, the -gdwarf32 selects the 32-bit DWARF format
and the -gdwarf64 selects the 64-bit DWARF format. The default is target specific, on
most targets it is -gdwarf32 though. The 32-bit DWARF format is smaller, but can't
support more than 2GiB of debug information in any of the DWARF debug information
sections. The 64-bit DWARF format allows larger debug information and might not be well
supported by all consumers yet.

-gdescribe-dies
Add description attributes to some DWARF DIEs that have no name attribute, such as
artificial variables, external references and call site parameter DIEs.

-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.

-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for
conversion into a GDB index. This option is only useful with a linker that can produce
GDB index version 7.

-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types"
section instead of making them part of the ".debug_info" section. It is more efficient
to put them in a separate comdat section since the linker can then remove duplicates.
But not all DWARF consumers support ".debug_types" sections yet and on some objects
".debug_types" produces larger instead of smaller debugging information.

-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect
code generation to be appended to the DW_AT_producer attribute in DWARF debugging
information. The options are concatenated with spaces separating them from each other
and from the compiler version. It is enabled by default. See also -frecord-gcc-switches
for another way of storing compiler options into the object file.

-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with
-gdwarf-version. On most targets using non-conflicting DWARF extensions from later
standard versions is allowed.

-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with
-gdwarf-version.

-gas-loc-support
Inform the compiler that the assembler supports ".loc" directives. It may then use them
for the assembler to generate DWARF2+ line number tables.

This is generally desirable, because assembler-generated line-number tables are a lot
more compact than those the compiler can generate itself.

This option will be enabled by default if, at GCC configure time, the assembler was found
to support such directives.

-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number
tables are to be generated.

-gas-locview-support
Inform the compiler that the assembler supports "view" assignment and reset assertion
checking in ".loc" directives.

This option will be enabled by default if, at GCC configure time, the assembler was found
to support them.

-gno-as-locview-support
Force GCC to assign view numbers internally, if -gvariable-location-views are explicitly
requested.

-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information, rather than just file
and line. This option is enabled by default.

-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal representation at the beginning
of statements, and to keep them roughly in place throughout compilation, using them to
guide the output of "is_stmt" markers in the line number table. This is enabled by
default when compiling with optimization (-Os, -O1, -O2, ...), and outputting DWARF 2
debug information at the normal level.

-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers implied from the line
number table. This enables debug information consumers to inspect state at certain
points of the program, even if no instructions associated with the corresponding source
locations are present at that point. If the assembler lacks support for view numbers in
line number tables, this will cause the compiler to emit the line number table, which
generally makes them somewhat less compact. The augmented line number tables and
location lists are fully backward-compatible, so they can be consumed by debug
information consumers that are not aware of these augmentations, but they won't derive
any benefit from them either.

This is enabled by default when outputting DWARF 2 debug information at the normal level,
as long as there is assembler support, -fvar-tracking-assignments is enabled and
-gstrict-dwarf is not. When assembler support is not available, this may still be
enabled, but it will force GCC to output internal line number tables, and if
-ginternal-reset-location-views is not enabled, that will most certainly lead to silently
mismatching location views.

There is a proposed representation for view numbers that is not backward compatible with
the location list format introduced in DWARF 5, that can be enabled with
-gvariable-location-views=incompat5. This option may be removed in the future, is only
provided as a reference implementation of the proposed representation. Debug information
consumers are not expected to support this extended format, and they would be rendered
unable to decode location lists using it.

-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from location view lists. This
requires the compiler to have very accurate insn length estimates, which isn't always the
case, and it may cause incorrect view lists to be generated silently when using an
assembler that does not support location view lists. The GNU assembler will flag any
such error as a "view number mismatch". This is only enabled on ports that define a
reliable estimation function.

-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions. Location view tracking
markers are inserted at inlined entry points, so that address and view numbers can be
computed and output in debug information. This can be enabled independently of location
views, in which case the view numbers won't be output, but it can only be enabled along
with statement frontiers, and it is only enabled by default if location views are
enabled.

-gz[=type]
Produce compressed debug sections in DWARF format, if that is supported. If type is not
given, the default type depends on the capabilities of the assembler and linker used.
type may be one of none (don't compress debug sections), or zlib (use zlib compression in
ELF gABI format). If the linker doesn't support writing compressed debug sections, the
option is rejected. Otherwise, if the assembler does not support them, -gz is silently
ignored when producing object files.

-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation
source file matches the base name of file in which the struct is defined.

This option substantially reduces the size of debugging information, but at significant
potential loss in type information to the debugger. See -femit-struct-debug-reduced for
a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation
source file matches the base name of file in which the type is defined, unless the struct
is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential
loss in type information to the debugger. See -femit-struct-debug-baseonly for a more
aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates debug information. The
intent is to reduce duplicate struct debug information between different object files
within the same program.

This option is a detailed version of -femit-struct-debug-reduced and
-femit-struct-debug-baseonly, which serves for most needs.

The optional first word limits the specification to structs that are used directly (dir:)
or used indirectly (ind:). A struct type is used directly when it is the type of a
variable, member. Indirect uses arise through pointers to structs. That is, when use of
an incomplete struct is valid, the use is indirect. An example is struct one direct;
struct two * indirect;.

The optional second word limits the specification to ordinary structs (ord:) or generic
structs (gen:). Generic structs are a bit complicated to explain. For C++, these are
non-explicit specializations of template classes, or non-template classes within the
above. Other programming languages have generics, but -femit-struct-debug-detailed does
not yet implement them.

The third word specifies the source files for those structs for which the compiler should
emit debug information. The values none and any have the normal meaning. The value base
means that the base of name of the file in which the type declaration appears must match
the base of the name of the main compilation file. In practice, this means that when
compiling foo.c, debug information is generated for types declared in that file and
foo.h, but not other header files. The value sys means those types satisfying base or
declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is -femit-struct-debug-detailed=all.

This option works only with DWARF debug output.

-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section instead of using GAS
".cfi_*" directives.

-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug symbol output for types
that are nowhere used in the source file being compiled. Sometimes it is useful to have
GCC emit debugging information for all types declared in a compilation unit, regardless
of whether or not they are actually used in that compilation unit, for example if, in the
debugger, you want to cast a value to a type that is not actually used in your program
(but is declared). More often, however, this results in a significant amount of wasted
space.

Options That Control Optimization
These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and
to make debugging produce the expected results. Statements are independent: if you stop the
program with a breakpoint between statements, you can then assign a new value to any variable
or change the program counter to any other statement in the function and get exactly the
results you expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or
code size at the expense of compilation time and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program. Compiling
multiple files at once to a single output file mode allows the compiler to use information
gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag
are listed in this section.

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command
line, even if individual optimization flags are specified. Similarly, -Og suppresses many
optimization passes.

Depending on the target and how GCC was configured, a slightly different set of optimizations
may be enabled at each -O level than those listed here. You can invoke GCC with -Q
--help=optimizers to find out the exact set of optimizations that are enabled at each level.

-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a
large function.

With -O, the compiler tries to reduce code size and execution time, without performing
any optimizations that take a great deal of compilation time.

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim
-fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fforward-propagate
-fguess-branch-probability -fif-conversion -fif-conversion2
-finline-functions-called-once -fipa-modref -fipa-profile -fipa-pure-const
-fipa-reference -fipa-reference-addressable -fmerge-constants -fmove-loop-invariants
-fmove-loop-stores -fomit-frame-pointer -freorder-blocks -fshrink-wrap
-fshrink-wrap-separate -fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-pta
-ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time

-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve
a space-speed tradeoff. As compared to -O, this option increases both compilation time
and the performance of the generated code.

-O2 turns on all optimization flags specified by -O1. It also turns on the following
optimization flags:

-falign-functions -falign-jumps -falign-labels -falign-loops -fcaller-saves
-fcode-hoisting -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations -ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining -fipa-bit-cp -fipa-cp
-fipa-icf -fipa-ra -fipa-sra -fipa-vrp -fisolate-erroneous-paths-dereference
-flra-remat -foptimize-sibling-calls -foptimize-strlen -fpartial-inlining -fpeephole2
-freorder-blocks-algorithm=stc -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fsched-interblock
-fsched-spec -fstore-merging -fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce
-ftree-loop-vectorize -ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap

Please note the warning under -fgcse about invoking -O2 on programs that use computed
gotos.

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later
versions, -D_FORTIFY_SOURCE=3, is set by default, and is activated when -O is set to 2 or
higher. This enables additional compile-time and run-time checks for several libc
functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

NOTE: In Debian 13 and Ubuntu 24.04 and later versions, -D_TIME_BITS=64 together with
-D_FILE_OFFSET_BITS=64 is set by default on the 32bit architectures armel, armhf, hppa,
m68k, mips, mipsel, powerpc and sh4.

-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the
following optimization flags:

-fgcse-after-reload -fipa-cp-clone -floop-interchange -floop-unroll-and-jam -fpeel-loops
-fpredictive-commoning -fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic -fversion-loops-for-strides

-O0 Reduce compilation time and make debugging produce the expected results. This is the
default.

-Os Optimize for size. -Os enables all -O2 optimizations except those that often increase
code size:

-falign-functions -falign-jumps -falign-labels -falign-loops -fprefetch-loop-arrays
-freorder-blocks-algorithm=stc

It also enables -finline-functions, causes the compiler to tune for code size rather than
execution speed, and performs further optimizations designed to reduce code size.

-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also
enables optimizations that are not valid for all standard-compliant programs. It turns
on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays, unless
-fmax-stack-var-size is specified, and -fno-protect-parens. It turns off
-fsemantic-interposition.

-Og Optimize debugging experience. -Og should be the optimization level of choice for the
standard edit-compile-debug cycle, offering a reasonable level of optimization while
maintaining fast compilation and a good debugging experience. It is a better choice than
-O0 for producing debuggable code because some compiler passes that collect debug
information are disabled at -O0.

Like -O0, -Og completely disables a number of optimization passes so that individual
options controlling them have no effect. Otherwise -Og enables all -O1 optimization
flags except for those that may interfere with debugging:

-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion -fif-conversion2
-finline-functions-called-once -fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra

-Oz Optimize aggressively for size rather than speed. This may increase the number of
instructions executed if those instructions require fewer bytes to encode. -Oz behaves
similarly to -Os including enabling most -O2 optimizations.

If you use multiple -O options, with or without level numbers, the last such option is the
one that is effective.

Options of the form -fflag specify machine-independent flags. Most flags have both positive
and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of
the forms is listed---the one you typically use. You can figure out the other form by either
removing no- or adding it.

The following options control specific optimizations. They are either activated by -O
options or are related to ones that are. You can use the following flags in the rare cases
when "fine-tuning" of optimizations to be performed is desired.

-fno-defer-pop
For machines that must pop arguments after a function call, always pop the arguments as
soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default;
this allows the compiler to let arguments accumulate on the stack for several function
calls and pop them all at once.

-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions
and checks if the result can be simplified. If loop unrolling is active, two passes are
performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.

-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast
enables floating-point expression contraction such as forming of fused multiply-add
operations if the target has native support for them. -ffp-contract=on enables floating-
point expression contraction if allowed by the language standard. This is currently not
implemented and treated equal to -ffp-contract=off.

The default is -ffp-contract=fast.

-fomit-frame-pointer
Omit the frame pointer in functions that don't need one. This avoids the instructions to
save, set up and restore the frame pointer; on many targets it also makes an extra
register available.

On some targets this flag has no effect because the standard calling sequence always uses
a frame pointer, so it cannot be omitted.

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all
functions. Several targets always omit the frame pointer in leaf functions.

Enabled by default at -O1 and higher.

-foptimize-sibling-calls
Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen", "strchr" or "strcpy") and
their "_FORTIFY_SOURCE" counterparts into faster alternatives.

Enabled at levels -O2, -O3.

-fno-inline
Do not expand any functions inline apart from those marked with the "always_inline"
attribute. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the "noinline"
attribute.

-finline-small-functions
Integrate functions into their callers when their body is smaller than expected function
call code (so overall size of program gets smaller). The compiler heuristically decides
which functions are simple enough to be worth integrating in this way. This inlining
applies to all functions, even those not declared inline.

Enabled at levels -O2, -O3, -Os.

-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to
previous inlining. This option has any effect only when inlining itself is turned on by
the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler
heuristically decides which functions are worth integrating in this way.

If all calls to a given function are integrated, and the function is declared "static",
then the function is normally not output as assembler code in its own right.

Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.

-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they
are not marked "inline". If a call to a given function is integrated, then the function
is not output as assembler code in its own right.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.

-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than
the function call overhead early before doing -fprofile-generate instrumentation and real
inlining pass. Doing so makes profiling significantly cheaper and usually inlining
faster on programs having large chains of nested wrapper functions.

Enabled by default.

-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters
and replacement of parameters passed by reference by parameters passed by value.

Enabled at levels -O2, -O3 and -Os.

-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows
coarse control of this limit. n is the size of functions that can be inlined in number
of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be specified
individually by using --param name=value. The -finline-limit=n option sets some of these
parameters as follows:

max-inline-insns-single
is set to n/2.

max-inline-insns-auto
is set to n/2.

See below for a documentation of the individual parameters controlling inlining and for
the defaults of these parameters.

Note: there may be no value to -finline-limit that results in default behavior.

Note: pseudo instruction represents, in this particular context, an abstract measurement
of function's size. In no way does it represent a count of assembly instructions and as
such its exact meaning might change from one release to an another.

-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which applies only to
functions that are declared using the "dllexport" attribute or declspec.

-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if
the function has been inlined into all of its callers. This switch does not affect
functions using the "extern inline" extension in GNU C90. In C++, emit any and all
inline functions into the object file.

-fkeep-static-functions
Emit "static" functions into the object file, even if the function is never used.

-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the
variables aren't referenced.

GCC enables this option by default. If you want to force the compiler to check if a
variable is referenced, regardless of whether or not optimization is turned on, use the
-fno-keep-static-consts option.

-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants)
across compilation units.

This option is the default for optimized compilation if the assembler and linker support
it. Use -fno-merge-constants to inhibit this behavior.

Enabled at levels -O1, -O2, -O3, -Os.

-fmerge-all-constants
Attempt to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants this considers
e.g. even constant initialized arrays or initialized constant variables with integral or
floating-point types. Languages like C or C++ require each variable, including multiple
instances of the same variable in recursive calls, to have distinct locations, so using
this option results in non-conforming behavior.

-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass
looks at innermost loops and reorders their instructions by overlapping different
iterations.

-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves allowed. By
setting this flag certain anti-dependences edges are deleted, which triggers the
generation of reg-moves based on the life-range analysis. This option is effective only
with -fmodulo-sched enabled.

-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to use "decrement and branch"
instructions on a count register instead of instruction sequences that decrement a
register, compare it against zero, and then branch based upon the result. This option is
only meaningful on architectures that support such instructions, which include x86,
PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg option doesn't remove the
decrement and branch instructions from the generated instruction stream introduced by
other optimization passes.

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant
function contain the function's address explicitly.

This option results in less efficient code, but some strange hacks that alter the
assembler output may be confused by the optimizations performed when this option is not
used.

The default is -ffunction-cse

-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized
to zero into BSS. This can save space in the resulting code.

This option turns off this behavior because some programs explicitly rely on variables
going to the data section---e.g., so that the resulting executable can find the beginning
of that section and/or make assumptions based on that.

The default is -fzero-initialized-in-bss.

-fthread-jumps
Perform optimizations that check to see if a jump branches to a location where another
comparison subsumed by the first is found. If so, the first branch is redirected to
either the destination of the second branch or a point immediately following it,
depending on whether the condition is known to be true or false.

Enabled at levels -O1, -O2, -O3, -Os.

-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a 32-bit
system, split the registers apart and allocate them independently. This normally
generates better code for those types, but may make debugging more difficult.

Enabled at levels -O1, -O2, -O3, -Os.

-fsplit-wide-types-early
Fully split wide types early, instead of very late. This option has no effect unless
-fsplit-wide-types is turned on.

This is the default on some targets.

-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when the target
of the jump is not reached by any other path. For example, when CSE encounters an "if"
statement with an "else" clause, CSE follows the jump when the condition tested is false.

Enabled at levels -O2, -O3, -Os.

-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally
skip over blocks. When CSE encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the body of the "if".

Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are performed.

Enabled at levels -O2, -O3, -Os.

-fgcse
Perform a global common subexpression elimination pass. This pass also performs global
constant and copy propagation.

Note: When compiling a program using computed gotos, a GCC extension, you may get better
run-time performance if you disable the global common subexpression elimination pass by
adding -fno-gcse to the command line.

Enabled at levels -O2, -O3, -Os.

-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads
that are only killed by stores into themselves. This allows a loop containing a
load/store sequence to be changed to a load outside the loop, and a copy/store within the
loop.

Enabled by default when -fgcse is enabled.

-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression
elimination. This pass attempts to move stores out of loops. When used in conjunction
with -fgcse-lm, loops containing a load/store sequence can be changed to a load before
the loop and a store after the loop.

Not enabled at any optimization level.

-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates
redundant loads that come after stores to the same memory location (both partial and full
redundancies).

Not enabled at any optimization level.

-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after
reload. The purpose of this pass is to clean up redundant spilling.

Enabled by -O3, -fprofile-use and -fauto-profile.

-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to derive bounds for the
number of iterations of a loop. This assumes that loop code does not invoke undefined
behavior by for example causing signed integer overflows or out-of-bound array accesses.
The bounds for the number of iterations of a loop are used to guide loop unrolling and
peeling and loop exit test optimizations. This option is enabled by default.

-funconstrained-commons
This option tells the compiler that variables declared in common blocks (e.g. Fortran)
may later be overridden with longer trailing arrays. This prevents certain optimizations
that depend on knowing the array bounds.

-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and
saves code size. The resulting code may or may not perform better than without cross-
jumping.

Enabled at levels -O2, -O3, -Os.

-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always
skipped on architectures that do not have instructions to support this. Enabled by
default at -O1 and higher on architectures that support this.

-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher.

-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher.

-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This includes use
of conditional moves, min, max, set flags and abs instructions, and some tricks doable by
standard arithmetics. The use of conditional execution on chips where it is available is
controlled by -fif-conversion2.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-
less equivalents.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and destructors: one for a
base subobject, one for a complete object, and one for a virtual destructor that calls
operator delete afterwards. For a hierarchy with virtual bases, the base and complete
variants are clones, which means two copies of the function. With this option, the base
and complete variants are changed to be thunks that call a common implementation.

Enabled by -Os.

-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code or data
element resides at address zero. This option enables simple constant folding
optimizations at all optimization levels. In addition, other optimization passes in GCC
use this flag to control global dataflow analyses that eliminate useless checks for null
pointers; these assume that a memory access to address zero always results in a trap, so
that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note however that in some environments this assumption is not true. Use
-fno-delete-null-pointer-checks to disable this optimization for programs that depend on
that behavior.

This option is enabled by default on most targets. On Nios II ELF, it defaults to off.
On AVR and MSP430, this option is completely disabled.

Passes that use the dataflow information are enabled independently at different
optimization levels.

-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done both within
a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and
interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.

-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct calls. Based on the
analysis of the type inheritance graph, determine for a given call the set of likely
targets. If the set is small, preferably of size 1, change the call into a conditional
deciding between direct and indirect calls. The speculative calls enable more
optimizations, such as inlining. When they seem useless after further optimization, they
are converted back into original form.

-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when running the link-
time optimizer in local transformation mode. This option enables more devirtualization
but significantly increases the size of streamed data. For this reason it is disabled by
default.

-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.

Enabled at levels -O2, -O3, -Os.

-free
Attempt to remove redundant extension instructions. This is especially helpful for the
x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to
their lower 32-bit half.

Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.

-fno-lifetime-dse
In C++ the value of an object is only affected by changes within its lifetime: when the
constructor begins, the object has an indeterminate value, and any changes during the
lifetime of the object are dead when the object is destroyed. Normally dead store
elimination will take advantage of this; if your code relies on the value of the object
storage persisting beyond the lifetime of the object, you can use this flag to disable
this optimization. To preserve stores before the constructor starts (e.g. because your
operator new clears the object storage) but still treat the object as dead after the
destructor, you can use -flifetime-dse=1. The default behavior can be explicitly
selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.

-flive-range-shrinkage
Attempt to decrease register pressure through register live range shrinkage. This is
helpful for fast processors with small or moderate size register sets.

-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator. The
algorithm argument can be priority, which specifies Chow's priority coloring, or CB,
which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for
all architectures, but for those targets that do support it, it is the default because it
generates better code.

-fira-region=region
Use specified regions for the integrated register allocator. The region argument should
be one of the following:

all Use all loops as register allocation regions. This can give the best results for
machines with a small and/or irregular register set.

mixed
Use all loops except for loops with small register pressure as the regions. This
value usually gives the best results in most cases and for most architectures, and is
enabled by default when compiling with optimization for speed (-O, -O2, ...).

one Use all functions as a single region. This typically results in the smallest code
size, and is enabled by default for -Os or -O0.

-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist
expressions. This option usually results in smaller code, but it can slow the compiler
down.

This option is enabled at level -Os for all targets.

-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop invariants.
This option usually results in generation of faster and smaller code on machines with
large register files (>= 32 registers), but it can slow the compiler down.

This option is enabled at level -O3 for some targets.

-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a
call. Each hard register gets a separate stack slot, and as a result function stack
frames are larger.

-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that
does not get a hard register gets a separate stack slot, and as a result function stack
frames are larger.

-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading values of spilled
pseudos, LRA tries to rematerialize (recalculate) values if it is profitable.

Enabled at levels -O2, -O3, -Os.

-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit
instruction slots available after delayed branch instructions.

Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.

-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate
execution stalls due to required data being unavailable. This helps machines that have
slow floating point or memory load instructions by allowing other instructions to be
issued until the result of the load or floating-point instruction is required.

Enabled at levels -O2, -O3.

-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling
after register allocation has been done. This is especially useful on machines with a
relatively small number of registers and where memory load instructions take more than
one cycle.

Enabled at levels -O2, -O3, -Os.

-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is normally enabled when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec
Disable speculative motion of non-load instructions, which is normally enabled when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-pressure
Enable register pressure sensitive insn scheduling before register allocation. This only
makes sense when scheduling before register allocation is enabled, i.e. with
-fschedule-insns or at -O2 or higher. Usage of this option can improve the generated
code and decrease its size by preventing register pressure increase above the number of
available hard registers and subsequent spills in register allocation.

-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns
into the ready list during the second scheduling pass. -fno-sched-stalled-insns means
that no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on
how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is
equivalent to -fsched-stalled-insns=1.

-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency on a stalled insn that
is a candidate for premature removal from the queue of stalled insns. This has an effect
only during the second scheduling pass, and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.

-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling. This allows motion
across basic block boundaries, resulting in faster schedules. This option is
experimental, as not all machine descriptions used by GCC model the CPU closely enough to
avoid unreliable results from the algorithm.

This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.

-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that
belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions
on the critical path. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.

-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors
speculative instructions with greater dependency weakness. This is enabled by default
when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.

-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction
belonging to a basic block with greater size or frequency. This is enabled by default
when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.

-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the
instruction that is less dependent on the last instruction scheduled. This is enabled by
default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.

-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the
instruction that has more instructions depending on it. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.

-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a loop is modulo
scheduled, later scheduling passes may change its schedule. Use this option to control
that behavior.

-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs
instead of the first scheduler pass.

-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective scheduling runs
instead of the second scheduler pass.

-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling. This option
has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned
on.

-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops. This
option has no effect unless -fsel-sched-pipelining is turned on.

-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This
means that for symbols exported from the DSO, the compiler cannot perform interprocedural
propagation, inlining and other optimizations in anticipation that the function or
variable in question may change. While this feature is useful, for example, to rewrite
memory allocation functions by a debugging implementation, it is expensive in the terms
of code quality. With -fno-semantic-interposition the compiler assumes that if
interposition happens for functions the overwriting function will have precisely the same
semantics (and side effects). Similarly if interposition happens for variables, the
constructor of the variable will be the same. The flag has no effect for functions
explicitly declared inline (where it is never allowed for interposition to change
semantics) and for symbols explicitly declared weak.

-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at
the top of the function. This flag is enabled by default at -O and higher.

-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately, so that those parts
are only executed when needed. This option is on by default, but has no effect unless
-fshrink-wrap is also turned on and the target supports this.

-fcaller-saves
Enable allocation of values to registers that are clobbered by function calls, by
emitting extra instructions to save and restore the registers around such calls. Such
allocation is done only when it seems to result in better code.

This option is always enabled by default on certain machines, usually those which have no
call-preserved registers to use instead.

Enabled at levels -O2, -O3, -Os.

-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to
find ways to combine them.

Enabled by default at -O1 and higher.

-fipa-ra
Use caller save registers for allocation if those registers are not used by any called
function. In that case it is not necessary to save and restore them around calls. This
is only possible if called functions are part of same compilation unit as current
function and they are compiled before it.

Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be
instrumented for profiling (-p, or -pg) or if callee's register usage cannot be known
exactly (this happens on targets that do not expose prologues and epilogues in RTL).

-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less stack space, even if
that makes the program slower. This option implies setting the large-stack-frame
parameter to 100 and the large-stack-frame-growth parameter to 400.

-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O1 and higher.

-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation of expressions
executed on all paths to the function exit as early as possible. This is especially
useful as a code size optimization, but it often helps for code speed as well. This flag
is enabled by default at -O2 and higher.

-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default
at -O2 and -O3.

-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by
default at -O3.

-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O1 and higher.

-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE
is that FRE only considers expressions that are computed on all paths leading to the
redundant computation. This analysis is faster than PRE, though it exposes fewer
redundancies. This flag is enabled by default at -O1 and higher.

-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by
default at -O1 and higher.

-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the loads are from
adjacent locations in the same structure and the target architecture has a conditional
move instruction. This flag is enabled by default at -O2 and higher.

-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations.
This flag is enabled by default at -O1 and higher.

-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at -O1 and higher.

-fipa-reference
Discover which static variables do not escape the compilation unit. Enabled by default
at -O1 and higher.

-fipa-reference-addressable
Discover read-only, write-only and non-addressable static variables. Enabled by default
at -O1 and higher.

-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by default.

-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and reference
analysis. This option can cause excessive memory and compile-time usage on large
compilation units. It is not enabled by default at any optimization level.

-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold
functions are marked as cold. Also functions executed once (such as "cold", "noreturn",
static constructors or destructors) are identified. Cold functions and loop less parts of
functions executed once are then optimized for size. Enabled by default at -O1 and
higher.

-fipa-modref
Perform interprocedural mod/ref analysis. This optimization analyzes the side effects of
functions (memory locations that are modified or referenced) and enables better
optimization across the function call boundary. This flag is enabled by default at -O1
and higher.

-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the program to
determine when values passed to functions are constants and then optimizes accordingly.
This optimization can substantially increase performance if the application has constants
passed to functions. This flag is enabled by default at -O2, -Os and -O3. It is also
enabled by -fprofile-use and -fauto-profile.

-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger. When
enabled, interprocedural constant propagation performs function cloning when externally
visible function can be called with constant arguments. Because this optimization can
create multiple copies of functions, it may significantly increase code size (see --param
ipa-cp-unit-growth=value). This flag is enabled by default at -O3. It is also enabled
by -fprofile-use and -fauto-profile.

-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation. This flag is enabled
by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is
enabled.

-fipa-vrp
When enabled, perform interprocedural propagation of value ranges. This flag is enabled
by default at -O2. It requires that -fipa-cp is enabled.

-fipa-icf
Perform Identical Code Folding for functions and read-only variables. The optimization
reduces code size and may disturb unwind stacks by replacing a function by equivalent one
with a different name. The optimization works more effectively with link-time
optimization enabled.

Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on
different levels and thus the optimizations are not same - there are equivalences that
are found only by GCC and equivalences found only by Gold.

This flag is enabled by default at -O2 and -Os.

-flive-patching=level
Control GCC's optimizations to produce output suitable for live-patching.

If the compiler's optimization uses a function's body or information extracted from its
body to optimize/change another function, the latter is called an impacted function of
the former. If a function is patched, its impacted functions should be patched too.

The impacted functions are determined by the compiler's interprocedural optimizations.
For example, a caller is impacted when inlining a function into its caller, cloning a
function and changing its caller to call this new clone, or extracting a function's
pureness/constness information to optimize its direct or indirect callers, etc.

Usually, the more IPA optimizations enabled, the larger the number of impacted functions
for each function. In order to control the number of impacted functions and more easily
compute the list of impacted function, IPA optimizations can be partially enabled at two
different levels.

The level argument should be one of the following:

inline-clone
Only enable inlining and cloning optimizations, which includes inlining, cloning,
interprocedural scalar replacement of aggregates and partial inlining. As a result,
when patching a function, all its callers and its clones' callers are impacted,
therefore need to be patched as well.

-flive-patching=inline-clone disables the following optimization flags:
-fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions
-fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const
-fipa-reference-addressable -fipa-stack-alignment -fipa-modref

inline-only-static
Only enable inlining of static functions. As a result, when patching a static
function, all its callers are impacted and so need to be patched as well.

In addition to all the flags that -flive-patching=inline-clone disables,
-flive-patching=inline-only-static disables the following additional optimization
flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp

When -flive-patching is specified without any value, the default value is inline-clone.

This flag is disabled by default.

Note that -flive-patching is not supported with link-time optimization (-flto).

-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to dereferencing a null
pointer. Isolate those paths from the main control flow and turn the statement with
erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and
higher and depends on -fdelete-null-pointer-checks also being enabled.

-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a null value being used
in a way forbidden by a "returns_nonnull" or "nonnull" attribute. Isolate those paths
from the main control flow and turn the statement with erroneous or undefined behavior
into a trap. This is not currently enabled, but may be enabled by -O2 in the future.

-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O1 and
higher.

-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer
alignment information. This pass only operates on local scalar variables and is enabled
by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled.

-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates
on local scalar variables and is enabled by default at -O1 and higher.

-fssa-backprop
Propagate information about uses of a value up the definition chain in order to simplify
the definitions. For example, this pass strips sign operations if the sign of a value
never matters. The flag is enabled by default at -O1 and higher.

-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is
enabled by default at -O1 and higher, except for -Og.

-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar
array. This flag is enabled by default at -O2 and higher.

-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the other.
This optimization is known as tail merging or cross jumping. This flag is enabled by
default at -O2 and higher. The compilation time in this pass can be limited using max-
tail-merge-comparisons parameter and max-tail-merge-iterations parameter.

-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O1 and
higher.

-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions that may
set "errno" but are otherwise free of side effects. This flag is enabled by default at
-O2 and higher if -Os is not also specified.

-ffinite-loops
Assume that a loop with an exit will eventually take the exit and not loop indefinitely.
This allows the compiler to remove loops that otherwise have no side-effects, not
considering eventual endless looping as such.

This option is enabled by default at -O2 for C++ with -std=c++11 or higher.

-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy
elimination, range propagation and expression simplification) based on a dominator tree
traversal. This also performs jump threading (to reduce jumps to jumps). This flag is
enabled by default at -O1 and higher.

-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a memory
location that is later overwritten by another store without any intervening loads. In
this case the earlier store can be deleted. This flag is enabled by default at -O1 and
higher.

-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases
effectiveness of code motion optimizations. It also saves one jump. This flag is
enabled by default at -O1 and higher. It is not enabled for -Os, since it usually
increases code size.

-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O1 and higher.

-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as -floop-nest-optimize. To use this code
transformation, GCC has to be configured with --with-isl to enable the Graphite loop
transformation infrastructure.

-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the
polyhedral representation and transform it back to gimple. Using -fgraphite-identity we
can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some
minimal optimizations are also performed by the code generator isl, like index splitting
and dead code elimination in loops.

-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop nest optimizer based on
the Pluto optimization algorithms. It calculates a loop structure optimized for data-
locality and parallelism. This option is experimental.

-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized.
Parallelize all the loops that can be analyzed to not contain loop carried dependences
without checking that it is profitable to parallelize the loops.

-ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt to reduce copying
by coalescing versions of different user-defined variables, instead of just compiler
temporaries. This may severely limit the ability to debug an optimized program compiled
with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA
coalescing of user variables. This option is enabled by default if optimization is
enabled, and it does very little otherwise.

-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents.
The intent is to remove control-flow from the innermost loops in order to improve the
ability of the vectorization pass to handle these loops. This is enabled by default if
vectorization is enabled.

-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop bodies
and allow further loop optimizations, like parallelization or vectorization, to take
place. For example, the loop

DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO

is transformed to

DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and
-fauto-profile.

-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library.
This flag is enabled by default at -O2 and higher, and by -fprofile-use and
-fauto-profile.

This pass distributes the initialization loops and generates a call to memset zero. For
example, the loop

DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO

is transformed to

DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO

and the initialization loop is transformed into a call to memset zero. This flag is
enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

-floop-interchange
Perform loop interchange outside of graphite. This flag can improve cache performance on
loop nest and allow further loop optimizations, like vectorization, to take place. For
example, the loop

for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];

is transformed to

for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and
-fauto-profile.

-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop nest this unrolls the
outer loop by some factor and fuses the resulting multiple inner loops. This flag is
enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that are hard to
handle at RTL level (function calls, operations that expand to nontrivial sequences of
insns). With -funswitch-loops it also moves operands of conditions that are invariant
out of the loop, so that we can use just trivial invariantness analysis in loop
unswitching. The pass also includes store motion.

-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number
of iterations requires complicated analysis. Later optimizations then may determine the
number easily. Useful especially in connection with unrolling.

-ftree-scev-cprop
Perform final value replacement. If a variable is modified in a loop in such a way that
its value when exiting the loop can be determined using only its initial value and the
number of loop iterations, replace uses of the final value by such a computation,
provided it is sufficiently cheap. This reduces data dependencies and may allow further
simplifications. Enabled by default at -O1 and higher.

-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging
and induction variable elimination) on trees.

-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only
possible for loops whose iterations are independent and can be arbitrarily reordered.
The optimization is only profitable on multiprocessor machines, for loops that are CPU-
intensive, rather than constrained e.g. by memory bandwidth. This option implies
-pthread, and thus is only supported on targets that have support for -pthread.

-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at
-O1 and higher, except for -Og.

-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with
scalars to prevent committing structures to memory too early. This flag is enabled by
default at -O1 and higher, except for -Og.

-fstore-merging
Perform merging of narrow stores to consecutive memory addresses. This pass merges
contiguous stores of immediate values narrower than a word into fewer wider stores to
reduce the number of instructions. This is enabled by default at -O2 and higher as well
as -Os.

-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single
def temporaries are replaced at their use location with their defining expression. This
results in non-GIMPLE code, but gives the expanders much more complex trees to work on
resulting in better RTL generation. This is enabled by default at -O1 and higher.

-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related expressions
involving multiplications and replaces them by less expensive calculations when possible.
This is enabled by default at -O1 and higher.

-ftree-vectorize
Perform vectorization on trees. This flag enables -ftree-loop-vectorize and
-ftree-slp-vectorize if not explicitly specified.

-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O2 and by
-ftree-vectorize, -fprofile-use, and -fauto-profile.

-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O2 and by
-ftree-vectorize, -fprofile-use, and -fauto-profile.

-ftrivial-auto-var-init=choice
Initialize automatic variables with either a pattern or with zeroes to increase the
security and predictability of a program by preventing uninitialized memory disclosure
and use. GCC still considers an automatic variable that doesn't have an explicit
initializer as uninitialized, -Wuninitialized and -Wanalyzer-use-of-uninitialized-value
will still report warning messages on such automatic variables and the compiler will
perform optimization as if the variable were uninitialized. With this option, GCC will
also initialize any padding of automatic variables that have structure or union types to
zeroes. However, the current implementation cannot initialize automatic variables that
are declared between the controlling expression and the first case of a "switch"
statement. Using -Wtrivial-auto-var-init to report all such cases.

The three values of choice are:

* uninitialized doesn't initialize any automatic variables. This is C and C++'s
default.

* pattern Initialize automatic variables with values which will likely transform logic
bugs into crashes down the line, are easily recognized in a crash dump and without
being values that programmers can rely on for useful program semantics. The current
value is byte-repeatable pattern with byte "0xFE". The values used for pattern
initialization might be changed in the future.

* zero Initialize automatic variables with zeroes.

The default is uninitialized.

You can control this behavior for a specific variable by using the variable attribute
"uninitialized".

-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument should be one of
unlimited, dynamic, cheap or very-cheap. With the unlimited model the vectorized code-
path is assumed to be profitable while with the dynamic model a runtime check guards the
vectorized code-path to enable it only for iteration counts that will likely execute
faster than when executing the original scalar loop. The cheap model disables
vectorization of loops where doing so would be cost prohibitive for example due to
required runtime checks for data dependence or alignment but otherwise is equal to the
dynamic model. The very-cheap model only allows vectorization if the vector code would
entirely replace the scalar code that is being vectorized. For example, if each
iteration of a vectorized loop would only be able to handle exactly four iterations of
the scalar loop, the very-cheap model would only allow vectorization if the scalar
iteration count is known to be a multiple of four.

The default cost model depends on other optimization flags and is either dynamic or
cheap.

-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with the OpenMP simd
directive. The model argument should be one of unlimited, dynamic, cheap. All values of
model have the same meaning as described in -fvect-cost-model and by default a cost model
defined with -fvect-cost-model is used.

-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation
pass, but instead of values, ranges of values are propagated. This allows the optimizers
to remove unnecessary range checks like array bound checks and null pointer checks. This
is enabled by default at -O2 and higher. Null pointer check elimination is only done if
-fdelete-null-pointer-checks is enabled.

-fsplit-paths
Split paths leading to loop backedges. This can improve dead code elimination and common
subexpression elimination. This is enabled by default at -O3 and above.

-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations of the unrolled
loop using the value in the first iteration. This breaks long dependency chains, thus
improving efficiency of the scheduling passes.

A combination of -fweb and CSE is often sufficient to obtain the same effect. However,
that is not reliable in cases where the loop body is more complicated than a single basic
block. It also does not work at all on some architectures due to restrictions in the CSE
pass.

This optimization is enabled by default.

-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables when
unrolling a loop, which can result in superior code.

This optimization is enabled by default for PowerPC targets, but disabled by default
otherwise.

-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is
turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory
loads and stores) performed in previous iterations of loops.

This option is enabled at level -O3. It is also enabled by -fprofile-use and
-fauto-profile.

-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve
the performance of loops that access large arrays.

This option may generate better or worse code; results are highly dependent on the
structure of loops within the source code.

Disabled at level -Os.

-fno-printf-return-value
Do not substitute constants for known return value of formatted output functions such as
"sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not "printf" of "fprintf"). This
transformation allows GCC to optimize or even eliminate branches based on the known
return value of these functions called with arguments that are either constant, or whose
values are known to be in a range that makes determining the exact return value possible.
For example, when -fprintf-return-value is in effect, both the branch and the body of the
"if" statement (but not the call to "snprint") can be optimized away when "i" is a 32-bit
or smaller integer because the return value is guaranteed to be at most 8.

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...

The -fprintf-return-value option relies on other optimizations and yields best results
with -O2 and above. It works in tandem with the -Wformat-overflow and
-Wformat-truncation options. The -fprintf-return-value option is enabled by default.

-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between
-fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some
targets use one, some use the other, a few use both.

-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.

-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.

GCC uses heuristics to guess branch probabilities if they are not provided by profiling
feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If
some branch probabilities are specified by "__builtin_expect", then the heuristics are
used to guess branch probabilities for the rest of the control flow graph, taking the
"__builtin_expect" info into account. The interactions between the heuristics and
"__builtin_expect" can be complex, and in some cases, it may be useful to disable the
heuristics so that the effects of "__builtin_expect" are easier to understand.

It is also possible to specify expected probability of the expression with
"__builtin_expect_with_probability" built-in function.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches
and improve code locality.

Enabled at levels -O1, -O2, -O3, -Os.

-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The algorithm argument can be
simple, which does not increase code size (except sometimes due to secondary effects like
alignment), or stc, the "software trace cache" algorithm, which tries to put all often
executed code together, minimizing the number of branches executed by making extra copies
of code.

The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.

-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce
number of taken branches, partitions hot and cold basic blocks into separate sections of
the assembly and .o files, to improve paging and cache locality performance.

This optimization is automatically turned off in the presence of exception handling or
unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce
sections, for functions with a user-defined section attribute and on any architecture
that does not support named sections. When -fsplit-stack is used this option is not
enabled by default (to avoid linker errors), but may be enabled explicitly (if using a
working linker).

Enabled for x86 at levels -O2, -O3, -Os.

-freorder-functions
Reorder functions in the object file in order to improve code locality. This is
implemented by using special subsections ".text.hot" for most frequently executed
functions and ".text.unlikely" for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must place them
in a reasonable way.

This option isn't effective unless you either provide profile feedback (see
-fprofile-arcs for details) or manually annotate functions with "hot" or "cold"
attributes.

Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language
being compiled. For C (and C++), this activates optimizations based on the type of
expressions. In particular, an object of one type is assumed never to reside at the same
address as an object of a different type, unless the types are almost the same. For
example, an "unsigned int" can alias an "int", but not a "void*" or a "double". A
character type may alias any other type.

Pay special attention to code like this:

union a_union {
int i;
double d;
};

int f() {
union a_union t;
t.d = 3.0;
return t.i;
}

The practice of reading from a different union member than the one most recently written
to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is
allowed, provided the memory is accessed through the union type. So, the code above
works as expected. However, this code might not:

int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}

Similarly, access by taking the address, casting the resulting pointer and dereferencing
the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

-fipa-strict-aliasing
Controls whether rules of -fstrict-aliasing are applied across function boundaries. Note
that if multiple functions gets inlined into a single function the memory accesses are no
longer considered to be crossing a function boundary.

The -fipa-strict-aliasing option is enabled by default and is effective only in
combination with -fstrict-aliasing.

-falign-functions
-falign-functions=n
-falign-functions=n:m
-falign-functions=n:m:n2
-falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater than or equal to n,
skipping up to m-1 bytes. This ensures that at least the first m bytes of the function
can be fetched by the CPU without crossing an n-byte alignment boundary.

If m is not specified, it defaults to n.

Examples: -falign-functions=32 aligns functions to the next 32-byte boundary,
-falign-functions=24 aligns to the next 32-byte boundary only if this can be done by
skipping 23 bytes or less, -falign-functions=32:7 aligns to the next 32-byte boundary
only if this can be done by skipping 6 bytes or less.

The second pair of n2:m2 values allows you to specify a secondary alignment:
-falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by
skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be
done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.

Some assemblers only support this flag when n is a power of two; in that case, it is
rounded up.

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are
not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n
option value is 65536.

Enabled at levels -O2, -O3.

-flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily overaligning
functions. It attempts to instruct the assembler to align by the amount specified by
-falign-functions, but not to skip more bytes than the size of the function.

-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.

Parameters of this option are analogous to the -falign-functions option.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not
aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then
their values are used instead.

If n is not specified or is zero, use a machine-dependent default which is very likely to
be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2
Align loops to a power-of-two boundary. If the loops are executed many times, this makes
up for any execution of the dummy padding instructions.

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned.
The maximum allowed n option value is 65536.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2
Align branch targets to a power-of-two boundary, for branch targets where the targets can
only be reached by jumping. In this case, no dummy operations need be executed.

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n
option value is 65536.

Enabled at levels -O2, -O3.

-fno-allocation-dce
Do not remove unused C++ allocations in dead code elimination.

-fallow-store-data-races
Allow the compiler to perform optimizations that may introduce new data races on stores,
without proving that the variable cannot be concurrently accessed by other threads. Does
not affect optimization of local data. It is safe to use this option if it is known that
global data will not be accessed by multiple threads.

Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-
conversions that may cause a value that was already in memory to be re-written with that
same value. Such re-writing is safe in a single threaded context but may be unsafe in a
multi-threaded context. Note that on some processors, if-conversions may be required in
order to enable vectorization.

Enabled at level -Ofast.

-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has no effect, while
-fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.

Enabled by default.

-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements. Output them in the
same order that they appear in the input file. When this option is used, unreferenced
static variables are not removed. This option is intended to support existing code that
relies on a particular ordering. For new code, it is better to use attributes when
possible.

-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors
is explicitly requested. Additionally -fno-toplevel-reorder implies
-fno-section-anchors.

-funreachable-traps
With this option, the compiler turns calls to "__builtin_unreachable" into traps, instead
of using them for optimization. This also affects any such calls implicitly generated by
the compiler.

This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable,
but does not affect the values of those options. If -fsanitize=unreachable is enabled,
that option takes priority over this one.

This option is enabled by default at -O0 and -Og.

-fweb
Constructs webs as commonly used for register allocation purposes and assign each web
individual pseudo register. This allows the register allocation pass to operate on
pseudos directly, but also strengthens several other optimization passes, such as CSE,
loop optimizer and trivial dead code remover. It can, however, make debugging
impossible, since variables no longer stay in a "home register".

Enabled by default with -funroll-loops.

-fwhole-program
Assume that the current compilation unit represents the whole program being compiled.
All public functions and variables with the exception of "main" and those merged by
attribute "externally_visible" become static functions and in effect are optimized more
aggressively by interprocedural optimizers.

With -flto this option has a limited use. In most cases the precise list of symbols used
or exported from the binary is known the resolution info passed to the link-time
optimizer by the linker plugin. It is still useful if no linker plugin is used or during
incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).

-flto[=n]
This option runs the standard link-time optimizer. When invoked with source code, it
generates GIMPLE (one of GCC's internal representations) and writes it to special ELF
sections in the object file. When the object files are linked together, all the function
bodies are read from these ELF sections and instantiated as if they had been part of the
same translation unit.

To use the link-time optimizer, -flto and optimization options should be specified at
compile time and during the final link. It is recommended that you compile all the files
participating in the same link with the same options and also specify those options at
link time. For example:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC save a bytecode representation of GIMPLE into special
ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from
foo.o and bar.o, merges the two files into a single internal image, and compiles the
result as usual. Since both foo.o and bar.o are merged into a single image, this causes
all the interprocedural analyses and optimizations in GCC to work across the two files as
if they were a single one. This means, for example, that the inliner is able to inline
functions in bar.o into functions in foo.o and vice-versa.

Another (simpler) way to enable link-time optimization is:

gcc -o myprog -flto -O2 foo.c bar.c

The above generates bytecode for foo.c and bar.c, merges them together into a single
GIMPLE representation and optimizes them as usual to produce myprog.

The important thing to keep in mind is that to enable link-time optimizations you need to
use the GCC driver to perform the link step. GCC automatically performs link-time
optimization if any of the objects involved were compiled with the -flto command-line
option. You can always override the automatic decision to do link-time optimization by
passing -fno-lto to the link command.

To make whole program optimization effective, it is necessary to make certain whole
program assumptions. The compiler needs to know what functions and variables can be
accessed by libraries and runtime outside of the link-time optimized unit. When
supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information
to the compiler about used and externally visible symbols. When the linker plugin is not
available, -fwhole-program should be used to allow the compiler to make these
assumptions, which leads to more aggressive optimization decisions.

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file
is larger than a regular object file because it contains GIMPLE bytecodes and the usual
final code (see -ffat-lto-objects). This means that object files with LTO information
can be linked as normal object files; if -fno-lto is passed to the linker, no
interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is
enabled the compile stage is faster but you cannot perform a regular, non-LTO link on
them.

When producing the final binary, GCC only applies link-time optimizations to those files
that contain bytecode. Therefore, you can mix and match object files and libraries with
GIMPLE bytecodes and final object code. GCC automatically selects which files to
optimize in LTO mode and which files to link without further processing.

Generally, options specified at link time override those specified at compile time,
although in some cases GCC attempts to infer link-time options from the settings used to
compile the input files.

If you do not specify an optimization level option -O at link time, then GCC uses the
highest optimization level used when compiling the object files. Note that it is
generally ineffective to specify an optimization level option only at link time and not
at compile time, for two reasons. First, compiling without optimization suppresses
compiler passes that gather information needed for effective optimization at link time.
Second, some early optimization passes can be performed only at compile time and not at
link time.

There are some code generation flags preserved by GCC when generating bytecodes, as they
need to be used during the final link. Currently, the following options and their
settings are taken from the first object file that explicitly specifies them: -fcommon,
-fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following
scheme:

B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>

Certain ABI-changing flags are required to match in all compilation units, and trying to
override this at link time with a conflicting value is ignored. This includes options
such as -freg-struct-return and -fpcc-struct-return.

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or
-fno-strict-aliasing are passed through to the link stage and merged conservatively for
conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv
take precedence; and for example -ffp-contract=off takes precedence over
-ffp-contract=fast. You can override them at link time.

Diagnostic options such as -Wstringop-overflow are passed through to the link stage and
their setting matches that of the compile-step at function granularity. Note that this
matters only for diagnostics emitted during optimization. Note that code transforms such
as inlining can lead to warnings being enabled or disabled for regions if code not
consistent with the setting at compile time.

When you need to pass options to the assembler via -Wa or -Xassembler make sure to either
compile such translation units with -fno-lto or consistently use the same assembler
options on all translation units. You can alternatively also specify assembler options
at LTO link time.

To enable debug info generation you need to supply -g at compile time. If any of the
input files at link time were built with debug info generation enabled the link will
enable debug info generation as well. Any elaborate debug info settings like the dwarf
level -gdwarf-5 need to be explicitly repeated at the linker command line and mixing
different settings in different translation units is discouraged.

If LTO encounters objects with C linkage declared with incompatible types in separate
translation units to be linked together (undefined behavior according to ISO C99 6.2.7),
a non-fatal diagnostic may be issued. The behavior is still undefined at run time.
Similar diagnostics may be raised for other languages.

Another feature of LTO is that it is possible to apply interprocedural optimizations on
files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries and
-lgfortran is added to get the Fortran runtime libraries. In general, when mixing
languages in LTO mode, you should use the same link command options as when mixing
languages in a regular (non-LTO) compilation.

If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a,
it is possible to extract and use them in an LTO link if you are using a linker with
plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib
instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use
gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin
support. At link time, use the flag -fuse-linker-plugin to ensure that the library
participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a
and passes them on to the running GCC to make them part of the aggregated GIMPLE image to
be optimized.

If you are not using a linker with plugin support and/or do not enable the linker plugin,
then the objects inside libfoo.a are extracted and linked as usual, but they do not
participate in the LTO optimization process. In order to make a static library suitable
for both LTO optimization and usual linkage, compile its object files with -flto
-ffat-lto-objects.

Link-time optimizations do not require the presence of the whole program to operate. If
the program does not require any symbols to be exported, it is possible to combine -flto
and -fwhole-program to allow the interprocedural optimizers to use more aggressive
assumptions which may lead to improved optimization opportunities. Use of
-fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).

The current implementation of LTO makes no attempt to generate bytecode that is portable
between different types of hosts. The bytecode files are versioned and there is a strict
version check, so bytecode files generated in one version of GCC do not work with an
older or newer version of GCC.

Link-time optimization does not work well with generation of debugging information on
systems other than those using a combination of ELF and DWARF.

If you specify the optional n, the optimization and code generation done at link time is
executed in parallel using n parallel jobs by utilizing an installed make program. The
environment variable MAKE may be used to override the program used.

You can also specify -flto=jobserver to use GNU make's job server mode to determine the
number of parallel jobs. This is useful when the Makefile calling GCC is already
executing in parallel. You must prepend a + to the command recipe in the parent Makefile
for this to work. This option likely only works if MAKE is GNU make. Even without the
option value, GCC tries to automatically detect a running GNU make's job server.

Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to
autodetection of the number of CPU threads present in your system.

-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either
1to1 to specify a partitioning mirroring the original source files or balanced to specify
partitioning into equally sized chunks (whenever possible) or max to create new partition
for every symbol where possible. Specifying none as an algorithm disables partitioning
and streaming completely. The default value is balanced. While 1to1 can be used as an
workaround for various code ordering issues, the max partitioning is intended for
internal testing only. The value one specifies that exactly one partition should be used
while the value none bypasses partitioning and executes the link-time optimization step
directly from the WPA phase.

-flto-compression-level=n
This option specifies the level of compression used for intermediate language written to
LTO object files, and is only meaningful in conjunction with LTO mode (-flto). GCC
currently supports two LTO compression algorithms. For zstd, valid values are 0 (no
compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values
outside this range are clamped to either minimum or maximum of the supported values. If
the option is not given, a default balanced compression setting is used.

-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on
plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.

This option enables the extraction of object files with GIMPLE bytecode out of library
archives. This improves the quality of optimization by exposing more code to the link-
time optimizer. This information specifies what symbols can be accessed externally (by
non-LTO object or during dynamic linking). Resulting code quality improvements on
binaries (and shared libraries that use hidden visibility) are similar to
-fwhole-program. See -flto for a description of the effect of this flag and how to use
it.

This option is enabled by default when LTO support in GCC is enabled and GCC was
configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).

-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the
object code. This makes them usable for both LTO linking and normal linking. This option
is effective only when compiling with -flto and is ignored at link time.

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete
toolchain to be aware of LTO. It requires a linker with linker plugin support for basic
functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a
full-featured build environment (capable of building static libraries etc). GCC provides
the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With
non fat LTO makefiles need to be modified to use them.

Note that modern binutils provide plugin auto-load mechanism. Installing the linker
plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers
(gcc-ar, gcc-nm and gcc-ranlib).

The default is -fno-fat-lto-objects on targets with linker plugin support.

-fcompare-elim
After register allocation and post-register allocation instruction splitting, identify
arithmetic instructions that compute processor flags similar to a comparison operation
based on that arithmetic. If possible, eliminate the explicit comparison operation.

This pass only applies to certain targets that cannot explicitly represent the comparison
operation before register allocation is complete.

Enabled at levels -O1, -O2, -O3, -Os.

-fcprop-registers
After register allocation and post-register allocation instruction splitting, perform a
copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate
the copy.

Enabled at levels -O1, -O2, -O3, -Os.

-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be
inconsistent due to missed counter updates. When this option is specified, GCC uses
heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error
message when an inconsistent profile is detected.

This option is enabled by -fauto-profile.

-fprofile-partial-training
With "-fprofile-use" all portions of programs not executed during train run are optimized
agressively for size rather than speed. In some cases it is not practical to train all
possible hot paths in the program. (For example, program may contain functions specific
for a given hardware and trianing may not cover all hardware configurations program is
run on.) With "-fprofile-partial-training" profile feedback will be ignored for all
functions not executed during the train run leading them to be optimized as if they were
compiled without profile feedback. This leads to better performance when train run is not
representative but also leads to significantly bigger code.

-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following optimizations, many of
which are generally profitable only with profile feedback available:

-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt
-finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning
-fsplit-loops -funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-reorder-functions

Before you can use this option, you must first generate profiling information.

By default, GCC emits an error message if the feedback profiles do not match the source
code. This error can be turned into a warning by using -Wno-error=coverage-mismatch.
Note this may result in poorly optimized code. Additionally, by default, GCC also emits
a warning message if the feedback profiles do not exist (see -Wmissing-profile).

If path is specified, GCC looks at the path to find the profile feedback data files. See
-fprofile-dir.

-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the following optimizations,
many of which are generally profitable only with profile feedback available:

path is the name of a file containing AutoFDO profile information. If omitted, it
defaults to fbdata.afdo in the current directory.

Producing an AutoFDO profile data file requires running your program with the perf
utility on a supported GNU/Linux target system. For more information, see
<https://perf.wiki.kernel.org/>.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be
used by GCC. You must also supply the unstripped binary for your program to this tool.
See <https://github.com/google/autofdo>.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo

The following options control compiler behavior regarding floating-point arithmetic. These
options trade off between speed and correctness. All must be specifically enabled.

-ffloat-store
Do not store floating-point variables in registers, and inhibit other options that might
change whether a floating-point value is taken from a register or memory.

This option prevents undesirable excess precision on machines such as the 68000 where the
floating registers (of the 68881) keep more precision than a "double" is supposed to
have. Similarly for the x86 architecture. For most programs, the excess precision does
only good, but a few programs rely on the precise definition of IEEE floating point. Use
-ffloat-store for such programs, after modifying them to store all pertinent intermediate
computations into variables.

-fexcess-precision=style
This option allows further control over excess precision on machines where floating-point
operations occur in a format with more precision or range than the IEEE standard and
interchange floating-point types. By default, -fexcess-precision=fast is in effect; this
means that operations may be carried out in a wider precision than the types specified in
the source if that would result in faster code, and it is unpredictable when rounding to
the types specified in the source code takes place. When compiling C or C++, if
-fexcess-precision=standard is specified then excess precision follows the rules
specified in ISO C99 or C++; in particular, both casts and assignments cause values to be
rounded to their semantic types (whereas -ffloat-store only affects assignments). This
option is enabled by default for C or C++ if a strict conformance option such as -std=c99
or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless
of whether a strict conformance option is used.

-fexcess-precision=standard is not implemented for languages other than C or C++. On the
x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former
case, IEEE semantics apply without excess precision, and in the latter, rounding is
unpredictable.

-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only,
-fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.

This option causes the preprocessor macro "__FAST_MATH__" to be defined.

This option is not turned on by any -O option besides -Ofast since it can result in
incorrect output for programs that depend on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

-fno-math-errno
Do not set "errno" after calling math functions that are executed with a single
instruction, e.g., "sqrt". A program that relies on IEEE exceptions for math error
handling may want to use this flag for speed while maintaining IEEE arithmetic
compatibility.

This option is not turned on by any -O option since it can result in incorrect output for
programs that depend on an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs that do not require the
guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets "errno". There is therefore no reason for
the compiler to consider the possibility that it might, and -fno-math-errno is the
default.

-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and
results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it
may include libraries or startup files that change the default FPU control word or other
similar optimizations.

The default is -fno-unsafe-math-optimizations.

-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates
the ISO C and C++ language standard by possibly changing computation result. NOTE: re-
ordering may change the sign of zero as well as ignore NaNs and inhibit or create
underflow or overflow (and thus cannot be used on code that relies on rounding behavior
like "(x + 2**52) - 2**52". May also reorder floating-point comparisons and thus may not
be used when ordered comparisons are required. This option requires that both
-fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much
sense with -frounding-math. For Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if this
enables optimizations. For example "x / y" can be replaced with "x * (1/y)", which is
useful if "(1/y)" is subject to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.

The default is -fno-reciprocal-math.

-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results
are not NaNs or +-Infs.

The default is -fno-finite-math-only.

-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero.
IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then
prohibits simplification of expressions such as x+0.0 or 0.0*x (even with
-ffinite-math-only). This option implies that the sign of a zero result isn't
significant.

The default is -fsigned-zeros.

-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps.
These traps include division by zero, overflow, underflow, inexact result and invalid
operation. This option requires that -fno-signaling-nans be in effect. Setting this
option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.

This option should never be turned on by any -O option since it can result in incorrect
output for programs that depend on an exact implementation of IEEE or ISO
rules/specifications for math functions.

The default is -ftrapping-math.

Future versions of GCC may provide finer control of this setting using C99's
"FENV_ACCESS" pragma. This command-line option will be used along with -frounding-math
to specify the default state for "FENV_ACCESS".

-frounding-math
Disable transformations and optimizations that assume default floating-point rounding
behavior. This is round-to-zero for all floating point to integer conversions, and
round-to-nearest for all other arithmetic truncations. This option should be specified
for programs that change the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of floating-point
expressions at compile time (which may be affected by rounding mode) and arithmetic
transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC
optimizations that are affected by rounding mode. Future versions of GCC may provide
finer control of this setting using C99's "FENV_ACCESS" pragma. This command-line option
will be used along with -ftrapping-math to specify the default state for "FENV_ACCESS".

-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during
floating-point operations. Setting this option disables optimizations that may change
the number of exceptions visible with signaling NaNs. This option implies
-ftrapping-math.

This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC
optimizations that affect signaling NaN behavior.

-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and "trunc", and their
"float" and "long double" variants, to generate code that raises the "inexact" floating-
point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise
the "inexact" exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as
integrated into ISO C2X, does not allow these functions to do so.

The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C2X
or a later C standard is selected. This option does nothing unless -ftrapping-math is in
effect.

Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a
library function then the "inexact" exception may be raised if the library implementation
does not follow TS 18661.

-fsingle-precision-constant
Treat floating-point constants as single precision instead of implicitly converting them
to double-precision constants.

-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when
performing complex division. Also, there is no checking whether the result of a complex
multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in
that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math.

This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma.
Nevertheless, the option applies to all languages.

-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as
part of complex division, but there is no checking whether the result of a complex
multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in
that case.

The default is -fno-cx-fortran-rules.

The following options control optimizations that may improve performance, but are not enabled
by any -O options. This section includes experimental options that may produce broken code.

-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time
using -fbranch-probabilities, to improve optimizations based on the number of times each
branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc
execution counts to a file called sourcename.gcda for each source file. The information
in this data file is very dependent on the structure of the generated code, so you must
use the same source code and the same optimization options for both compilations. See
details about the file naming in -fprofile-arcs.

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN.
These can be used to improve optimization. Currently, they are only used in one place:
in reorg.cc, instead of guessing which path a branch is most likely to take, the
REG_BR_PROB values are used to exactly determine which path is taken more often.

Enabled by -fprofile-use and -fauto-profile.

-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of
expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered from profiling values of
expressions for usage in optimizations.

Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first time of execution of
a function and orders these functions in ascending order.

Enabled with -fprofile-use.

-fvpt
If combined with -fprofile-arcs, this option instructs the compiler to add code to gather
information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and actually performs the
optimizations based on them. Currently the optimizations include specialization of
division operations using the knowledge about the value of the denominator.

Enabled with -fprofile-use and -fauto-profile.

-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left
over after register allocation. This optimization most benefits processors with lots of
registers. Depending on the debug information format adopted by the target, however, it
can make debugging impossible, since variables no longer stay in a "home register".

Enabled by default with -funroll-loops.

-fschedule-fusion
Performs a target dependent pass over the instruction stream to schedule instructions of
same type together because target machine can execute them more efficiently if they are
adjacent to each other in the instruction flow.

Enabled at levels -O2, -O3, -Os.

-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the
control flow of the function allowing other optimizations to do a better job.

Enabled by -fprofile-use and -fauto-profile.

-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry
to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and
-frename-registers. It also turns on complete loop peeling (i.e. complete removal of
loops with a small constant number of iterations). This option makes code larger, and
may or may not make it run faster.

Enabled by -fprofile-use and -fauto-profile.

-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is
entered. This usually makes programs run more slowly. -funroll-all-loops implies the
same options as -funroll-loops.

-fpeel-loops
Peels loops for which there is enough information that they do not roll much (from
profile feedback or static analysis). It also turns on complete loop peeling (i.e.
complete removal of loops with small constant number of iterations).

Enabled by -O3, -fprofile-use, and -fauto-profile.

-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
and higher, except for -Og.

-fmove-loop-stores
Enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant
stores to after the end of the loop in exchange for carrying the stored value in a
register across the iteration. Note for this option to have an effect -ftree-loop-im has
to be enabled as well. Enabled at level -O1 and higher, except for -Og.

-fsplit-loops
Split a loop into two if it contains a condition that's always true for one side of the
iteration space and false for the other.

Enabled by -fprofile-use and -fauto-profile.

-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop
on both branches (modified according to result of the condition).

Enabled by -fprofile-use and -fauto-profile.

-fversion-loops-for-strides
If a loop iterates over an array with a variable stride, create another version of the
loop that assumes the stride is always one. For example:

for (int i = 0; i < n; ++i)
x[i * stride] = ...;

becomes:

if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;

This is particularly useful for assumed-shape arrays in Fortran where (for example) it
allows better vectorization assuming contiguous accesses. This flag is enabled by
default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target
supports arbitrary sections. The name of the function or the name of the data item
determines the section's name in the output file.

Use these options on systems where the linker can perform optimizations to improve
locality of reference in the instruction space. Most systems using the ELF object format
have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs)
based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may
lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug
information. There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so. When you
specify these options, the assembler and linker create larger object and executable files
and are also slower. These options affect code generation. They prevent optimizations
by the compiler and assembler using relative locations inside a translation unit since
the locations are unknown until link time. An example of such an optimization is
relaxing calls to short call instructions.

-fstdarg-opt
Optimize the prologue of variadic argument functions with respect to usage of those
arguments.

-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared "anchor"
symbols to address nearby objects. This transformation can help to reduce the number of
GOT entries and GOT accesses on some targets.

For example, the implementation of the following function "foo":

static int a, b, c;
int foo (void) { return a + b + c; }

usually calculates the addresses of all three variables, but if you compile it with
-fsection-anchors, it accesses the variables from a common anchor point instead. The
effect is similar to the following pseudocode (which isn't valid C):

int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}

Not all targets support this option.

-fzero-call-used-regs=choice
Zero call-used registers at function return to increase program security by either
mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage
through registers.

The possible values of choice are the same as for the "zero_call_used_regs" attribute.
The default is skip.

You can control this behavior for a specific function by using the function attribute
"zero_call_used_regs".

--param name=value
In some places, GCC uses various constants to control the amount of optimization that is
done. For example, GCC does not inline functions that contain more than a certain number
of instructions. You can control some of these constants on the command line using the
--param option.

The names of specific parameters, and the meaning of the values, are tied to the
internals of the compiler, and are subject to change without notice in future releases.

In order to get the minimal, maximal and default values of a parameter, use the
--help=param -Q options.

In each case, the value is an integer. The following choices of name are recognized for
all targets:

predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold (in
percent), then it is considered well predictable.

max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a block and replace
them with conditionally executed instructions. This parameter gives the maximum
number of instructions in a block which should be considered for if-conversion. The
compiler will also use other heuristics to decide whether if-conversion is likely to
be profitable.

max-rtl-if-conversion-predictable-cost
RTL if-conversion will try to remove conditional branches around a block and replace
them with conditionally executed instructions. These parameters give the maximum
permissible cost for the sequence that would be generated by if-conversion depending
on whether the branch is statically determined to be predictable or not. The units
for this parameter are the same as those for the GCC internal seq_cost metric. The
compiler will try to provide a reasonable default for this parameter using the
BRANCH_COST target macro.

max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping. The algorithm
used by -fcrossjumping is O(N^2) in the number of edges incoming to each block.
Increasing values mean more aggressive optimization, making the compilation time
increase with probably small improvement in executable size.

min-crossjump-insns
The minimum number of instructions that must be matched at the end of two blocks
before cross-jumping is performed on them. This value is ignored in the case where
all instructions in the block being cross-jumped from are matched.

max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping.
The expansion is relative to a jump instruction.

max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed
goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos
early in the compilation process, and unfactors them as late as possible. Only
computed jumps at the end of a basic blocks with no more than max-goto-duplication-
insns are unfactored.

max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to
fill a delay slot. If more than this arbitrary number of instructions are searched,
the time savings from filling the delay slot are minimal, so stop searching.
Increasing values mean more aggressive optimization, making the compilation time
increase with probably small improvement in execution time.

max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when
searching for a block with valid live register information. Increasing this
arbitrarily chosen value means more aggressive optimization, increasing the
compilation time. This parameter should be removed when the delay slot code is
rewritten to maintain the control-flow graph.

max-gcse-memory
The approximate maximum amount of memory in "kB" that can be allocated in order to
perform the global common subexpression elimination optimization. If more memory
than specified is required, the optimization is not done.

max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value for any
expression, then RTL PRE inserts or removes the expression and thus leaves partially
redundant computations in the instruction stream.

max-pending-list-length
The maximum number of pending dependencies scheduling allows before flushing the
current state and starting over. Large functions with few branches or calls can
create excessively large lists which needlessly consume memory and resources.

max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when modulo
scheduling a loop. Larger values can exponentially increase compilation time.

max-inline-functions-called-once-loop-depth
Maximal loop depth of a call considered by inline heuristics that tries to inline all
functions called once.

max-inline-functions-called-once-insns
Maximal estimated size of functions produced while inlining functions called once.

max-inline-insns-single
Several parameters control the tree inliner used in GCC. This number sets the
maximum number of instructions (counted in GCC's internal representation) in a single
function that the tree inliner considers for inlining. This only affects functions
declared inline and methods implemented in a class declaration (C++).

max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would
otherwise not be considered for inlining by the compiler are investigated. To those
functions, a different (more restrictive) limit compared to functions declared inline
can be applied (--param max-inline-insns-auto).

max-inline-insns-small
This is bound applied to calls which are considered relevant with
-finline-small-functions.

max-inline-insns-size
This is bound applied to calls which are optimized for size. Small growth may be
desirable to anticipate optimization oppurtunities exposed by inlining.

uninlined-function-insns
Number of instructions accounted by inliner for function overhead such as function
prologue and epilogue.

uninlined-function-time
Extra time accounted by inliner for function overhead such as time needed to execute
function prologue and epilogue.

inline-heuristics-hint-percent
The scale (in percents) applied to inline-insns-single, inline-insns-single-O2,
inline-insns-auto when inline heuristics hints that inlining is very profitable (will
enable later optimizations).

uninlined-thunk-insns
uninlined-thunk-time
Same as --param uninlined-function-insns and --param uninlined-function-time but
applied to function thunks.

inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this
threshold (in percent), the function can be inlined regardless of the limit on
--param max-inline-insns-single and --param max-inline-insns-auto.

large-function-insns
The limit specifying really large functions. For functions larger than this limit
after inlining, inlining is constrained by --param large-function-growth. This
parameter is useful primarily to avoid extreme compilation time caused by non-linear
algorithms used by the back end.

large-function-growth
Specifies maximal growth of large function caused by inlining in percents. For
example, parameter value 100 limits large function growth to 2.0 times the original
size.

large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units
larger than this limit is limited by --param inline-unit-growth. For small units
this might be too tight. For example, consider a unit consisting of function A that
is inline and B that just calls A three times. If B is small relative to A, the
growth of unit is 300\% and yet such inlining is very sane. For very large units
consisting of small inlineable functions, however, the overall unit growth limit is
needed to avoid exponential explosion of code size. Thus for smaller units, the size
is increased to --param large-unit-insns before applying --param inline-unit-growth.

lazy-modules
Maximum number of concurrently open C++ module files when lazy loading.

inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. For
example, parameter value 20 limits unit growth to 1.2 times the original size. Cold
functions (either marked cold via an attribute or by profile feedback) are not
accounted into the unit size.

ipa-cp-unit-growth
Specifies maximal overall growth of the compilation unit caused by interprocedural
constant propagation. For example, parameter value 10 limits unit growth to 1.1
times the original size.

ipa-cp-large-unit-insns
The size of translation unit that IPA-CP pass considers large.

large-stack-frame
The limit specifying large stack frames. While inlining the algorithm is trying to
not grow past this limit too much.

large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents. For
example, parameter value 1000 limits large stack frame growth to 11 times the
original size.

max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a self-recursive
inline function can grow into by performing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. For
functions not declared inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled; --param max-inline-insns-recursive-
auto applies instead.

max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.

--param max-inline-recursive-depth applies to functions declared inline. For
functions not declared inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled; --param max-inline-recursive-depth-
auto applies instead.

min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average
and can hurt for function having little recursion depth by increasing the prologue
size or complexity of function body to other optimizers.

When profile feedback is available (see -fprofile-generate) the actual recursion
depth can be guessed from the probability that function recurses via a given call
expression. This parameter limits inlining only to call expressions whose
probability exceeds the given threshold (in percents).

early-inlining-insns
Specify growth that the early inliner can make. In effect it increases the amount of
inlining for code having a large abstraction penalty.

max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds the number of nested
indirect calls the early inliner can resolve. Deeper chains are still handled by
late inlining.

comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility are shared
across multiple compilation units.

modref-max-bases
modref-max-refs
modref-max-accesses
Specifies the maximal number of base pointers, references and accesses stored for a
single function by mod/ref analysis.

modref-max-tests
Specifies the maxmal number of tests alias oracle can perform to disambiguate memory
locations using the mod/ref information. This parameter ought to be bigger than
--param modref-max-bases and --param modref-max-refs.

modref-max-depth
Specifies the maximum depth of DFS walk used by modref escape analysis. Setting to 0
disables the analysis completely.

modref-max-escape-points
Specifies the maximum number of escape points tracked by modref per SSA-name.

modref-max-adjustments
Specifies the maximum number the access range is enlarged during modref dataflow
analysis.

profile-func-internal-id
A parameter to control whether to use function internal id in profile database
lookup. If the value is 0, the compiler uses an id that is based on function
assembler name and filename, which makes old profile data more tolerant to source
changes such as function reordering etc.

min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized when
-ftree-vectorize is used. The number of iterations after vectorization needs to be
greater than the value specified by this option to allow vectorization.

gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be moved by GCSE
optimizations. This is currently supported only in the code hoisting pass. The
bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e.,
the expressions that have cost less than gcse-unrestricted-cost. Specifying 0
disables hoisting of simple expressions.

gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine instruction, at which
GCSE optimizations do not constrain the distance an expression can travel. This is
currently supported only in the code hoisting pass. The lesser the cost, the more
aggressive code hoisting is. Specifying 0 allows all expressions to travel
unrestricted distances.

max-hoist-depth
The depth of search in the dominator tree for expressions to hoist. This is used to
avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the
search, but may slow down compilation of huge functions.

max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to avoid
quadratic behavior in tree tail merging.

max-tail-merge-iterations
The maximum amount of iterations of the pass over the function. This is used to
limit compilation time in tree tail merging.

store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it is legal to do so.

max-stores-to-merge
The maximum number of stores to attempt to merge into wider stores in the store
merging pass.

max-store-chains-to-track
The maximum number of store chains to track at the same time in the attempt to merge
them into wider stores in the store merging pass.

max-stores-to-track
The maximum number of stores to track at the same time in the attemt to to merge them
into wider stores in the store merging pass.

max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled. If a loop is
unrolled, this parameter also determines how many times the loop code is unrolled.

max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a
loop may have to be unrolled. If a loop is unrolled, this parameter also determines
how many times the loop code is unrolled.

max-unroll-times
The maximum number of unrollings of a single loop.

max-peeled-insns
The maximum number of instructions that a loop may have to be peeled. If a loop is
peeled, this parameter also determines how many times the loop code is peeled.

max-peel-times
The maximum number of peelings of a single loop.

max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.

max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.

max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.

max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.

max-unswitch-insns
The maximum number of insns of an unswitched loop.

max-unswitch-depth
The maximum depth of a loop nest to be unswitched.

lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.

min-loop-cond-split-prob
When FDO profile information is available, min-loop-cond-split-prob specifies minimum
threshold for probability of semi-invariant condition statement to trigger loop
split.

iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which all candidates are
considered for each use in induction variable optimizations. If there are more
candidates than this, only the most relevant ones are considered to avoid quadratic
time complexity.

iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction
variable uses.

iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this value, always try to
remove unnecessary ivs from the set when adding a new one.

avg-loop-niter
Average number of iterations of a loop.

dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store elimination.
Larger values may result in larger compilation times.

dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per store. Larger values result in
larger compilation times and may result in more removed dead stores.

scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large
expressions slow the analyzer.

scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions analyzer.
Complex expressions slow the analyzer.

max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if conversion unless the loop
is marked with simd pragma.

vect-max-layout-candidates
The maximum number of possible vector layouts (such as permutations) to consider when
optimizing to-be-vectorized code.

vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop
versioning for alignment in the vectorizer.

vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop
versioning for alias in the vectorizer.

vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment for vectorizer. Value -1
means no limit.

max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm for analysis of
the number of iterations of the loop tries to evaluate.

hot-bb-count-fraction
The denominator n of fraction 1/n of the maximal execution count of a basic block in
the entire program that a basic block needs to at least have in order to be
considered hot. The default is 10000, which means that a basic block is considered
hot if its execution count is greater than 1/10000 of the maximal execution count. 0
means that it is never considered hot. Used in non-LTO mode.

hot-bb-count-ws-permille
The number of most executed permilles, ranging from 0 to 1000, of the profiled
execution of the entire program to which the execution count of a basic block must be
part of in order to be considered hot. The default is 990, which means that a basic
block is considered hot if its execution count contributes to the upper 990
permilles, or 99.0%, of the profiled execution of the entire program. 0 means that
it is never considered hot. Used in LTO mode.

hot-bb-frequency-fraction
The denominator n of fraction 1/n of the execution frequency of the entry block of a
function that a basic block of this function needs to at least have in order to be
considered hot. The default is 1000, which means that a basic block is considered
hot in a function if it is executed more frequently than 1/1000 of the frequency of
the entry block of the function. 0 means that it is never considered hot.

unlikely-bb-count-fraction
The denominator n of fraction 1/n of the number of profiled runs of the entire
program below which the execution count of a basic block must be in order for the
basic block to be considered unlikely executed. The default is 20, which means that
a basic block is considered unlikely executed if it is executed in fewer than 1/20,
or 5%, of the runs of the program. 0 means that it is always considered unlikely
executed.

max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases
where a function contains a single loop with known bound and another loop with
unknown bound. The known number of iterations is predicted correctly, while the
unknown number of iterations average to roughly 10. This means that the loop without
bounds appears artificially cold relative to the other one.

builtin-expect-probability
Control the probability of the expression having the specified value. This parameter
takes a percentage (i.e. 0 ... 100) as input.

builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string cmp call eligible for
inlining.

align-threshold
Select fraction of the maximal frequency of executions of a basic block in a function
to align the basic block.

align-loop-iterations
A loop expected to iterate at least the selected number of iterations is aligned.

tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of
executed instructions is covered. This limits unnecessary code size expansion.

The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is
available. The real profiles (as opposed to statically estimated ones) are much less
balanced allowing the threshold to be larger value.

tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is a
rather artificial limit, as most of the duplicates are eliminated later in cross
jumping, so it may be set to much higher values than is the desired code growth.

tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this
threshold (in percent).

tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than this threshold.

Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-
probability-feedback is used for compilation with profile feedback and tracer-min-
branch-probability compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to make tracer effective.

stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as 2 raised to num
bytes. Higher values may reduce the number of explicit probes, but a value larger
than the operating system provided guard will leave code vulnerable to stack clash
style attacks.

stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated. This param
controls the maximum distance between probes into the stack as 2 raised to num bytes.
Higher values may reduce the number of explicit probes, but a value larger than the
operating system provided guard will leave code vulnerable to stack clash style
attacks.

max-cse-path-length
The maximum number of basic blocks on path that CSE considers.

max-cse-insns
The maximum number of instructions CSE processes before flushing.

ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter
specifies the minimum percentage by which the garbage collector's heap should be
allowed to expand between collections. Tuning this may improve compilation speed; it
has no effect on code generation.

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If
"getrlimit" is available, the notion of "RAM" is the smallest of actual RAM and
"RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to calculate RAM on a particular
platform, the lower bound of 30% is used. Setting this parameter and ggc-min-
heapsize to zero causes a full collection to occur at every opportunity. This is
extremely slow, but can be useful for debugging.

ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect
garbage. The first collection occurs after the heap expands by ggc-min-expand%
beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has
no effect on code generation.

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that
RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four
megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to
calculate RAM on a particular platform, the lower bound is used. Setting this
parameter very large effectively disables garbage collection. Setting this parameter
and ggc-min-expand to zero causes a full collection to occur at every opportunity.

max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent
register. Increasing values mean more aggressive optimization, making the
compilation time increase with probably slightly better performance.

max-cselib-memory-locations
The maximum number of memory locations cselib should take into account. Increasing
values mean more aggressive optimization, making the compilation time increase with
probably slightly better performance.

max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider
at any given time during the first scheduling pass. Increasing values mean more
thorough searches, making the compilation time increase with probably little benefit.

max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling.

max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the
selective scheduler.

max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling.

max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the
selective scheduler.

min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock
speculative scheduling.

max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of 0
disables region extensions.

max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion.

sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative
insns are scheduled.

sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state across
it.

sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory
locations.

selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth of
search for available instructions.

selsched-max-sched-times
The maximum number of times that an instruction is scheduled during selective
scheduling. This is the limit on the number of iterations through which the
instruction may be pipelined.

selsched-insns-to-rename
The maximum number of best instructions in the ready list that are considered for
renaming in the selective scheduler.

sms-min-sc
The minimum value of stage count that swing modulo scheduler generates.

max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in
combiner for a pseudo register as last known value of that register.

max-combine-insns
The maximum number of instructions the RTL combiner tries to combine.

integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler's
memory usage and increasing its speed. This sets the maximum value of a shared
integer constant.

ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing protection when
-fstack-protector is used.

This default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number
of functions protected by the stack protector.

min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not optimizing.

max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when
threading jumps.

max-jump-thread-paths
The maximum number of paths to consider when searching for jump threading
opportunities. When arriving at a block, incoming edges are only considered if the
number of paths to be searched so far multiplied by the number of incoming edges does
not exhaust the specified maximum number of paths to consider.

max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field sensitive manner during
pointer analysis.

prefetch-latency
Estimate on average number of instructions that are executed before prefetch
finishes. The distance prefetched ahead is proportional to this constant.
Increasing this number may also lead to less streams being prefetched (see
simultaneous-prefetches).

simultaneous-prefetches
Maximum number of prefetches that can run at the same time.

l1-cache-line-size
The size of cache line in L1 data cache, in bytes.

l1-cache-size
The size of L1 data cache, in kilobytes.

l2-cache-size
The size of L2 data cache, in kilobytes.

prefetch-dynamic-strides
Whether the loop array prefetch pass should issue software prefetch hints for strides
that are non-constant. In some cases this may be beneficial, though the fact the
stride is non-constant may make it hard to predict when there is clear benefit to
issuing these hints.

Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0
if prefetch hints should be issued only for strides that are known to be constant and
below prefetch-minimum-stride.

prefetch-minimum-stride
Minimum constant stride, in bytes, to start using prefetch hints for. If the stride
is less than this threshold, prefetch hints will not be issued.

This setting is useful for processors that have hardware prefetchers, in which case
there may be conflicts between the hardware prefetchers and the software prefetchers.
If the hardware prefetchers have a maximum stride they can handle, it should be used
here to improve the use of software prefetchers.

A value of -1 means we don't have a threshold and therefore prefetch hints can be
issued for any constant stride.

This setting is only useful for strides that are known and constant.

destructive-interference-size
constructive-interference-size
The values for the C++17 variables "std::hardware_destructive_interference_size" and
"std::hardware_constructive_interference_size". The destructive interference size is
the minimum recommended offset between two independent concurrently-accessed objects;
the constructive interference size is the maximum recommended size of contiguous
memory accessed together. Typically both will be the size of an L1 cache line for
the target, in bytes. For a generic target covering a range of L1 cache line sizes,
typically the constructive interference size will be the small end of the range and
the destructive size will be the large end.

The destructive interference size is intended to be used for layout, and thus has ABI
impact. The default value is not expected to be stable, and on some targets varies
with -mtune, so use of this variable in a context where ABI stability is important,
such as the public interface of a library, is strongly discouraged; if it is used in
that context, users can stabilize the value using this option.

The constructive interference size is less sensitive, as it is typically only used in
a static_assert to make sure that a type fits within a cache line.

See also -Winterference-size.

loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.

loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange to be profitable.

min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches to
enable prefetching in a loop.

prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory
references to enable prefetching in a loop.

use-canonical-types
Whether the compiler should use the "canonical" type system. Should always be 1,
which uses a more efficient internal mechanism for comparing types in C++ and
Objective-C++. However, if bugs in the canonical type system are causing compilation
failures, set this value to 0 to disable canonical types.

switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger than
switch-conversion-max-branch-ratio times the number of branches in the switch.

max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy
elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some
sorts of source code the enhanced partial redundancy elimination optimization can run
away, consuming all of the memory available on the host machine. This parameter sets
a limit on the length of the sets that are computed, which prevents the runaway
behavior. Setting a value of 0 for this parameter allows an unlimited set length.

rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically. When the limit hits the
innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are
value-numbered optimistically and the remaining ones not.

sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for redundancies for
loads and stores. If this limit is hit the search is aborted and the load or store
is not considered redundant. The number of queries is algorithmically limited to the
number of stores on all paths from the load to the function entry.

ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops
than the number given by this parameter, only at most the given number of the most
frequently-executed loops form regions for regional register allocation.

ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table, the table
can still require excessive amounts of memory for huge functions. If the conflict
table for a function could be more than the size in MB given by this parameter, the
register allocator instead uses a faster, simpler, and lower-quality algorithm that
does not require building a pseudo-register conflict table.

ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to
move loop invariants (see -O3). The number of available registers reserved for some
other purposes is given by this parameter. Default of the parameter is the best
found from numerous experiments.

ira-consider-dup-in-all-alts
Make IRA to consider matching constraint (duplicated operand number) heavily in all
available alternatives for preferred register class. If it is set as zero, it means
IRA only respects the matching constraint when it's in the only available alternative
with an appropriate register class. Otherwise, it means IRA will check all available
alternatives for preferred register class even if it has found some choice with an
appropriate register class and respect the found qualified matching constraint.

ira-simple-lra-insn-threshold
Approximate function insn number in 1K units triggering simple local RA.

lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns. This
optimization is called inheritance. EBB is used as a region to do this optimization.
The parameter defines a minimal fall-through edge probability in percentage used to
add BB to inheritance EBB in LRA. The default value was chosen from numerous runs of
SPEC2000 on x86-64.

loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and in amount
of needed compile-time memory, with very large loops. Loops with more basic blocks
than this parameter won't have loop invariant motion optimization performed on them.

loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops. This parameter limits
the number of data references in loops that are considered for data dependence
analysis. These large loops are no handled by the optimizations using loop data
dependencies.

max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow
analysis of any function. If this limit is exceeded with variable tracking at
assignments enabled, analysis for that function is retried without it, after removing
all debug insns from the function. If the limit is exceeded even without debug
insns, var tracking analysis is completely disabled for the function. Setting the
parameter to zero makes it unlimited.

max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or
debug temporaries to value expressions. This trades compilation time for more
complete debug information. If this is set too low, value expressions that are
available and could be represented in debug information may end up not being used;
setting this higher may enable the compiler to find more complex debug expressions,
but compile time and memory use may grow.

max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid
complexity explosion at inlining or expanding to RTL. If a function has more such
gimple stmts than the set limit, such stmts will be dropped from the inlined copy of
a function, and from its RTL expansion.

min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below the
parameter is reserved exclusively for debug insns created by
-fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it
if the reserved range is exhausted.

ipa-sra-deref-prob-threshold
IPA-SRA replaces a pointer which is known not be NULL with one or more new parameters
only when the probability (in percent, relative to function entry) of it being
dereferenced is higher than this parameter.

ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when
their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of
the original pointer parameter.

ipa-sra-ptrwrap-growth-factor
Additional maximum allowed growth of total size of new parameters that ipa-sra
replaces a pointer to an aggregate with, if it points to a local variable that the
caller only writes to and passes it as an argument to other functions.

ipa-sra-max-replacements
Maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it is also the
maximum number of replacements of a formal parameter.

sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar
parts of aggregates with uses of independent scalar variables. These parameters
control the maximum size, in storage units, of aggregate which is considered for
replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size
(sra-max-scalarization-size-Osize) respectively.

sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA)
will track, per one local variable, in order to facilitate copy propagation.

tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this parameter
specifies the size in bytes after which variables are saved with the logging
functions as opposed to save/restore code sequence pairs. This option only applies
when using -fgnu-tm.

graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of
parameters in a Static Control Part (SCoP) is bounded. A value of zero can be used
to lift the bound. A variable whose value is unknown at compilation time and defined
outside a SCoP is a parameter of the SCoP.

loop-block-tile-size
Loop blocking or strip mining transforms, enabled with -floop-block or
-floop-strip-mine, strip mine each loop in the loop nest by a given number of
iterations. The strip length can be changed using the loop-block-tile-size
parameter.

ipa-jump-function-lookups
Specifies number of statements visited during jump function offset discovery.

ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's
parameter in order to propagate them and perform devirtualization. ipa-cp-value-
list-size is the maximum number of values and types it stores per one formal
parameter of a function.

ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics and performs
those cloning opportunities with scores that exceed ipa-cp-eval-threshold.

ipa-cp-max-recursive-depth
Maximum depth of recursive cloning for self-recursive function.

ipa-cp-min-recursive-probability
Recursive cloning only when the probability of call being executed exceeds the
parameter.

ipa-cp-profile-count-base
When using -fprofile-use option, IPA-CP will consider the measured execution count of
a call graph edge at this percentage position in their histogram as the basis for its
heuristics calculation.

ipa-cp-recursive-freq-factor
The number of times interprocedural copy propagation expects recursive functions to
call themselves.

ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they are evaluated for
cloning.

ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another function will
receive when they are evaluated for cloning.

ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values passed in an aggregate.
ipa-max-agg-items controls the maximum number of such values per one parameter.

ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number of iterations
of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score
of the candidate.

ipa-max-loop-predicates
The maximum number of different predicates IPA will use to describe when loops in a
function have known properties.

ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis in order to
track values pointed to by function parameters. In order not spend too much time
analyzing huge functions, it gives up and consider all memory clobbered after
examining ipa-max-aa-steps statements modifying memory.

ipa-max-switch-predicate-bounds
Maximal number of boundary endpoints of case ranges of switch statement. For switch
exceeding this limit, IPA-CP will not construct cloning cost predicate, which is used
to estimate cloning benefit, for default case of the switch statement.

ipa-max-param-expr-ops
IPA-CP will analyze conditional statement that references some function parameter to
estimate benefit for cloning upon certain constant value. But if number of
operations in a parameter expression exceeds ipa-max-param-expr-ops, the expression
is treated as complicated one, and is not handled by IPA analysis.

lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The number
of partitions should exceed the number of CPUs used for compilation.

lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions). This prevents
expenses of splitting very small programs into too many partitions.

lto-max-partition
Size of max partition for WHOPR (in estimated instructions). to provide an upper
bound for individual size of partition. Meant to be used only with balanced
partitioning.

lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming.

cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup
fails for an identifier.

sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative
to a statement's original block to allow statement sinking of a statement. Larger
numbers result in more aggressive statement sinking. A small positive adjustment is
applied for statements with memory operands as those are even more profitable so
sink.

max-stores-to-sink
The maximum number of conditional store pairs that can be sunk. Set to 0 if either
vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is
disabled.

case-values-threshold
The smallest number of different values for which it is best to use a jump-table
instead of a tree of conditional branches. If the value is 0, use the default for
the machine.

jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding into a jump table (in percent).
The parameter is used when optimizing for size.

jump-table-max-growth-ratio-for-speed
The maximum code size growth ratio when expanding into a jump table (in percent).
The parameter is used when optimizing for speed.

tree-reassoc-width
Set the maximum number of instructions executed in parallel in reassociated tree.
This parameter overrides target dependent heuristics used by default if has non zero
value.

sched-pressure-algorithm
Choose between the two available implementations of -fsched-pressure. Algorithm 1 is
the original implementation and is the more likely to prevent instructions from being
reordered. Algorithm 2 was designed to be a compromise between the relatively
conservative approach taken by algorithm 1 and the rather aggressive approach taken
by the default scheduler. It relies more heavily on having a regular register file
and accurate register pressure classes. See haifa-sched.cc in the GCC sources for
more details.

The default choice depends on the target.

max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when seeking a
basis for a new straight-line strength reduction candidate.

asan-globals
Enable buffer overflow detection for global objects. This kind of protection is
enabled by default if you are using -fsanitize=address option. To disable global
objects protection use --param asan-globals=0.

asan-stack
Enable buffer overflow detection for stack objects. This kind of protection is
enabled by default when using -fsanitize=address. To disable stack protection use
--param asan-stack=0 option.

asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind of protection is
enabled by default when using -fsanitize=address. To disable memory reads protection
use --param asan-instrument-reads=0.

asan-instrument-writes
Enable buffer overflow detection for memory writes. This kind of protection is
enabled by default when using -fsanitize=address. To disable memory writes
protection use --param asan-instrument-writes=0 option.

asan-memintrin
Enable detection for built-in functions. This kind of protection is enabled by
default when using -fsanitize=address. To disable built-in functions protection use
--param asan-memintrin=0.

asan-use-after-return
Enable detection of use-after-return. This kind of protection is enabled by default
when using the -fsanitize=address option. To disable it use --param
asan-use-after-return=0.

Note: By default the check is disabled at run time. To enable it, add
"detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS.

asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is greater or equal to
this number, use callbacks instead of inline checks. E.g. to disable inline code use
--param asan-instrumentation-with-call-threshold=0.

asan-kernel-mem-intrinsic-prefix
If nonzero, prefix calls to "memcpy", "memset" and "memmove" with __asan_ or
__hwasan_ for -fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively.

hwasan-instrument-stack
Enable hwasan instrumentation of statically sized stack-allocated variables. This
kind of instrumentation is enabled by default when using -fsanitize=hwaddress and
disabled by default when using -fsanitize=kernel-hwaddress. To disable stack
instrumentation use --param hwasan-instrument-stack=0, and to enable it use --param
hwasan-instrument-stack=1.

hwasan-random-frame-tag
When using stack instrumentation, decide tags for stack variables using a
deterministic sequence beginning at a random tag for each frame. With this parameter
unset tags are chosen using the same sequence but beginning from 1. This is enabled
by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress.
To disable it use --param hwasan-random-frame-tag=0.

hwasan-instrument-allocas
Enable hwasan instrumentation of dynamically sized stack-allocated variables. This
kind of instrumentation is enabled by default when using -fsanitize=hwaddress and
disabled by default when using -fsanitize=kernel-hwaddress. To disable
instrumentation of such variables use --param hwasan-instrument-allocas=0, and to
enable it use --param hwasan-instrument-allocas=1.

hwasan-instrument-reads
Enable hwasan checks on memory reads. Instrumentation of reads is enabled by default
for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking
memory reads use --param hwasan-instrument-reads=0.

hwasan-instrument-writes
Enable hwasan checks on memory writes. Instrumentation of writes is enabled by
default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable
checking memory writes use --param hwasan-instrument-writes=0.

hwasan-instrument-mem-intrinsics
Enable hwasan instrumentation of builtin functions. Instrumentation of these builtin
functions is enabled by default for both -fsanitize=hwaddress and
-fsanitize=kernel-hwaddress. To disable instrumentation of builtin functions use
--param hwasan-instrument-mem-intrinsics=0.

use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to this number, directly
poison (or unpoison) shadow memory instead of using run-time callbacks.

tsan-distinguish-volatile
Emit special instrumentation for accesses to volatiles.

tsan-instrument-func-entry-exit
Emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit().

max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on a finite state
automaton jump thread path.

threader-debug
threader-debug=[none|all] Enables verbose dumping of the threader solver.

parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.

parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops (static, dynamic,
guided, auto, runtime).

parloops-min-per-thread
The minimum number of iterations per thread of an innermost parallelized loop for
which the parallelized variant is preferred over the single threaded one. Note that
for a parallelized loop nest the minimum number of iterations of the outermost loop
per thread is two.

max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names in things like fold
routines. One level of recursion corresponds to following a use-def chain.

max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a must-def specifying the
dynamic type of an object that invokes a virtual call we may be able to devirtualize
speculatively.

evrp-sparse-threshold
Maximum number of basic blocks before EVRP uses a sparse cache.

ranger-debug
Specifies the type of debug output to be issued for ranges.

evrp-switch-limit
Specifies the maximum number of switch cases before EVRP ignores a switch.

unroll-jam-min-percent
The minimum percentage of memory references that must be optimized away for the
unroll-and-jam transformation to be considered profitable.

unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled by the unroll-and-jam
transformation.

max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be generated by the RTL if-
conversion pass for a branch that is considered unpredictable.

max-variable-expansions-in-unroller
If -fvariable-expansion-in-unroller is used, the maximum number of times that an
individual variable will be expanded during loop unrolling.

partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in percent relative to entry BB
of the function) to make partial inlining happen.

max-tracked-strlens
Maximum number of strings for which strlen optimization pass will track string
lengths.

gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy elimination after reload.

gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count that permit performing
redundancy elimination after reload.

max-loop-header-insns
The maximum number of insns in loop header duplicated by the copy loop headers pass.

vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector size.

vect-partial-vector-usage
Controls when the loop vectorizer considers using partial vector loads and stores as
an alternative to falling back to scalar code. 0 stops the vectorizer from ever
using partial vector loads and stores. 1 allows partial vector loads and stores if
vectorization removes the need for the code to iterate. 2 allows partial vector
loads and stores in all loops. The parameter only has an effect on targets that
support partial vector loads and stores.

vect-inner-loop-cost-factor
The maximum factor which the loop vectorizer applies to the cost of statements in an
inner loop relative to the loop being vectorized. The factor applied is the maximum
of the estimated number of iterations of the inner loop and this parameter. The
default value of this parameter is 50.

vect-induction-float
Enable loop vectorization of floating point inductions.

avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.

sms-loop-average-count-threshold
A threshold on the average loop count considered by the swing modulo scheduler.

sms-dfa-history
The number of cycles the swing modulo scheduler considers when checking conflicts
using DFA.

graphite-allow-codegen-errors
Whether codegen errors should be ICEs when -fchecking.

sms-max-ii-factor
A factor for tuning the upper bound that swing modulo scheduler uses for scheduling a
loop.

lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered during spilling a non-reload
pseudo.

max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing exponentiation by a real
constant.

max-dse-active-local-stores
Maximum number of active local stores in RTL dead store elimination.

asan-instrument-allocas
Enable asan allocas/VLAs protection.

max-iterations-computation-cost
Bound on the cost of an expression to compute the number of iterations.

max-isl-operations
Maximum number of isl operations, 0 means unlimited.

graphite-max-arrays-per-scop
Maximum number of arrays per scop.

max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be added.

fsm-scale-path-stmts
Scale factor to apply to the number of statements in a threading path crossing a loop
backedge when comparing to --param=max-jump-thread-duplication-stmts.

uninit-control-dep-attempts
Maximum number of nested calls to search for control dependencies during
uninitialized variable analysis.

uninit-max-chain-len
Maximum number of predicates anded for each predicate ored in the normalized
predicate chain.

uninit-max-num-chains
Maximum number of predicates ored in the normalized predicate chain.

sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the
model looks into; at ' ' only enable instruction sorting heuristic.

loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can have before the loop
versioning pass considers it too big to copy.

loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can have before the loop
versioning pass considers it too big to copy, discounting any instructions in inner
loops that directly benefit from versioning.

ssa-name-def-chain-limit
The maximum number of SSA_NAME assignments to follow in determining a property of a
variable such as its value. This limits the number of iterations or recursive calls
GCC performs when optimizing certain statements or when determining their validity
prior to issuing diagnostics.

store-merging-max-size
Maximum size of a single store merging region in bytes.

hash-table-verification-limit
The number of elements for which hash table verification is done for each searched
element.

max-find-base-term-values
Maximum number of VALUEs handled during a single find_base_term call.

analyzer-max-enodes-per-program-point
The maximum number of exploded nodes per program point within the analyzer, before
terminating analysis of that point.

analyzer-max-constraints
The maximum number of constraints per state.

analyzer-min-snodes-for-call-summary
The minimum number of supernodes within a function for the analyzer to consider
summarizing its effects at call sites.

analyzer-max-enodes-for-full-dump
The maximum depth of exploded nodes that should appear in a dot dump before switching
to a less verbose format.

analyzer-max-recursion-depth
The maximum number of times a callsite can appear in a call stack within the
analyzer, before terminating analysis of a call that would recurse deeper.

analyzer-max-svalue-depth
The maximum depth of a symbolic value, before approximating the value as unknown.

analyzer-max-infeasible-edges
The maximum number of infeasible edges to reject before declaring a diagnostic as
infeasible.

gimple-fe-computed-hot-bb-threshold
The number of executions of a basic block which is considered hot. The parameter is
used only in GIMPLE FE.

analyzer-bb-explosion-factor
The maximum number of 'after supernode' exploded nodes within the analyzer per
supernode, before terminating analysis.

ranger-logical-depth
Maximum depth of logical expression evaluation ranger will look through when
evaluating outgoing edge ranges.

ranger-recompute-depth
Maximum depth of instruction chains to consider for recomputation in the outgoing
range calculator.

relation-block-limit
Maximum number of relations the oracle will register in a basic block.

min-pagesize
Minimum page size for warning purposes.

openacc-kernels
Specify mode of OpenACC `kernels' constructs handling. With
--param=openacc-kernels=decompose, OpenACC `kernels' constructs are decomposed into
parts, a sequence of compute constructs, each then handled individually. This is
work in progress. With --param=openacc-kernels=parloops, OpenACC `kernels'
constructs are handled by the parloops pass, en bloc. This is the current default.

openacc-privatization
Control whether the -fopt-info-omp-note and applicable -fdump-tree-*-details options
emit OpenACC privatization diagnostics. With --param=openacc-privatization=quiet,
don't diagnose. This is the current default. With
--param=openacc-privatization=noisy, do diagnose.

The following choices of name are available on AArch64 targets:

aarch64-sve-compare-costs
When vectorizing for SVE, consider using "unpacked" vectors for smaller elements and
use the cost model to pick the cheapest approach. Also use the cost model to choose
between SVE and Advanced SIMD vectorization.

Using unpacked vectors includes storing smaller elements in larger containers and
accessing elements with extending loads and truncating stores.

aarch64-float-recp-precision
The number of Newton iterations for calculating the reciprocal for float type. The
precision of division is proportional to this param when division approximation is
enabled. The default value is 1.

aarch64-double-recp-precision
The number of Newton iterations for calculating the reciprocal for double type. The
precision of division is propotional to this param when division approximation is
enabled. The default value is 2.

aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization. Accepts values from 0 to 4,
inclusive.

0 Use the default heuristics.

1 Use only Advanced SIMD for auto-vectorization.

2 Use only SVE for auto-vectorization.

3 Use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed
equal.

4 Use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.

The default value is 0.

aarch64-loop-vect-issue-rate-niters
The tuning for some AArch64 CPUs tries to take both latencies and issue rates into
account when deciding whether a loop should be vectorized using SVE, vectorized using
Advanced SIMD, or not vectorized at all. If this parameter is set to n, GCC will not
use this heuristic for loops that are known to execute in fewer than n Advanced SIMD
iterations.

aarch64-vect-unroll-limit
The vectorizer will use available tuning information to determine whether it would be
beneficial to unroll the main vectorized loop and by how much. This parameter set's
the upper bound of how much the vectorizer will unroll the main loop. The default
value is four.

The following choices of name are available on i386 and x86_64 targets:

x86-stlf-window-ninsns
Instructions number above which STFL stall penalty can be compensated.

x86-stv-max-visits
The maximum number of use and def visits when discovering a STV chain before the
discovery is aborted.

Program Instrumentation Options
GCC supports a number of command-line options that control adding run-time instrumentation to
the code it normally generates. For example, one purpose of instrumentation is collect
profiling statistics for use in finding program hot spots, code coverage analysis, or
profile-guided optimizations. Another class of program instrumentation is adding run-time
checking to detect programming errors like invalid pointer dereferences or out-of-bounds
array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable
hijacking. There is also a general hook which can be used to implement other forms of
tracing or function-level instrumentation for debug or program analysis purposes.

-p
-pg Generate extra code to write profile information suitable for the analysis program prof
(for -p) or gprof (for -pg). You must use this option when compiling the source files
you want data about, and you must also use it when linking.

You can use the function attribute "no_instrument_function" to suppress profiling of
individual functions when compiling with these options.

-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program
records how many times each branch and call is executed and how many times it is taken or
returns. On targets that support constructors with priority support, profiling properly
handles constructors, destructors and C++ constructors (and destructors) of classes which
are used as a type of a global variable.

When the compiled program exits it saves this data to a file called auxname.gcda for each
source file. The data may be used for profile-directed optimizations
(-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object
file's auxname is generated from the name of the output file, if explicitly specified and
it is not the final executable, otherwise it is the basename of the source file. In both
cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for
output file specified as -o dir/foo.o).

Note that if a command line directly links source files, the corresponding .gcda files
will be prefixed with the unsuffixed name of the output file. E.g. "gcc a.c b.c -o
binary" would generate binary-a.gcda and binary-b.gcda files.

--coverage
This option is used to compile and link code instrumented for coverage analysis. The
option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when
linking). See the documentation for those options for more details.

* Compile the source files with -fprofile-arcs plus optimization and code generation
options. For test coverage analysis, use the additional -ftest-coverage option. You
do not need to profile every source file in a program.

* Compile the source files additionally with -fprofile-abs-path to create absolute path
names in the .gcno files. This allows gcov to find the correct sources in projects
where compilations occur with different working directories.

* Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).

* Run the program on a representative workload to generate the arc profile information.
This may be repeated any number of times. You can run concurrent instances of your
program, and provided that the file system supports locking, the data files will be
correctly updated. Unless a strict ISO C dialect option is in effect, "fork" calls
are detected and correctly handled without double counting.

Moreover, an object file can be recompiled multiple times and the corresponding .gcda
file merges as long as the source file and the compiler options are unchanged.

* For profile-directed optimizations, compile the source files again with the same
optimization and code generation options plus -fbranch-probabilities.

* For test coverage analysis, use gcov to produce human readable information from the
.gcno and .gcda files. Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph,
then finds a spanning tree for the graph. Only arcs that are not on the spanning tree
have to be instrumented: the compiler adds code to count the number of times that these
arcs are executed. When an arc is the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a new basic block must be
created to hold the instrumentation code.

-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program
coverage. Each source file's note file is called auxname.gcno. Refer to the
-fprofile-arcs option above for a description of auxname and instructions on how to
generate test coverage data. Coverage data matches the source files more closely if you
do not optimize.

-fprofile-abs-path
Automatically convert relative source file names to absolute path names in the .gcno
files. This allows gcov to find the correct sources in projects where compilations occur
with different working directories.

-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects
only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs
and used by -fprofile-use and -fbranch-probabilities and its related options. Both
absolute and relative paths can be used. By default, GCC uses the current directory as
path, thus the profile data file appears in the same directory as the object file. In
order to prevent the file name clashing, if the object file name is not an absolute path,
we mangle the absolute path of the sourcename.gcda file and use it as the file name of a
.gcda file. See details about the file naming in -fprofile-arcs. See similar option
-fprofile-note.

When an executable is run in a massive parallel environment, it is recommended to save
profile to different folders. That can be done with variables in path that are exported
during run-time:

%p process ID.

%q{VAR}
value of environment variable VAR

-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile useful for
later recompilation with profile feedback based optimization. You must use
-fprofile-generate both when compiling and when linking your program.

The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions,
and -fipa-bit-cp.

If path is specified, GCC looks at the path to find the profile feedback data files. See
-fprofile-dir.

To optimize the program based on the collected profile information, use -fprofile-use.

-fprofile-info-section
-fprofile-info-section=name
Register the profile information in the specified section instead of using a
constructor/destructor. The section name is name if it is specified, otherwise the
section name defaults to ".gcov_info". A pointer to the profile information generated by
-fprofile-arcs is placed in the specified section for each translation unit. This option
disables the profile information registration through a constructor and it disables the
profile information processing through a destructor. This option is not intended to be
used in hosted environments such as GNU/Linux. It targets freestanding environments (for
example embedded systems) with limited resources which do not support
constructors/destructors or the C library file I/O.

The linker could collect the input sections in a continuous memory block and define start
and end symbols. A GNU linker script example which defines a linker output section
follows:

.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}

The program could dump the profiling information registered in this linker set for
example like this:

#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>

extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];

static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;

for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}

static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}

static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}

static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;

/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));

while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}

int
main (void)
{
dump_gcov_info ();
return 0;
}

The merge-stream subcommand of gcov-tool may be used to deserialize the data stream
generated by the "__gcov_filename_to_gcfn" and "__gcov_info_to_gcda" functions and merge
the profile information into .gcda files on the host filesystem.

-fprofile-note=path
If path is specified, GCC saves .gcno file into path location. If you combine the option
with multiple source files, the .gcno file will be overwritten.

-fprofile-prefix-path=path
This option can be used in combination with profile-generate=profile_dir and
profile-use=profile_dir to inform GCC where is the base directory of built source tree.
By default profile_dir will contain files with mangled absolute paths of all object files
in the built project. This is not desirable when directory used to build the
instrumented binary differs from the directory used to build the binary optimized with
profile feedback because the profile data will not be found during the optimized build.
In such setups -fprofile-prefix-path=path with path pointing to the base directory of the
build can be used to strip the irrelevant part of the path and keep all file names
relative to the main build directory.

-fprofile-prefix-map=old=new
When compiling files residing in directory old, record profiling information (with
--coverage) describing them as if the files resided in directory new instead. See also
-ffile-prefix-map and -fcanon-prefix-map.

-fprofile-update=method
Alter the update method for an application instrumented for profile feedback based
optimization. The method argument should be one of single, atomic or prefer-atomic. The
first one is useful for single-threaded applications, while the second one prevents
profile corruption by emitting thread-safe code.

Warning: When an application does not properly join all threads (or creates an detached
thread), a profile file can be still corrupted.

Using prefer-atomic would be transformed either to atomic, when supported by a target, or
to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is
present in the command line.

-fprofile-filter-files=regex
Instrument only functions from files whose name matches any of the regular expressions
(separated by semi-colons).

For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and
all C files starting with 'module'.

-fprofile-exclude-files=regex
Instrument only functions from files whose name does not match any of the regular
expressions (separated by semi-colons).

For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files
that are located in the /usr/ folder.

-fprofile-reproducible=[multithreaded|parallel-runs|serial]
Control level of reproducibility of profile gathered by "-fprofile-generate". This makes
it possible to rebuild program with same outcome which is useful, for example, for
distribution packages.

With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is
reproducible provided the trained program behaves the same at each invocation of the
train run, it is not multi-threaded and profile data streaming is always done in the same
order. Note that profile streaming happens at the end of program run but also before
"fork" function is invoked.

Note that it is quite common that execution counts of some part of programs depends, for
example, on length of temporary file names or memory space randomization (that may affect
hash-table collision rate). Such non-reproducible part of programs may be annotated by
"no_instrument_function" function attribute. gcov-dump with -l can be used to dump
gathered data and verify that they are indeed reproducible.

With -fprofile-reproducible=parallel-runs collected profile stays reproducible regardless
the order of streaming of the data into gcda files. This setting makes it possible to
run multiple instances of instrumented program in parallel (such as with "make -j"). This
reduces quality of gathered data, in particular of indirect call profiling.

-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access instructions are
instrumented to detect out-of-bounds and use-after-free bugs. The option enables
-fsanitize-address-use-after-scope. See
<https://github.com/google/sanitizers/wiki/AddressSanitizer> for more details. The run-
time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to
"help=1", the available options are shown at startup of the instrumented program. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a
list of supported options. The option cannot be combined with -fsanitize=thread or
-fsanitize=hwaddress. Note that the only target -fsanitize=hwaddress is currently
supported on is AArch64.

To get more accurate stack traces, it is possible to use options such as -O0, -O1, or -Og
(which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which
prevents optimizing sibling and tail recursive calls; this option is implicit for -O0,
-O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions).
Since multiple runs of the program may yield backtraces with different addresses due to
ASLR (Address Space Layout Randomization), it may be desirable to turn ASLR off. On
Linux, this can be achieved with setarch `uname -m` -R ./prog.

-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kernel-sanitizers> for more details.

-fsanitize=hwaddress
Enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the
top byte of a pointer to allow the detection of memory errors with a low memory overhead.
Memory access instructions are instrumented to detect out-of-bounds and use-after-free
bugs. The option enables -fsanitize-address-use-after-scope. See
<https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html> for more
details. The run-time behavior can be influenced using the HWASAN_OPTIONS environment
variable. When set to "help=1", the available options are shown at startup of the
instrumented program. The option cannot be combined with -fsanitize=thread or
-fsanitize=address, and is currently only available on AArch64.

-fsanitize=kernel-hwaddress
Enable Hardware-assisted AddressSanitizer for compilation of the Linux kernel. Similar
to -fsanitize=kernel-address but using an alternate instrumentation method, and similar
to -fsanitize=hwaddress but with instrumentation differences necessary for compiling the
Linux kernel. These differences are to avoid hwasan library initialization calls and to
account for the stack pointer having a different value in its top byte.

Note: This option has different defaults to the -fsanitize=hwaddress. Instrumenting the
stack and alloca calls are not on by default but are still possible by specifying the
command-line options --param hwasan-instrument-stack=1 and --param
hwasan-instrument-allocas=1 respectively. Using a random frame tag is not implemented for
kernel instrumentation.

-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer operands. The option must be
combined with either -fsanitize=kernel-address or -fsanitize=address The option cannot be
combined with -fsanitize=thread. Note: By default the check is disabled at run time. To
enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS.
Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers
are non-null.

-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be combined with either
-fsanitize=kernel-address or -fsanitize=address The option cannot be combined with
-fsanitize=thread. Note: By default the check is disabled at run time. To enable it,
add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using
"detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are
non-null.

-fsanitize=shadow-call-stack
Enable ShadowCallStack, a security enhancement mechanism used to protect programs against
return address overwrites (e.g. stack buffer overflows.) It works by saving a function's
return address to a separately allocated shadow call stack in the function prologue and
restoring the return address from the shadow call stack in the function epilogue.
Instrumentation only occurs in functions that need to save the return address to the
stack.

Currently it only supports the aarch64 platform. It is specifically designed for linux
kernels that enable the CONFIG_SHADOW_CALL_STACK option. For the user space programs,
runtime support is not currently provided in libc and libgcc. Users who want to use this
feature in user space need to provide their own support for the runtime. It should be
noted that this may cause the ABI rules to be broken.

On aarch64, the instrumentation makes use of the platform register "x18". This generally
means that any code that may run on the same thread as code compiled with ShadowCallStack
must be compiled with the flag -ffixed-x18, otherwise functions compiled without
-ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer.

Also, because there is no userspace runtime support, code compiled with ShadowCallStack
cannot use exception handling. Use -fno-exceptions to turn off exceptions.

See <https://clang.llvm.org/docs/ShadowCallStack.html> for more details.

-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions are
instrumented to detect data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. The run-
time behavior can be influenced using the TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported
options. The option cannot be combined with -fsanitize=address, -fsanitize=leak.

Note that sanitized atomic builtins cannot throw exceptions when operating on invalid
memory addresses with non-call exceptions (-fnon-call-exceptions).

-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only matters for linking of
executables. The executable is linked against a library that overrides "malloc" and
other allocator functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more
details. The run-time behavior can be influenced using the LSAN_OPTIONS environment
variable. The option cannot be combined with -fsanitize=thread.

-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various
computations are instrumented to detect undefined behavior at runtime. See
<https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. The
run-time behavior can be influenced using the UBSAN_OPTIONS environment variable.
Current suboptions are:

-fsanitize=shift
This option enables checking that the result of a shift operation is not undefined.
Note that what exactly is considered undefined differs slightly between C and C++, as
well as between ISO C90 and C99, etc. This option has two suboptions,
-fsanitize=shift-base and -fsanitize=shift-exponent.

-fsanitize=shift-exponent
This option enables checking that the second argument of a shift operation is not
negative and is smaller than the precision of the promoted first argument.

-fsanitize=shift-base
If the second argument of a shift operation is within range, check that the result of
a shift operation is not undefined. Note that what exactly is considered undefined
differs slightly between C and C++, as well as between ISO C90 and C99, etc.

-fsanitize=integer-divide-by-zero
Detect integer division by zero.

-fsanitize=unreachable
With this option, the compiler turns the "__builtin_unreachable" call into a
diagnostics message call instead. When reaching the "__builtin_unreachable" call,
the behavior is undefined.

-fsanitize=vla-bound
This option instructs the compiler to check that the size of a variable length array
is positive.

-fsanitize=null
This option enables pointer checking. Particularly, the application built with this
option turned on will issue an error message when it tries to dereference a NULL
pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer,
or if a method is invoked on an object pointed by a NULL pointer.

-fsanitize=return
This option enables return statement checking. Programs built with this option
turned on will issue an error message when the end of a non-void function is reached
without actually returning a value. This option works in C++ only.

-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check that the result of
"+", "*", and both unary and binary "-" does not overflow in the signed arithmetics.
This also detects "INT_MIN / -1" signed division. Note, integer promotion rules must
be taken into account. That is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

-fsanitize=bounds
This option enables instrumentation of array bounds. Various out of bounds accesses
are detected. Flexible array members, flexible array member-like arrays, and
initializers of variables with static storage are not instrumented, with the
exception of flexible array member-like arrays for which "-fstrict-flex-arrays" or
"-fstrict-flex-arrays=" options or "strict_flex_array" attributes say they shouldn't
be treated like flexible array member-like arrays.

-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most out of bounds
accesses are detected, including flexible array member-like arrays. Initializers of
variables with static storage are not instrumented.

-fsanitize=alignment
This option enables checking of alignment of pointers when they are dereferenced, or
when a reference is bound to insufficiently aligned target, or when a method or
constructor is invoked on insufficiently aligned object.

-fsanitize=object-size
This option enables instrumentation of memory references using the
"__builtin_dynamic_object_size" function. Various out of bounds pointer accesses are
detected.

-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar options,
-fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since
floating-point division by zero can be a legitimate way of obtaining infinities and
NaNs.

-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion checking. We check
that the result of the conversion does not overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option
does not work well with "FE_INVALID" exceptions enabled.

-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether null values are not
passed to arguments marked as requiring a non-null value by the "nonnull" function
attribute.

-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in functions marked with
"returns_nonnull" function attribute, to detect returning of null values from such
functions.

-fsanitize=bool
This option enables instrumentation of loads from bool. If a value other than 0/1 is
loaded, a run-time error is issued.

-fsanitize=enum
This option enables instrumentation of loads from an enum type. If a value outside
the range of values for the enum type is loaded, a run-time error is issued.

-fsanitize=vptr
This option enables instrumentation of C++ member function calls, member accesses and
some conversions between pointers to base and derived classes, to verify the
referenced object has the correct dynamic type.

-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If the pointer
arithmetics overflows, a run-time error is issued.

-fsanitize=builtin
This option enables instrumentation of arguments to selected builtin functions. If
an invalid value is passed to such arguments, a run-time error is issued. E.g.
passing 0 as the argument to "__builtin_ctz" or "__builtin_clz" invokes undefined
behavior and is diagnosed by this option.

Note that sanitizers tend to increase the rate of false positive warnings, most notably
those around -Wmaybe-uninitialized. We recommend against combining -Werror and [the use
of] sanitizers.

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives
a diagnostic message. This currently works only for the C family of languages.

-fno-sanitize=all
This option disables all previously enabled sanitizers. -fsanitize=all is not allowed,
as some sanitizers cannot be used together.

-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is
useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.

-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si may contain wildcards.

-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers mentioned in comma-
separated list of opts. Enabling this option for a sanitizer component causes it to
attempt to continue running the program as if no error happened. This means multiple
runtime errors can be reported in a single program run, and the exit code of the program
may indicate success even when errors have been reported. The -fno-sanitize-recover=
option can be used to alter this behavior: only the first detected error is reported and
program then exits with a non-zero exit code.

Currently this feature only works for -fsanitize=undefined (and its suboptions except for
-fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and
-fsanitize=address. For these sanitizers error recovery is turned on by default, except
-fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and
-fno-sanitize-recover=all is also accepted, the former enables recovery for all
sanitizers that support it, the latter disables recovery for all sanitizers that support
it.

Even if a recovery mode is turned on the compiler side, it needs to be also enabled on
the runtime library side, otherwise the failures are still fatal. The runtime library
defaults to "halt_on_error=0" for ThreadSanitizer and UndefinedBehaviorSanitizer, while
default value for AddressSanitizer is "halt_on_error=1". This can be overridden through
setting the "halt_on_error" flag in the corresponding environment variable.

Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying
an opts list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope bugs. The option sets
-fstack-reuse to none.

-fsanitize-trap[=opts]
The -fsanitize-trap= option instructs the compiler to report for sanitizers mentioned in
comma-separated list of opts undefined behavior using "__builtin_trap" rather than a
"libubsan" library routine. If this option is enabled for certain sanitizer, it takes
precedence over the -fsanitizer-recover= for that sanitizer, "__builtin_trap" will be
emitted and be fatal regardless of whether recovery is enabled or disabled using
-fsanitize-recover=.

The advantage of this is that the "libubsan" library is not needed and is not linked in,
so this is usable even in freestanding environments.

Currently this feature works with -fsanitize=undefined (and its suboptions except for
-fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and
-fsanitize=bounds-strict. "-fsanitize-trap=all" can be also specified, which enables it
for "undefined" suboptions, -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If
"-fsanitize-trap=undefined" or "-fsanitize-trap=all" is used and "-fsanitize=vptr" is
enabled on the command line, the instrumentation is silently ignored as the
instrumentation always needs "libubsan" support, -fsanitize-trap=vptr is not allowed.

-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option is deprecated equivalent of
-fsanitize-trap=all.

-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a call to
"__sanitizer_cov_trace_pc" into every basic block.

-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts a call to
"__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4"
or "__sanitizer_cov_trace_cmp8" for integral comparison with both operands variable or
"__sanitizer_cov_trace_const_cmp1", "__sanitizer_cov_trace_const_cmp2",
"__sanitizer_cov_trace_const_cmp4" or "__sanitizer_cov_trace_const_cmp8" for integral
comparison with one operand constant, "__sanitizer_cov_trace_cmpf" or
"__sanitizer_cov_trace_cmpd" for float or double comparisons and
"__sanitizer_cov_trace_switch" for switch statements.

-fcf-protection=[full|branch|return|none|check]
Enable code instrumentation of control-flow transfers to increase program security by
checking that target addresses of control-flow transfer instructions (such as indirect
function call, function return, indirect jump) are valid. This prevents diverting the
flow of control to an unexpected target. This is intended to protect against such
threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming
(COP/JOP).

The value "branch" tells the compiler to implement checking of validity of control-flow
transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The
value "return" implements checking of validity at the point of returning from a function.
The value "full" is an alias for specifying both "branch" and "return". The value "none"
turns off instrumentation.

The value "check" is used for the final link with link-time optimization (LTO). An error
is issued if LTO object files are compiled with different -fcf-protection values. The
value "check" is ignored at the compile time.

The macro "__CET__" is defined when -fcf-protection is used. The first bit of "__CET__"
is set to 1 for the value "branch" and the second bit of "__CET__" is set to 1 for the
"return".

You can also use the "nocf_check" attribute to identify which functions and calls should
be skipped from instrumentation.

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow
Enforcement Technology (CET) which works for i686 processor or newer.

NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C,
C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found.

-fharden-compares
For every logical test that survives gimple optimizations and is not the condition in a
conditional branch (for example, conditions tested for conditional moves, or to store in
boolean variables), emit extra code to compute and verify the reversed condition, and to
call "__builtin_trap" if the results do not match. Use with
-fharden-conditional-branches to cover all conditionals.

-fharden-conditional-branches
For every non-vectorized conditional branch that survives gimple optimizations, emit
extra code to compute and verify the reversed condition, and to call "__builtin_trap" if
the result is unexpected. Use with -fharden-compares to cover all conditionals.

-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is
done by adding a guard variable to functions with vulnerable objects. This includes
functions that call "alloca", and functions with buffers larger than or equal to 8 bytes.
The guards are initialized when a function is entered and then checked when the function
exits. If a guard check fails, an error message is printed and the program exits. Only
variables that are actually allocated on the stack are considered, optimized away
variables or variables allocated in registers don't count.

-fstack-protector-all
Like -fstack-protector except that all functions are protected.

-fstack-protector-strong
Like -fstack-protector but includes additional functions to be protected --- those that
have local array definitions, or have references to local frame addresses. Only
variables that are actually allocated on the stack are considered, optimized away
variables or variables allocated in registers don't count.

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default
for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding
are found.

-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have the "stack_protect"
attribute.

-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should
specify this flag if you are running in an environment with multiple threads, but you
only rarely need to specify it in a single-threaded environment since stack overflow is
automatically detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the operating system
or the language runtime must do that. The switch causes generation of code to ensure
that they see the stack being extended.

You can additionally specify a string parameter: no means no checking, generic means
force the use of old-style checking, specific means use the best checking method and is
equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the
compiler but comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are always allocated dynamically
if their size exceeds a fixed threshold. Note this may change the semantics of some
code.

2. Fixed limit on the size of the static frame of functions: when it is topped by a
particular function, stack checking is not reliable and a warning is issued by the
compiler.

3. Inefficiency: because of both the modified allocation strategy and the generic
implementation, code performance is hampered.

Note that old-style stack checking is also the fallback method for specific if no target
support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack
overflows. specific is an excellent choice when compiling Ada code. It is not generally
sufficient to protect against stack-clash attacks. To protect against those you want
-fstack-clash-protection.

-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this option is enabled, the
compiler will only allocate one page of stack space at a time and each page is accessed
immediately after allocation. Thus, it prevents allocations from jumping over any stack
guard page provided by the operating system.

Most targets do not fully support stack clash protection. However, on those targets
-fstack-clash-protection will protect dynamic stack allocations.
-fstack-clash-protection may also provide limited protection for static stack allocations
if the target supports -fstack-check=specific.

NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default
for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found.

-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the
value of a register or the address of a symbol. If a larger stack is required, a signal
is raised at run time. For most targets, the signal is raised before the stack overruns
the boundary, so it is possible to catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you
can use the flags -fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this
may only work with the GNU linker.

You can locally override stack limit checking by using the "no_stack_limit" function
attribute.

-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting
program has a discontiguous stack which can only overflow if the program is unable to
allocate any more memory. This is most useful when running threaded programs, as it is
no longer necessary to calculate a good stack size to use for each thread. This is
currently only implemented for the x86 targets running GNU/Linux.

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there
may not be much stack space available for the latter code to run. If compiling all code,
including library code, with -fsplit-stack is not an option, then the linker can fix up
these calls so that the code compiled without -fsplit-stack always has a large stack.
Support for this is implemented in the gold linker in GNU binutils release 2.21 and
later.

-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns on (or off, if using
-fvtable-verify=none) the security feature that verifies at run time, for every virtual
call, that the vtable pointer through which the call is made is valid for the type of the
object, and has not been corrupted or overwritten. If an invalid vtable pointer is
detected at run time, an error is reported and execution of the program is immediately
halted.

This option causes run-time data structures to be built at program startup, which are
used for verifying the vtable pointers. The options std and preinit control the timing
of when these data structures are built. In both cases the data structures are built
before execution reaches "main". Using -fvtable-verify=std causes the data structures to
be built after shared libraries have been loaded and initialized.
-fvtable-verify=preinit causes them to be built before shared libraries have been loaded
and initialized.

If this option appears multiple times in the command line with different values
specified, none takes highest priority over both std and preinit; preinit takes priority
over std.

-fvtv-debug
When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes
debug versions of the runtime functions for the vtable verification feature to be called.
This flag also causes the compiler to log information about which vtable pointers it
finds for each class. This information is written to a file named vtv_set_ptr_data.log
in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the
current working directory otherwise.

Note: This feature appends data to the log file. If you want a fresh log file, be sure
to delete any existing one.

-fvtv-counts
This is a debugging flag. When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, this causes the compiler to keep track of the total number of
virtual calls it encounters and the number of verifications it inserts. It also counts
the number of calls to certain run-time library functions that it inserts and logs this
information for each compilation unit. The compiler writes this information to a file
named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR
if that is defined or the current working directory otherwise. It also counts the size
of the vtable pointer sets for each class, and writes this information to
vtv_class_set_sizes.log in the same directory.

Note: This feature appends data to the log files. To get fresh log files, be sure to
delete any existing ones.

-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function
entry and just before function exit, the following profiling functions are called with
the address of the current function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current function, so the call site
information may not be available to the profiling functions otherwise.)

void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);

The first argument is the address of the start of the current function, which may be
looked up exactly in the symbol table.

This instrumentation is also done for functions expanded inline in other functions. The
profiling calls indicate where, conceptually, the inline function is entered and exited.
This means that addressable versions of such functions must be available. If all your
uses of a function are expanded inline, this may mean an additional expansion of code
size. If you use "extern inline" in your C code, an addressable version of such
functions must be provided. (This is normally the case anyway, but if you get lucky and
the optimizer always expands the functions inline, you might have gotten away without
providing static copies.)

A function may be given the attribute "no_instrument_function", in which case this
instrumentation is not done. This can be used, for example, for the profiling functions
listed above, high-priority interrupt routines, and any functions from which the
profiling functions cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).

-finstrument-functions-once
This is similar to -finstrument-functions, but the profiling functions are called only
once per instrumented function, i.e. the first profiling function is called after the
first entry into the instrumented function and the second profiling function is called
before the exit corresponding to this first entry.

The definition of "once" for the purpose of this option is a little vague because the
implementation is not protected against data races. As a result, the implementation only
guarantees that the profiling functions are called at least once per process and at most
once per thread, but the calls are always paired, that is to say, if a thread calls the
first function, then it will call the second function, unless it never reaches the exit
of the instrumented function.

-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the description of
-finstrument-functions). If the file that contains a function definition matches with
one of file, then that function is not instrumented. The match is done on substrings: if
the file parameter is a substring of the file name, it is considered to be a match.

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or
include/sys.

If, for some reason, you want to include letter , in one of sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the
option).

-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but this option sets the
list of function names to be excluded from instrumentation. The function name to be
matched is its user-visible name, such as "vector<int> blah(const vector<int> &)", not
the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done on
substrings: if the sym parameter is a substring of the function name, it is considered to
be a match. For C99 and C++ extended identifiers, the function name must be given in
UTF-8, not using universal character names.

-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with the function entry point
before the Mth NOP. If M is omitted, it defaults to 0 so the function entry points to
the address just at the first NOP. The NOP instructions reserve extra space which can be
used to patch in any desired instrumentation at run time, provided that the code segment
is writable. The amount of space is controllable indirectly via the number of NOPs; the
NOP instruction used corresponds to the instruction emitted by the internal GCC back-end
interface "gen_nop". This behavior is target-specific and may also depend on the
architecture variant and/or other compilation options.

For run-time identification, the starting addresses of these areas, which correspond to
their respective function entries minus M, are additionally collected in the
"__patchable_function_entries" section of the resulting binary.

Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes
precedence over command-line option -fpatchable-function-entry=N,M. This can be used to
increase the area size or to remove it completely on a single function. If "N=0", no pad
location is recorded.

The NOP instructions are inserted at---and maybe before, depending on M---the function
entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function
with dual entry points, the local entry point is this function entry address.

The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function
with dual entry points, the supported values for M are 0, 2, 6 and 14.

Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual
compilation.

If you use the -E option, nothing is done except preprocessing. Some of these options make
sense only together with -E because they cause the preprocessor output to be unsuitable for
actual compilation.

In addition to the options listed here, there are a number of options to control search paths
for include files documented in Directory Options. Options to control preprocessor
diagnostics are listed in Warning Options.

-D name
Predefine name as a macro, with definition 1.

-D name=definition
The contents of definition are tokenized and processed as if they appeared during
translation phase three in a #define directive. In particular, the definition is
truncated by embedded newline characters.

If you are invoking the preprocessor from a shell or shell-like program you may need to
use the shell's quoting syntax to protect characters such as spaces that have a meaning
in the shell syntax.

If you wish to define a function-like macro on the command line, write its argument list
with surrounding parentheses before the equals sign (if any). Parentheses are meaningful
to most shells, so you should quote the option. With sh and csh,
-D'name(args...)=definition' works.

-D and -U options are processed in the order they are given on the command line. All
-imacros file and -include file options are processed after all -D and -U options.

-U name
Cancel any previous definition of name, either built in or provided with a -D option.

-include file
Process file as if "#include "file"" appeared as the first line of the primary source
file. However, the first directory searched for file is the preprocessor's working
directory instead of the directory containing the main source file. If not found there,
it is searched for in the remainder of the "#include "..."" search chain as normal.

If multiple -include options are given, the files are included in the order they appear
on the command line.

-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away.
Macros it defines remain defined. This allows you to acquire all the macros from a
header without also processing its declarations.

All files specified by -imacros are processed before all files specified by -include.

-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined
macros remain defined.

-pthread
Define additional macros required for using the POSIX threads library. You should use
this option consistently for both compilation and linking. This option is supported on
GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.

-M Instead of outputting the result of preprocessing, output a rule suitable for make
describing the dependencies of the main source file. The preprocessor outputs one make
rule containing the object file name for that source file, a colon, and the names of all
the included files, including those coming from -include or -imacros command-line
options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name
of the source file with any suffix replaced with object file suffix and with any leading
directory parts removed. If there are many included files then the rule is split into
several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid
mixing such debug output with the dependency rules you should explicitly specify the
dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT.
Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

-MM Like -M but do not mention header files that are found in system header directories, nor
header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive
does not in itself determine whether that header appears in -MM dependency output.

-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF
switch is given the preprocessor sends the rules to the same place it would send
preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency
output file.

If file is -, then the dependencies are written to stdout.

-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes
missing header files are generated files and adds them to the dependency list without
raising an error. The dependency filename is taken directly from the "#include"
directive without prepending any path. -MG also suppresses preprocessed output, as a
missing header file renders this useless.

This feature is used in automatic updating of makefiles.

-Mno-modules
Disable dependency generation for compiled module interfaces.

-MP This option instructs CPP to add a phony target for each dependency other than the main
file, causing each to depend on nothing. These dummy rules work around errors make gives
if you remove header files without updating the Makefile to match.

This is typical output:

test.o: test.c test.h

test.h:

-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the
name of the main input file, deletes any directory components and any file suffix such as
.c, and appends the platform's usual object suffix. The result is the target.

An -MT option sets the target to be exactly the string you specify. If you want multiple
targets, you can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

-MQ target
Same as -MT, but it quotes any characters which are special to Make.
-MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ.

-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines
file based on whether an -o option is given. If it is, the driver uses its argument but
with a suffix of .d, otherwise it takes the name of the input file, removes any directory
components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the
dependency output file, but if used without -E, each -o is understood to specify a target
object file.

Since -E is not implied, -MD can be used to generate a dependency output file as a side
effect of the compilation process.

-MMD
Like -MD except mention only user header files, not system header files.

-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This
suppresses things like macro expansion, trigraph conversion, escaped newline splicing,
and processing of most directives. The preprocessor still recognizes and removes
comments, so that you can pass a file preprocessed with -C to the compiler without
problems. In this mode the integrated preprocessor is little more than a tokenizer for
the front ends.

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi.
These are the extensions that GCC uses for preprocessed files created by -save-temps.

-fdirectives-only
When preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the -E and -fpreprocessed options.

With -E, preprocessing is limited to the handling of directives such as "#define",
"#ifdef", and "#error". Other preprocessor operations, such as macro expansion and
trigraph conversion are not performed. In addition, the -dD option is implicitly
enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled.
Macros such as "__LINE__", which are contextually dependent, are handled normally. This
enables compilation of files previously preprocessed with "-E -fdirectives-only".

With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This
enables full preprocessing of files previously preprocessed with "-E -fdirectives-only".

-fdollars-in-identifiers
Accept $ in identifiers.

-fextended-identifiers
Accept universal character names and extended characters in identifiers. This option is
enabled by default for C99 (and later C standard versions) and C++.

-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.

-fmax-include-depth=depth
Set the maximum depth of the nested #include. The default is 200.

-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column
numbers in warnings or errors, even if tabs appear on the line. If the value is less
than 1 or greater than 100, the option is ignored. The default is 8.

-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit
diagnostic about the current macro expansion stack when a compilation error occurs in a
macro expansion. Using this option makes the preprocessor and the compiler consume more
memory. The level parameter can be used to choose the level of precision of token
location tracking thus decreasing the memory consumption if necessary. Value 0 of level
de-activates this option. Value 1 tracks tokens locations in a degraded mode for the sake
of minimal memory overhead. In this mode all tokens resulting from the expansion of an
argument of a function-like macro have the same location. Value 2 tracks tokens locations
completely. This value is the most memory hungry. When this option is given no argument,
the default parameter value is 2.

Note that "-ftrack-macro-expansion=2" is activated by default.

-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the "__FILE__" and
"__BASE_FILE__" macros as if the files resided in directory new instead. This can be
used to change an absolute path to a relative path by using . for new which can result in
more reproducible builds that are location independent. This option also affects
"__builtin_FILE()" during compilation. See also -ffile-prefix-map and
-fcanon-prefix-map.

-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is
UTF-8. charset can be any encoding supported by the system's "iconv" library routine.

-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The
default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the
width of "wchar_t" and the big-endian or little-endian byte order being used for code
generation. As with -fexec-charset, charset can be any encoding supported by the
system's "iconv" library routine; however, you will have problems with encodings that do
not fit exactly in "wchar_t".

-finput-charset=charset
Set the input character set, used for translation from the character set of the input
file to the source character set used by GCC. If the locale does not specify, or GCC
cannot get this information from the locale, the default is UTF-8. This can be
overridden by either the locale or this command-line option. Currently the command-line
option takes precedence if there's a conflict. charset can be any encoding supported by
the system's "iconv" library routine.

-fpch-deps
When using precompiled headers, this flag causes the dependency-output flags to also list
the files from the precompiled header's dependencies. If not specified, only the
precompiled header are listed and not the files that were used to create it, because
those files are not consulted when a precompiled header is used.

-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special
"#pragma", "#pragma GCC pch_preprocess "filename"" in the output to mark the place where
the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC
recognizes this "#pragma" and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really
suitable as input to GCC. It is switched on by -save-temps.

You should not write this "#pragma" in your own code, but it is safe to edit the filename
if the PCH file is available in a different location. The filename may be absolute or it
may be relative to GCC's current directory.

-fworking-directory
Enable generation of linemarkers in the preprocessor output that let the compiler know
the current working directory at the time of preprocessing. When this option is enabled,
the preprocessor emits, after the initial linemarker, a second linemarker with the
current working directory followed by two slashes. GCC uses this directory, when it's
present in the preprocessed input, as the directory emitted as the current working
directory in some debugging information formats. This option is implicitly enabled if
debugging information is enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in the command line, this option has
no effect, since no "#line" directives are emitted whatsoever.

-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred
to the older form -A predicate(answer), which is still supported, because it does not use
shell special characters.

-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.

-C Do not discard comments. All comments are passed through to the output file, except for
comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to
treat comments as tokens in their own right. For example, comments appearing at the
start of what would be a directive line have the effect of turning that line into an
ordinary source line, since the first token on the line is no longer a #.

-CC Do not discard comments, including during macro expansion. This is like -C, except that
comments contained within macros are also passed through to the output file where the
macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style
comments inside a macro to be converted to C-style comments. This is to prevent later
use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

-P Inhibit generation of linemarkers in the output from the preprocessor. This might be
useful when running the preprocessor on something that is not C code, and will be sent to
a program which might be confused by the linemarkers.

-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C
preprocessors. See the GNU CPP manual for details.

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these
options are only supported with the -E switch, or when invoking CPP explicitly.

-trigraphs
Support ISO C trigraphs. These are three-character sequences, all starting with ??, that
are defined by ISO C to stand for single characters. For example, ??/ stands for \, so
'??/n' is a character constant for a newline.

The nine trigraphs and their replacements are

Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~

By default, GCC ignores trigraphs, but in standard-conforming modes it converts them.
See the -std and -ansi options.

-remap
Enable special code to work around file systems which only permit very short file names,
such as MS-DOS.

-H Print the name of each header file used, in addition to other normal activities. Each
name is indented to show how deep in the #include stack it is. Precompiled header files
are also printed, even if they are found to be invalid; an invalid precompiled header
file is printed with ...x and a valid one with ...! .

-dletters
Says to make debugging dumps during compilation as specified by letters. The flags
documented here are those relevant to the preprocessor. Other letters are interpreted by
the compiler proper, or reserved for future versions of GCC, and so are silently ignored.
If you specify letters whose behavior conflicts, the result is undefined.

-dM Instead of the normal output, generate a list of #define directives for all the
macros defined during the execution of the preprocessor, including predefined macros.
This gives you a way of finding out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for
-fdump-rtl-mach.

-dD Like -dM except in two respects: it does not include the predefined macros, and it
outputs both the #define directives and the result of preprocessing. Both kinds of
output go to the standard output file.

-dN Like -dD, but emit only the macro names, not their expansions.

-dI Output #include directives in addition to the result of preprocessing.

-dU Like -dD except that only macros that are expanded, or whose definedness is tested in
preprocessor directives, are output; the output is delayed until the use or test of
the macro; and #undef directives are also output for macros tested but undefined at
the time.

-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP or with -E, it dumps
debugging information about location maps. Every token in the output is preceded by the
dump of the map its location belongs to.

When used from GCC without -E, this option has no effect.

-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly through to
the preprocessor. If option contains commas, it is split into multiple options at the
commas. However, many options are modified, translated or interpreted by the compiler
driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase.
The preprocessor's direct interface is undocumented and subject to change, so whenever
possible you should avoid using -Wp and let the driver handle the options instead.

-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific
preprocessor options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice,
once for the option and once for the argument.

-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By default, GCC performs
preprocessing as an integrated part of input tokenization and parsing. If this option is
provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and
Objective-C, respectively) is instead invoked twice, once for preprocessing only and once
for actual compilation of the preprocessed input. This option may be useful in
conjunction with the -B or -wrapper options to specify an alternate preprocessor or
perform additional processing of the program source between normal preprocessing and
compilation.

-flarge-source-files
Adjust GCC to expect large source files, at the expense of slower compilation and higher
memory usage.

Specifically, GCC normally tracks both column numbers and line numbers within source
files and it normally prints both of these numbers in diagnostics. However, once it has
processed a certain number of source lines, it stops tracking column numbers and only
tracks line numbers. This means that diagnostics for later lines do not include column
numbers. It also means that options like -Wmisleading-indentation cease to work at that
point, although the compiler prints a note if this happens. Passing -flarge-source-files
significantly increases the number of source lines that GCC can process before it stops
tracking columns.

Passing Options to the Assembler
You can pass options to the assembler.

-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into
multiple options at the commas.

-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific
assembler options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice,
once for the option and once for the argument.

Options for Linking
These options come into play when the compiler links object files into an executable output
file. They are meaningless if the compiler is not doing a link step.

object-file-name
A file name that does not end in a special recognized suffix is considered to name an
object file or library. (Object files are distinguished from libraries by the linker
according to the file contents.) If linking is done, these object files are used as
input to the linker.

-c
-S
-E If any of these options is used, then the linker is not run, and object file names should
not be used as arguments.

-flinker-output=type
This option controls code generation of the link-time optimizer. By default the linker
output is automatically determined by the linker plugin. For debugging the compiler and
if incremental linking with a non-LTO object file is desired, it may be useful to control
the type manually.

If type is exec, code generation produces a static binary. In this case -fpic and -fpie
are both disabled.

If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC
is preserved, but not enabled automatically. This allows to build shared libraries
without position-independent code on architectures where this is possible, i.e. on x86.

If type is pie, code generation produces an -fpie executable. This results in similar
optimizations as exec except that -fpie is not disabled if specified at compilation time.

If type is rel, the compiler assumes that incremental linking is done. The sections
containing intermediate code for link-time optimization are merged, pre-optimized, and
output to the resulting object file. In addition, if -ffat-lto-objects is specified,
binary code is produced for future non-LTO linking. The object file produced by
incremental linking is smaller than a static library produced from the same object files.
At link time the result of incremental linking also loads faster than a static library
assuming that the majority of objects in the library are used.

Finally nolto-rel configures the compiler for incremental linking where code generation
is forced, a final binary is produced, and the intermediate code for later link-time
optimization is stripped. When multiple object files are linked together the resulting
code is better optimized than with link-time optimizations disabled (for example, cross-
module inlining happens), but most of benefits of whole program optimizations are lost.

During the incremental link (by -r) the linker plugin defaults to rel. With current
interfaces to GNU Binutils it is however not possible to incrementally link LTO objects
and non-LTO objects into a single mixed object file. If any of object files in
incremental link cannot be used for link-time optimization, the linker plugin issues a
warning and uses nolto-rel. To maintain whole program optimization, it is recommended to
link such objects into static library instead. Alternatively it is possible to use H.J.
Lu's binutils with support for mixed objects.

-fuse-ld=bfd
Use the bfd linker instead of the default linker.

-fuse-ld=gold
Use the gold linker instead of the default linker.

-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.

-fuse-ld=mold
Use the Modern Linker (mold) instead of the default linker.

-llibrary
-l library
Search the library named library when linking. (The second alternative with the library
as a separate argument is only for POSIX compliance and is not recommended.)

The -l option is passed directly to the linker by GCC. Refer to your linker
documentation for exact details. The general description below applies to the GNU
linker.

The linker searches a standard list of directories for the library. The directories
searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a.
Some targets also support shared libraries, which typically have names like
liblibrary.so. If both static and shared libraries are found, the linker gives
preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option; the linker searches and
processes libraries and object files in the order they are specified. Thus, foo.o -lz
bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions
in z, those functions may not be loaded.

-lobjc
You need this special case of the -l option in order to link an Objective-C or
Objective-C++ program.

-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries
are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.

-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify
are passed to the linker, and options specifying linkage of the system libraries, such as
-static-libgcc or -shared-libgcc, are ignored. The standard startup files are used
normally, unless -nostartfiles is used.

The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These
entries are usually resolved by entries in libc. These entry points should be supplied
through some other mechanism when this option is specified.

-nolibc
Do not use the C library or system libraries tightly coupled with it when linking. Still
link with the startup files, libgcc or toolchain provided language support libraries such
as libgnat, libgfortran or libstdc++ unless options preventing their inclusion are used
as well. This typically removes -lc from the link command line, as well as system
libraries that normally go with it and become meaningless when absence of a C library is
assumed, for example -lpthread or -lm in some configurations. This is intended for bare-
board targets when there is indeed no C library available.

-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files
and only the libraries you specify are passed to the linker, and options specifying
linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a
library of internal subroutines which GCC uses to overcome shortcomings of particular
machines, or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other standard libraries.
In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify
-lgcc as well. This ensures that you have no unresolved references to internal GCC
library subroutines. (An example of such an internal subroutine is "__main", used to
ensure C++ constructors are called.)

-nostdlib++
Do not implicitly link with standard C++ libraries.

-e entry
--entry=entry
Specify that the program entry point is entry. The argument is interpreted by the
linker; the GNU linker accepts either a symbol name or an address.

-pie
Produce a dynamically linked position independent executable on targets that support it.
For predictable results, you must also specify the same set of options used for
compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.

-no-pie
Don't produce a dynamically linked position independent executable.

-static-pie
Produce a static position independent executable on targets that support it. A static
position independent executable is similar to a static executable, but can be loaded at
any address without a dynamic linker. For predictable results, you must also specify the
same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you
specify this linker option.

-pthread
Link with the POSIX threads library. This option is supported on GNU/Linux targets, most
other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this
option also sets flags for the preprocessor, so it should be used consistently for both
compilation and linking.

-r Produce a relocatable object as output. This is also known as partial linking.

-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This
instructs the linker to add all symbols, not only used ones, to the dynamic symbol table.
This option is needed for some uses of "dlopen" or to allow obtaining backtraces from
within a program.

-s Remove all symbol table and relocation information from the executable.

-static
On systems that support dynamic linking, this overrides -pie and prevents linking with
the shared libraries. On other systems, this option has no effect.

-shared
Produce a shared object which can then be linked with other objects to form an
executable. Not all systems support this option. For predictable results, you must also
specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions)
when you specify this linker option.[1]

-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either
the shared or static version, respectively. If no shared version of libgcc was built
when the compiler was configured, these options have no effect.

There are several situations in which an application should use the shared libgcc instead
of the static version. The most common of these is when the application wishes to throw
and catch exceptions across different shared libraries. In that case, each of the
libraries as well as the application itself should use the shared libgcc.

Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared
library or a main executable, because C++ programs typically use exceptions, so this is
the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you may find that they
are not always linked with the shared libgcc. If GCC finds, at its configuration time,
that you have a non-GNU linker or a GNU linker that does not support option
--eh-frame-hdr, it links the shared version of libgcc into shared libraries by default.
Otherwise, it takes advantage of the linker and optimizes away the linking with the
shared version of libgcc, linking with the static version of libgcc by default. This
allows exceptions to propagate through such shared libraries, without incurring
relocation costs at library load time.

However, if a library or main executable is supposed to throw or catch exceptions, you
must link it using the G++ driver, or using the option -shared-libgcc, such that it is
linked with the shared libgcc.

-static-libasan
When the -fsanitize=address option is used to link a program, the GCC driver
automatically links against libasan. If libasan is available as a shared library, and
the -static option is not used, then this links against the shared version of libasan.
The -static-libasan option directs the GCC driver to link libasan statically, without
necessarily linking other libraries statically.

-static-libtsan
When the -fsanitize=thread option is used to link a program, the GCC driver automatically
links against libtsan. If libtsan is available as a shared library, and the -static
option is not used, then this links against the shared version of libtsan. The
-static-libtsan option directs the GCC driver to link libtsan statically, without
necessarily linking other libraries statically.

-static-liblsan
When the -fsanitize=leak option is used to link a program, the GCC driver automatically
links against liblsan. If liblsan is available as a shared library, and the -static
option is not used, then this links against the shared version of liblsan. The
-static-liblsan option directs the GCC driver to link liblsan statically, without
necessarily linking other libraries statically.

-static-libubsan
When the -fsanitize=undefined option is used to link a program, the GCC driver
automatically links against libubsan. If libubsan is available as a shared library, and
the -static option is not used, then this links against the shared version of libubsan.
The -static-libubsan option directs the GCC driver to link libubsan statically, without
necessarily linking other libraries statically.

-static-libstdc++
When the g++ program is used to link a C++ program, it normally automatically links
against libstdc++. If libstdc++ is available as a shared library, and the -static option
is not used, then this links against the shared version of libstdc++. That is normally
fine. However, it is sometimes useful to freeze the version of libstdc++ used by the
program without going all the way to a fully static link. The -static-libstdc++ option
directs the g++ driver to link libstdc++ statically, without necessarily linking other
libraries statically.

-symbolic
Bind references to global symbols when building a shared object. Warn about any
unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker
defs). Only a few systems support this option.

-T script
Use script as the linker script. This option is supported by most systems using the GNU
linker. On some targets, such as bare-board targets without an operating system, the -T
option may be required when linking to avoid references to undefined symbols.

-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific
linker options that GCC does not recognize.

If you want to pass an option that takes a separate argument, you must use -Xlinker
twice, once for the option and once for the argument. For example, to pass -assert
definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to
write -Xlinker "-assert definitions", because this passes the entire string as a single
argument, which is not what the linker expects.

When using the GNU linker, it is usually more convenient to pass arguments to linker
options using the option=value syntax than as separate arguments. For example, you can
specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other
linkers may not support this syntax for command-line options.

-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into
multiple options at the commas. You can use this syntax to pass an argument to the
option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When
using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used.
To disable, use -Wl,-z,norelro.

-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it.
You can use -u multiple times with different symbols to force loading of additional
library modules.

-z keyword
-z is passed directly on to the linker along with the keyword keyword. See the section in
the documentation of your linker for permitted values and their meanings.

Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of
the compiler:

-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for header files during
preprocessing. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by
the sysroot prefix; see --sysroot and -isysroot.

Directories specified with -iquote apply only to the quote form of the directive,
"#include "file"". Directories specified with -I, -isystem, or -idirafter apply to
lookup for both the "#include "file"" and "#include <file>" directives.

You can specify any number or combination of these options on the command line to search
for header files in several directories. The lookup order is as follows:

1. For the quote form of the include directive, the directory of the current file is
searched first.

2. For the quote form of the include directive, the directories specified by -iquote
options are searched in left-to-right order, as they appear on the command line.

3. Directories specified with -I options are scanned in left-to-right order.

4. Directories specified with -isystem options are scanned in left-to-right order.

5. Standard system directories are scanned.

6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since
these directories are searched before the standard system header file directories.
However, you should not use this option to add directories that contain vendor-supplied
system header files; use -isystem for that.

The -isystem and -idirafter options also mark the directory as a system directory, so
that it gets the same special treatment that is applied to the standard system
directories.

If a standard system include directory, or a directory specified with -isystem, is also
specified with -I, the -I option is ignored. The directory is still searched but as a
system directory at its normal position in the system include chain. This is to ensure
that GCC's procedure to fix buggy system headers and the ordering for the "#include_next"
directive are not inadvertently changed. If you really need to change the search order
for system directories, use the -nostdinc and/or -isystem options.

-I- Split the include path. This option has been deprecated. Please use -iquote instead for
-I directories before the -I- and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers
requested with "#include "file""; they are not searched for "#include <file>". If
additional directories are specified with -I options after the -I-, those directories are
searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the
first search directory for "#include "file"". There is no way to override this effect of
-I-.

-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix
represents a directory, you should include the final /.

-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting
directory to the include search path. -iwithprefixbefore puts it in the same place -I
would; -iwithprefix puts it where -idirafter would.

-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for
Darwin targets, where it applies to both header files and libraries). See the --sysroot
option for more information.

-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.

-nostdinc
Do not search the standard system directories for header files. Only the directories
explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the
directory of the current file, if appropriate) are searched.

-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still
search the other standard directories. (This option is used when building the C++
library.)

-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name instead of
-fplugin=path/name.so. This option is not meant to be used by the user, but only passed
by the driver.

-Ldir
Add directory dir to the list of directories to be searched for -l.

-Bprefix
This option specifies where to find the executables, libraries, include files, and data
files of the compiler itself.

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It
tries prefix as a prefix for each program it tries to run, both with and without
machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If
that name is not found, or if -B is not specified, the driver tries two standard
prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file
name that is found, the unmodified program name is searched for using the directories
specified in your PATH environment variable.

The compiler checks to see if the path provided by -B refers to a directory, and if
necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the
linker, because the compiler translates these options into -L options for the linker.
They also apply to include files in the preprocessor, because the compiler translates
these options into -isystem options for the preprocessor. In this case, the compiler
appends include to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if
needed. If it is not found there, the two standard prefixes above are tried, and that is
all. The file is left out of the link if it is not found by those means.

Another way to specify a prefix much like the -B prefix is to use the environment
variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in
the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-
strapping the compiler.

-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path
absolute when generating a relative prefix.

--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the
compiler normally searches for headers in /usr/include and libraries in /usr/lib, it
instead searches dir/usr/include and dir/usr/lib.

If you use both this option and the -isysroot option, then the --sysroot option applies
to libraries, but the -isysroot option applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option.
If your linker does not support this option, the header file aspect of --sysroot still
works, but the library aspect does not.

--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified with --sysroot,
depending on the other options used, so that headers may for example be found in
dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of
such a suffix.

Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo.
In the table below, only one of the forms is listed---the one that is not the default. You
can figure out the other form by either removing no- or adding it.

-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto variables and
compiler generated temporaries. reuse_level can be all, named_vars, or none. all enables
stack reuse for all local variables and temporaries, named_vars enables the reuse only
for user defined local variables with names, and none disables stack reuse completely.
The default value is all. The option is needed when the program extends the lifetime of a
scoped local variable or a compiler generated temporary beyond the end point defined by
the language. When a lifetime of a variable ends, and if the variable lives in memory,
the optimizing compiler has the freedom to reuse its stack space with other temporaries
or scoped local variables whose live range does not overlap with it. Legacy code
extending local lifetime is likely to break with the stack reuse optimization.

For example,

int *p;
{
int local1;

p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}

if (*p == 10) // out of scope use of local1
{

}

Another example:

struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};

A *ap;

void foo(const A& ar)
{
ap = &ar;
}

void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns

{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}

The lifetime of a compiler generated temporary is well defined by the C++ standard. When
a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing
compiler has the freedom to reuse its stack space with other temporaries or scoped local
variables whose live range does not overlap with it. However some of the legacy code
relies on the behavior of older compilers in which temporaries' stack space is not
reused, the aggressive stack reuse can lead to runtime errors. This option is used to
control the temporary stack reuse optimization.

-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication
operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv
-fwrapv on the command-line results in -fwrapv being effective. Note that only active
options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in
-ftrapv being effective.

-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition,
subtraction and multiplication wraps around using twos-complement representation. This
flag enables some optimizations and disables others. The options -ftrapv and -fwrapv
override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv
being effective. Note that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being effective.

-fwrapv-pointer
This option instructs the compiler to assume that pointer arithmetic overflow on addition
and subtraction wraps around using twos-complement representation. This flag disables
some optimizations which assume pointer overflow is invalid.

-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv
-fwrapv-pointer.

-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For
some targets, this implies GCC generates frame unwind information for all functions,
which can produce significant data size overhead, although it does not affect execution.
If you do not specify this option, GCC enables it by default for languages like C++ that
normally require exception handling, and disables it for languages like C that do not
normally require it. However, you may need to enable this option when compiling C code
that needs to interoperate properly with exception handlers written in C++. You may also
wish to disable this option if you are compiling older C++ programs that don't use
exception handling.

-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this
requires platform-specific runtime support that does not exist everywhere. Moreover, it
only allows trapping instructions to throw exceptions, i.e. memory references or
floating-point instructions. It does not allow exceptions to be thrown from arbitrary
signal handlers such as "SIGALRM". This enables -fexceptions.

-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't otherwise contribute to
the execution of the program can be optimized away. This does not affect calls to
functions except those with the "pure" or "const" attributes. This option is enabled by
default for the Ada and C++ compilers, as permitted by the language specifications.
Optimization passes that cause dead exceptions to be removed are enabled independently at
different optimization levels.

-funwind-tables
Similar to -fexceptions, except that it just generates any needed static data, but does
not affect the generated code in any other way. You normally do not need to enable this
option; instead, a language processor that needs this handling enables it on your behalf.

-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target machine. The table is
exact at each instruction boundary, so it can be used for stack unwinding from
asynchronous events (such as debugger or garbage collector).

-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++ compiler uses the
"STB_GNU_UNIQUE" binding to make sure that definitions of template static data members
and static local variables in inline functions are unique even in the presence of
"RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different
"RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing
with the other one about the binding of the symbol. But this causes "dlclose" to be
ignored for affected DSOs; if your program relies on reinitialization of a DSO via
"dlclose" and "dlopen", you can use -fno-gnu-unique.

-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in
registers. This convention is less efficient, but it has the advantage of allowing
intercallability between GCC-compiled files and files compiled with other compilers,
particularly the Portable C Compiler (pcc).

The precise convention for returning structures in memory depends on the target
configuration macros.

Short structures and unions are those whose size and alignment match that of some integer
type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with
code compiled with the -freg-struct-return switch. Use it to conform to a non-default
application binary interface.

-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient
for small structures than -fpcc-struct-return.

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to
whichever convention is standard for the target. If there is no standard convention, GCC
defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler.
In those cases, we can choose the standard, and we chose the more efficient register
return alternative.

Warning: code compiled with the -freg-struct-return switch is not binary compatible with
code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default
application binary interface.

-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of
possible values. Specifically, the "enum" type is equivalent to the smallest integer
type that has enough room.

Warning: the -fshort-enums switch causes GCC to generate code that is not binary
compatible with code generated without that switch. Use it to conform to a non-default
application binary interface.

-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned int" instead of the
default for the target. This option is useful for building programs to run under WINE.

Warning: the -fshort-wchar switch causes GCC to generate code that is not binary
compatible with code generated without that switch. Use it to conform to a non-default
application binary interface.

-fcommon
In C code, this option controls the placement of global variables defined without an
initializer, known as tentative definitions in the C standard. Tentative definitions are
distinct from declarations of a variable with the "extern" keyword, which do not allocate
storage.

The default is -fno-common, which specifies that the compiler places uninitialized global
variables in the BSS section of the object file. This inhibits the merging of tentative
definitions by the linker so you get a multiple-definition error if the same variable is
accidentally defined in more than one compilation unit.

The -fcommon places uninitialized global variables in a common block. This allows the
linker to resolve all tentative definitions of the same variable in different compilation
units to the same object, or to a non-tentative definition. This behavior is
inconsistent with C++, and on many targets implies a speed and code size penalty on
global variable references. It is mainly useful to enable legacy code to link without
errors.

-fno-ident
Ignore the "#ident" directive.

-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if
the function is split in the middle, and the two halves are placed at locations far apart
in memory. This option is used when compiling crtstuff.c; you should not need to use it
for anything else.

-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable.
This option is generally only of use to those who actually need to read the generated
assembly code (perhaps while debugging the compiler itself).

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful
when comparing two assembler files.

The added comments include:

* information on the compiler version and command-line options,

* the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE,

* hints on which high-level expressions correspond to the various assembly instruction
operands.

For example, given this C source file:

int test (int n)
{
int i;
int total = 0;

for (i = 0; i < n; i++)
total += i * i;

return total;
}

compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -

gcc -S test.c -fverbose-asm -Os -o -

gives output similar to this:

.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]

.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits

The comments are intended for humans rather than machines and hence the precise format of
the comments is subject to change.

-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be recorded into the
object file that is being created. This switch is only implemented on some targets and
the exact format of the recording is target and binary file format dependent, but it
usually takes the form of a section containing ASCII text. This switch is related to the
-fverbose-asm switch, but that switch only records information in the assembler output
file as comments, so it never reaches the object file. See also -grecord-gcc-switches
for another way of storing compiler options into the object file.

-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if
supported for the target machine. Such code accesses all constant addresses through a
global offset table (GOT). The dynamic loader resolves the GOT entries when the program
starts (the dynamic loader is not part of GCC; it is part of the operating system). If
the GOT size for the linked executable exceeds a machine-specific maximum size, you get
an error message from the linker indicating that -fpic does not work; in that case,
recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64 and
32k on the m68k and RS/6000. The x86 has no such limit.)

Position-independent code requires special support, and therefore works only on certain
machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code
generated for the IBM RS/6000 is always position-independent.

When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.

-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic
linking and avoiding any limit on the size of the global offset table. This option makes
a difference on AArch64, m68k, PowerPC and SPARC.

Position-independent code requires special support, and therefore works only on certain
machines.

When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.

-fpie
-fPIE
These options are similar to -fpic and -fPIC, but the generated position-independent code
can be only linked into executables. Usually these options are used to compile code that
will be linked using the -pie GCC option.

-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the
value 1 for -fpie and 2 for -fPIE.

-fno-plt
Do not use the PLT for external function calls in position-independent code. Instead,
load the callee address at call sites from the GOT and branch to it. This leads to more
efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On
architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific
register, this gives more register allocation freedom to the compiler. Lazy binding
requires use of the PLT; with -fno-plt all external symbols are resolved at load time.

Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT
for specific external functions.

In position-dependent code, a few targets also convert calls to functions that are marked
to not use the PLT to use the GOT instead.

-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than
other code generation strategies. This option is of use in conjunction with -fpic or
-fPIC for building code that forms part of a dynamic linker and cannot reference the
address of a jump table. On some targets, jump tables do not require a GOT and this
option is not needed.

-fno-bit-tests
Do not use bit tests for switch statements even where it would be more efficient than
other code generation strategies.

-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it
(except perhaps as a stack pointer, frame pointer or in some other fixed role).

reg must be the name of a register. The register names accepted are machine-specific and
are defined in the "REGISTER_NAMES" macro in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way choice.

-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function
calls. It may be allocated for temporaries or variables that do not live across a call.
Functions compiled this way do not save and restore the register reg.

It is an error to use this flag with the frame pointer or stack pointer. Use of this
flag for other registers that have fixed pervasive roles in the machine's execution model
produces disastrous results.

This flag does not have a negative form, because it specifies a three-way choice.

-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be
allocated even for temporaries or variables that live across a call. Functions compiled
this way save and restore the register reg if they use it.

A different sort of disaster results from the use of this flag for a register in which
function values may be returned.

This flag does not have a negative form, because it specifies a three-way choice.

-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a
value is specified (which must be a small power of two), pack structure members according
to this value, representing the maximum alignment (that is, objects with default
alignment requirements larger than this are output potentially unaligned at the next
fitting location.

Warning: the -fpack-struct switch causes GCC to generate code that is not binary
compatible with code generated without that switch. Additionally, it makes the code
suboptimal. Use it to conform to a non-default application binary interface.

-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C
symbols are represented in the object file. One use is to help link with legacy assembly
code.

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary
compatible with code generated without that switch. Use it to conform to a non-default
application binary interface. Not all targets provide complete support for this switch.

-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of
global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is
subject to optimization: the compiler may use a more efficient model for symbols not
visible outside of the translation unit, or if -fpic is not given on the command line.

The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

-ftrampolines
For targets that normally need trampolines for nested functions, always generate them
instead of using descriptors. Otherwise, for targets that do not need them, like for
example HP-PA or IA-64, do nothing.

A trampoline is a small piece of code that is created at run time on the stack when the
address of a nested function is taken, and is used to call the nested function
indirectly. Therefore, it requires the stack to be made executable in order for the
program to work properly.

-fno-trampolines is enabled by default on a language by language basis to let the
compiler avoid generating them, if it computes that this is safe, and replace them with
descriptors. Descriptors are made up of data only, but the generated code must be
prepared to deal with them. As of this writing, -fno-trampolines is enabled by default
only for Ada.

Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are
not binary compatible if nested functions are present. This option must therefore be
used on a program-wide basis and be manipulated with extreme care.

For languages other than Ada, the "-ftrampolines" and "-fno-trampolines" options
currently have no effect, and trampolines are always generated on platforms that need
them for nested functions.

-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified option---all symbols are
marked with this unless overridden within the code. Using this feature can very
substantially improve linking and load times of shared object libraries, produce more
optimized code, provide near-perfect API export and prevent symbol clashes. It is
strongly recommended that you use this in any shared objects you distribute.

Despite the nomenclature, default always means public; i.e., available to be linked
against from outside the shared object. protected and internal are pretty useless in
real-world usage so the only other commonly used option is hidden. The default if
-fvisibility isn't specified is default, i.e., make every symbol public.

A good explanation of the benefits offered by ensuring ELF symbols have the correct
visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be
found at <https://www.akkadia.org/drepper/>)---however a superior solution made possible
by this option to marking things hidden when the default is public is to make the default
hidden and mark things public. This is the norm with DLLs on Windows and with
-fvisibility=hidden and "__attribute__ ((visibility("default")))" instead of
"__declspec(dllexport)" you get almost identical semantics with identical syntax. This
is a great boon to those working with cross-platform projects.

For those adding visibility support to existing code, you may find "#pragma GCC
visibility" of use. This works by you enclosing the declarations you wish to set
visibility for with (for example) "#pragma GCC visibility push(hidden)" and "#pragma GCC
visibility pop". Bear in mind that symbol visibility should be viewed as part of the API
interface contract and thus all new code should always specify visibility when it is not
the default; i.e., declarations only for use within the local DSO should always be marked
explicitly as hidden as so to avoid PLT indirection overheads---making this abundantly
clear also aids readability and self-documentation of the code. Note that due to ISO C++
specification requirements, "operator new" and "operator delete" must always be of
default visibility.

Be aware that headers from outside your project, in particular system headers and headers
from any other library you use, may not be expecting to be compiled with visibility other
than the default. You may need to explicitly say "#pragma GCC visibility push(default)"
before including any such headers.

"extern" declarations are not affected by -fvisibility, so a lot of code can be
recompiled with -fvisibility=hidden with no modifications. However, this means that
calls to "extern" functions with no explicit visibility use the PLT, so it is more
effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the
compiler which "extern" declarations should be treated as hidden.

Note that -fvisibility does affect C++ vague linkage entities. This means that, for
instance, an exception class that is be thrown between DSOs must be explicitly marked
with default visibility so that the type_info nodes are unified between the DSOs.

An overview of these techniques, their benefits and how to use them is at
<https://gcc.gnu.org/wiki/Visibility>.

-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields,
although the compiler usually honors those types anyway) should use a single access of
the width of the field's type, aligned to a natural alignment if possible. For example,
targets with memory-mapped peripheral registers might require all such accesses to be 16
bits wide; with this flag you can declare all peripheral bit-fields as "unsigned short"
(assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead
of, perhaps, a more efficient 32-bit access.

If this option is disabled, the compiler uses the most efficient instruction. In the
previous example, that might be a 32-bit load instruction, even though that accesses
bytes that do not contain any portion of the bit-field, or memory-mapped registers
unrelated to the one being updated.

In some cases, such as when the "packed" attribute is applied to a structure field, it
may not be possible to access the field with a single read or write that is correctly
aligned for the target machine. In this case GCC falls back to generating multiple
accesses rather than code that will fault or truncate the result at run time.

Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to
touch non bit-field members. It is therefore recommended to define all bits of the
field's type as bit-field members.

The default value of this option is determined by the application binary interface for
the target processor.

-fsync-libcalls
This option controls whether any out-of-line instance of the "__sync" family of functions
may be used to implement the C++11 "__atomic" family of functions.

The default value of this option is enabled, thus the only useful form of the option is
-fno-sync-libcalls. This option is used in the implementation of the libatomic runtime
library.

GCC Developer Options
This section describes command-line options that are primarily of interest to GCC developers,
including options to support compiler testing and investigation of compiler bugs and compile-
time performance problems. This includes options that produce debug dumps at various points
in the compilation; that print statistics such as memory use and execution time; and that
print information about GCC's configuration, such as where it searches for libraries. You
should rarely need to use any of these options for ordinary compilation and linking tasks.

Many developer options that cause GCC to dump output to a file take an optional =filename
suffix. You can specify stdout or - to dump to standard output, and stderr for standard
error.

If =filename is omitted, a default dump file name is constructed by concatenating the base
dump file name, a pass number, phase letter, and pass name. The base dump file name is the
name of output file produced by the compiler if explicitly specified and not an executable;
otherwise it is the source file name. The pass number is determined by the order passes are
registered with the compiler's pass manager. This is generally the same as the order of
execution, but passes registered by plugins, target-specific passes, or passes that are
otherwise registered late are numbered higher than the pass named final, even if they are
executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-
specific), r (RTL), or t (tree). The files are created in the directory of the output file.

-fcallgraph-info
-fcallgraph-info=MARKERS
Makes the compiler output callgraph information for the program, on a per-object-file
basis. The information is generated in the common VCG format. It can be decorated with
additional, per-node and/or per-edge information, if a list of comma-separated markers is
additionally specified. When the "su" marker is specified, the callgraph is decorated
with stack usage information; it is equivalent to -fstack-usage. When the "da" marker is
specified, the callgraph is decorated with information about dynamically allocated
objects.

When compiling with -flto, no callgraph information is output along with the object file.
At LTO link time, -fcallgraph-info may generate multiple callgraph information files next
to intermediate LTO output files.

-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified by letters. This is
used for debugging the RTL-based passes of the compiler.

Some -dletters switches have different meaning when -E is used for preprocessing.

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are
the possible letters for use in pass and letters, and their meanings:

-fdump-rtl-alignments
Dump after branch alignments have been computed.

-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.

-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have
auto inc or auto dec instructions.

-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.

-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.

-fdump-rtl-bbro
Dump after block reordering.

-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load
optimization passes.

-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.

-fdump-rtl-combine
Dump after the RTL instruction combination pass.

-fdump-rtl-compgotos
Dump after duplicating the computed gotos.

-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if
conversion passes.

-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.

-fdump-rtl-csa
Dump after combining stack adjustments.

-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression
elimination passes.

-fdump-rtl-dce
Dump after the standalone dead code elimination passes.

-fdump-rtl-dbr
Dump after delayed branch scheduling.

-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store
elimination passes.

-fdump-rtl-eh
Dump after finalization of EH handling code.

-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.

-fdump-rtl-expand
Dump after RTL generation.

-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward
propagation passes.

-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common
subexpression elimination.

-fdump-rtl-init-regs
Dump after the initialization of the registers.

-fdump-rtl-initvals
Dump after the computation of the initial value sets.

-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.

-fdump-rtl-ira
Dump after iterated register allocation.

-fdump-rtl-jump
Dump after the second jump optimization.

-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.

-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.

-fdump-rtl-mode_sw
Dump after removing redundant mode switches.

-fdump-rtl-rnreg
Dump after register renumbering.

-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.

-fdump-rtl-peephole2
Dump after the peephole pass.

-fdump-rtl-postreload
Dump after post-reload optimizations.

-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.

-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block
scheduling passes.

-fdump-rtl-ree
Dump after sign/zero extension elimination.

-fdump-rtl-seqabstr
Dump after common sequence discovery.

-fdump-rtl-shorten
Dump after shortening branches.

-fdump-rtl-sibling
Dump after sibling call optimizations.

-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction splitting.

-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.

-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers to the x87's stack-
like registers. This pass is only run on x86 variants.

-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg
expansion passes.

-fdump-rtl-unshare
Dump after all rtl has been unshared.

-fdump-rtl-vartrack
Dump after variable tracking.

-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.

-fdump-rtl-web
Dump after live range splitting.

-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.

-da
-fdump-rtl-all
Produce all the dumps listed above.

-dA Annotate the assembler output with miscellaneous debugging information.

-dD Dump all macro definitions, at the end of preprocessing, in addition to normal
output.

-dH Produce a core dump whenever an error occurs.

-dp Annotate the assembler output with a comment indicating which pattern and alternative
is used. The length and cost of each instruction are also printed.

-dP Dump the RTL in the assembler output as a comment before each instruction. Also
turns on -dp annotation.

-dx Just generate RTL for a function instead of compiling it. Usually used with
-fdump-rtl-expand.

-fdump-debug
Dump debugging information generated during the debug generation phase.

-fdump-earlydebug
Dump debugging information generated during the early debug generation phase.

-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use
diff on debugging dumps for compiler invocations with different compiler binaries and/or
different text / bss / data / heap / stack / dso start locations.

-freport-bug
Collect and dump debug information into a temporary file if an internal compiler error
(ICE) occurs.

-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output. This makes
it more feasible to use diff on debugging dumps for compiler invocations with different
options, in particular with and without -g.

-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers for the
links to the previous and next instructions in a sequence.

-fdump-ipa-switch
-fdump-ipa-switch-options
Control the dumping at various stages of inter-procedural analysis language tree to a
file. The file name is generated by appending a switch specific suffix to the source
file name, and the file is created in the same directory as the output file. The
following dumps are possible:

all Enables all inter-procedural analysis dumps.

cgraph
Dumps information about call-graph optimization, unused function removal, and
inlining decisions.

inline
Dump after function inlining.

Additionally, the options -optimized, -missed, -note, and -all can be provided, with the
same meaning as for -fopt-info, defaulting to -optimized.

For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that
were inlined, along with callsites that were not inlined.

By default, the dump will contain messages about successful optimizations (equivalent to
-optimized) together with low-level details about the analysis.

-fdump-lang
Dump language-specific information. The file name is made by appending .lang to the
source file name.

-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The options and filename portions
behave as described in the -fdump-tree option. The following switch values are accepted:

all Enable all language-specific dumps.

class
Dump class hierarchy information. Virtual table information is emitted unless 'slim'
is specified. This option is applicable to C++ only.

module
Dump module information. Options lineno (locations), graph (reachability), blocks
(clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) &
vops (macros) may provide additional information. This option is applicable to C++
only.

raw Dump the raw internal tree data. This option is applicable to C++ only.

-fdump-passes
Print on stderr the list of optimization passes that are turned on and off by the current
command-line options.

-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file name is
generated by appending a suffix ending in .statistics to the source file name, and the
file is created in the same directory as the output file. If the -option form is used,
-stats causes counters to be summed over the whole compilation unit while -details dumps
every event as the passes generate them. The default with no option is to sum counters
for each function compiled.

-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the intermediate language tree to a
file. If the -options form is used, options is a list of - separated options which
control the details of the dump. Not all options are applicable to all dumps; those that
are not meaningful are ignored. The following options are available

address
Print the address of each node. Usually this is not meaningful as it changes
according to the environment and source file. Its primary use is for tying up a dump
file with a debug environment.

asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead
of "DECL_NAME". Its primary use is ease of use working backward from mangled names
in the assembly file.

slim
When dumping front-end intermediate representations, inhibit dumping of members of a
scope or body of a function merely because that scope has been reached. Only dump
such items when they are directly reachable by some other path.

When dumping pretty-printed trees, this option inhibits dumping the bodies of control
structures.

When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-
like representation.

raw Print a raw representation of the tree. By default, trees are pretty-printed into a
C-like representation.

details
Enable more detailed dumps (not honored by every dump option). Also include
information from the optimization passes.

stats
Enable dumping various statistics about the pass (not honored by every dump option).

blocks
Enable showing basic block boundaries (disabled in raw dumps).

graph
For each of the other indicated dump files (-fdump-rtl-pass), dump a representation
of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot.
Each function in the file is pretty-printed as a subgraph, so that GraphViz can
render them all in a single plot.

This option currently only works for RTL dumps, and the RTL is always dumped in slim
form.

vops
Enable showing virtual operands for every statement.

lineno
Enable showing line numbers for statements.

uid Enable showing the unique ID ("DECL_UID") for each variable.

verbose
Enable showing the tree dump for each statement.

eh Enable showing the EH region number holding each statement.

scev
Enable showing scalar evolution analysis details.

optimized
Enable showing optimization information (only available in certain passes).

missed
Enable showing missed optimization information (only available in certain passes).

note
Enable other detailed optimization information (only available in certain passes).

all Turn on all options, except raw, slim, verbose and lineno.

optall
Turn on all optimization options, i.e., optimized, missed, and note.

To determine what tree dumps are available or find the dump for a pass of interest follow
the steps below.

1. Invoke GCC with -fdump-passes and in the stderr output look for a code that
corresponds to the pass you are interested in. For example, the codes "tree-evrp",
"tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes.
The number at the end distinguishes distinct invocations of the same pass.

2. To enable the creation of the dump file, append the pass code to the -fdump- option
prefix and invoke GCC with it. For example, to enable the dump from the Early Value
Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. Optionally, you
may specify the name of the dump file. If you don't specify one, GCC creates as
described below.

3. Find the pass dump in a file whose name is composed of three components separated by
a period: the name of the source file GCC was invoked to compile, a numeric suffix
indicating the pass number followed by the letter t for tree passes (and the letter r
for RTL passes), and finally the pass code. For example, the Early VRP pass dump
might be in a file named myfile.c.038t.evrp in the current working directory. Note
that the numeric codes are not stable and may change from one version of GCC to
another.

-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the -options form is
used, options is a list of - separated option keywords to select the dump details and
optimizations.

The options can be divided into three groups:

1. options describing what kinds of messages should be emitted,

2. options describing the verbosity of the dump, and

3. options describing which optimizations should be included.

The options from each group can be freely mixed as they are non-overlapping. However, in
case of any conflicts, the later options override the earlier options on the command
line.

The following options control which kinds of messages should be emitted:

optimized
Print information when an optimization is successfully applied. It is up to a pass to
decide which information is relevant. For example, the vectorizer passes print the
source location of loops which are successfully vectorized.

missed
Print information about missed optimizations. Individual passes control which
information to include in the output.

note
Print verbose information about optimizations, such as certain transformations, more
detailed messages about decisions etc.

all Print detailed optimization information. This includes optimized, missed, and note.

The following option controls the dump verbosity:

internals
By default, only "high-level" messages are emitted. This option enables additional,
more detailed, messages, which are likely to only be of interest to GCC developers.

One or more of the following option keywords can be used to describe a group of
optimizations:

ipa Enable dumps from all interprocedural optimizations.

loop
Enable dumps from all loop optimizations.

inline
Enable dumps from all inlining optimizations.

omp Enable dumps from all OMP (Offloading and Multi Processing) optimizations.

vec Enable dumps from all vectorization optimizations.

optall
Enable dumps from all optimizations. This is a superset of the optimization groups
listed above.

If options is omitted, it defaults to optimized-optall, which means to dump messages
about successful optimizations from all the passes, omitting messages that are treated as
"internals".

If the filename is provided, then the dumps from all the applicable optimizations are
concatenated into the filename. Otherwise the dump is output onto stderr. Though
multiple -fopt-info options are accepted, only one of them can include a filename. If
other filenames are provided then all but the first such option are ignored.

Note that the output filename is overwritten in case of multiple translation units. If a
combined output from multiple translation units is desired, stderr should be used
instead.

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on
stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The
order of the optimization group names and message types listed after -fopt-info does not
matter.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all
the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output
file is allowed. In this case, only the first option takes effect and the subsequent
options are ignored. Thus only vec.miss is produced which contains dumps from the
vectorizer about missed opportunities.

-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for
those optimizations that support -fopt-info.

This option is experimental and the format of the data within the compressed JSON file is
subject to change.

It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection
of messages with source file, line number and column number, with the following
additional data for each message:

* the execution count of the code being optimized, along with metadata about whether
this was from actual profile data, or just an estimate, allowing consumers to
prioritize messages by code hotness,

* the function name of the code being optimized, where applicable,

* the "inlining chain" for the code being optimized, so that when a function is inlined
into several different places (which might themselves be inlined), the reader can
distinguish between the copies,

* objects identifying those parts of the message that refer to expressions, statements
or symbol-table nodes, which of these categories they are, and, when available, their
source code location,

* the GCC pass that emitted the message, and

* the location in GCC's own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting
implementation details of the optimization passes.

-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging
output the scheduler prints to the dump files.

For n greater than zero, -fsched-verbose outputs the same information as
-fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also output basic
block probabilities, detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions info. And for n over
four, -fsched-verbose also includes dependence info.

-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable optimization passes.
These options are intended for use for debugging GCC. Compiler users should use regular
options for enabling/disabling passes instead.

-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked
in the compiler multiple times, the pass name should be appended with a sequential
number starting from 1.

-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same pass is statically
invoked in the compiler multiple times, the pass name should be appended with a
sequential number starting from 1. range-list is a comma-separated list of function
ranges or assembler names. Each range is a number pair separated by a colon. The
range is inclusive in both ends. If the range is trivial, the number pair can be
simplified as a single number. If the function's call graph node's uid falls within
one of the specified ranges, the pass is disabled for that function. The uid is
shown in the function header of a dump file, and the pass names can be dumped by
using option -fdump-passes.

-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the description of option arguments.

-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked
in the compiler multiple times, the pass name should be appended with a sequential
number starting from 1.

-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument description and
examples.

-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options.

# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll

-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the compiler configuration.
-fchecking=2 enables further internal consistency checking that might affect code
generation.

-frandom-seed=string
This option provides a seed that GCC uses in place of random numbers in generating
certain symbol names that have to be different in every compiled file. It is also used
to place unique stamps in coverage data files and the object files that produce them.
You can use the -frandom-seed option to produce reproducibly identical object files.

The string can either be a number (decimal, octal or hex) or an arbitrary string (in
which case it's converted to a number by computing CRC32).

The string should be different for every file you compile.

-save-temps
Store the usual "temporary" intermediate files permanently; name them as auxiliary output
files, as specified described under -dumpbase and -dumpdir.

When used in combination with the -x command-line option, -save-temps is sensible enough
to avoid overwriting an input source file with the same extension as an intermediate
file. The corresponding intermediate file may be obtained by renaming the source file
before using -save-temps.

-save-temps=cwd
Equivalent to -save-temps -dumpdir ./.

-save-temps=obj
Equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output
file specified after the -o option, including any directory separators. If the -o option
is not used, the -save-temps=obj switch behaves like -save-temps=cwd.

-time[=file]
Report the CPU time taken by each subprocess in the compilation sequence. For C source
files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time", that is time spent executing the
program itself. The second number is "system time", time spent executing operating
system routines on behalf of the program. Both numbers are in seconds.

With the specification of an output file, the output is appended to the named file, and
it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" are moved before the program name, and the options
passed to the program are displayed, so that one can later tell what file was being
compiled, and with which options.

-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument is
omitted (or if file is "."), the name of the dump file is determined by appending ".gkd"
to the dump base name, see -dumpbase.

-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and
-fcompare-debug-second to the arguments passed to the second compilation. Dump the final
internal representation in both compilations, and print an error if they differ.

If the equal sign is omitted, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly
enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a
dash, then it is used for opts, otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to
-fno-compare-debug, which disables the dumping of the final representation and the second
compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say
-fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual
compilation (rather than preprocessing, assembly or linking). To get just a warning,
setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do.

-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by
-fcompare-debug, along with options to silence warnings, and omitting other options that
would cause the compiler to produce output to files or to standard output as a side
effect. Dump files and preserved temporary files are renamed so as to contain the ".gk"
additional extension during the second compilation, to avoid overwriting those generated
by the first.

When this option is passed to the compiler driver, it causes the first compilation to be
skipped, which makes it useful for little other than debugging the compiler proper.

-gtoggle
Turn off generation of debug info, if leaving out this option generates it, or turn it on
at level 2 otherwise. The position of this argument in the command line does not matter;
it takes effect after all other options are processed, and it does so only once, no
matter how many times it is given. This is mainly intended to be used with
-fcompare-debug.

-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.

-Q Makes the compiler print out each function name as it is compiled, and print some
statistics about each pass when it finishes.

-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it
finishes.

-ftime-report-details
Record the time consumed by infrastructure parts separately for each pass.

-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The
default value is 5. If the value n is greater or equal to 10, the dump output is sent to
stderr using the same format as n minus 10.

-flto-report
Prints a report with internal details on the workings of the link-time optimizer. The
contents of this report vary from version to version. It is meant to be useful to GCC
developers when processing object files in LTO mode (via -flto).

Disabled by default.

-flto-report-wpa
Like -flto-report, but only print for the WPA phase of link-time optimization.

-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it
finishes.

-fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for the WPA
phase only.

-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or
after interprocedural optimization.

-fmultiflags
This option enables multilib-aware "TFLAGS" to be used to build target libraries with
options different from those the compiler is configured to use by default, through the
use of specs

Like "TFLAGS", this allows the target libraries to be built for portable baseline
environments, while the compiler defaults to more demanding ones. That's useful because
users can easily override the defaults the compiler is configured to use to build their
own programs, if the defaults are not ideal for their target environment, whereas
rebuilding the runtime libraries is usually not as easy or desirable.

Unlike "TFLAGS", the use of specs enables different flags to be selected for different
multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after
configuring --with-specs=%{fmultiflags:...}.

This option is discarded by the driver once it's done processing driver self spec.

It is also useful to check that "TFLAGS" are being used to build all target libraries, by
configuring a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and
building the compiler and target libraries.

-fprofile-report
Makes the compiler print some statistics about consistency of the (estimated) profile and
effect of individual passes.

-fstack-usage
Makes the compiler output stack usage information for the program, on a per-function
basis. The filename for the dump is made by appending .su to the auxname. auxname is
generated from the name of the output file, if explicitly specified and it is not an
executable, otherwise it is the basename of the source file. An entry is made up of
three fields:

* The name of the function.

* A number of bytes.

* One or more qualifiers: "static", "dynamic", "bounded".

The qualifier "static" means that the function manipulates the stack statically: a fixed
number of bytes are allocated for the frame on function entry and released on function
exit; no stack adjustments are otherwise made in the function. The second field is this
fixed number of bytes.

The qualifier "dynamic" means that the function manipulates the stack dynamically: in
addition to the static allocation described above, stack adjustments are made in the body
of the function, for example to push/pop arguments around function calls. If the
qualifier "bounded" is also present, the amount of these adjustments is bounded at
compile time and the second field is an upper bound of the total amount of stack used by
the function. If it is not present, the amount of these adjustments is not bounded at
compile time and the second field only represents the bounded part.

-fstats
Emit statistics about front-end processing at the end of the compilation. This option is
supported only by the C++ front end, and the information is generally only useful to the
G++ development team.

-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.

-fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound. counter-value-list is a comma-
separated list of name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples
which sets the name of the counter and list of closed intervals. The lower_bound is
optional and is zero initialized if not set. For example, with
-fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for second, third,
fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for first
10 invocations.

-print-file-name=library
Print the full absolute name of the library file library that would be used when
linking---and don't do anything else. With this option, GCC does not compile or link
anything; it just prints the file name.

-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches
present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.

-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them.
The directory name is separated from the switches by ;, and each switch starts with an @
instead of the -, without spaces between multiple switches. This is supposed to ease
shell processing.

-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib
subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are
used, this is usually just ., if OS libraries are present in libsuffix sibling
directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present
in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.

-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib
subdirectory.

-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.

-print-libgcc-file-name
Same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with
libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

-print-search-dirs
Print the name of the configured installation directory and a list of program and library
directories gcc searches---and don't do anything else.

This is useful when gcc prints the error message installation problem, cannot exec cpp0:
No such file or directory. To resolve this you either need to put cpp0 and the other
compiler components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the
trailing /.

-print-sysroot
Print the target sysroot directory that is used during compilation. This is the target
sysroot specified either at configure time or using the --sysroot option, possibly with
an extra suffix that depends on compilation options. If no target sysroot is specified,
the option prints nothing.

-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error
if the compiler is not configured with such a suffix---and don't do anything else.

-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do
anything else.

-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and don't do anything else.
This is the compiler version used in filesystem paths and specs. Depending on how the
compiler has been configured it can be just a single number (major version), two numbers
separated by a dot (major and minor version) or three numbers separated by dots (major,
minor and patchlevel version).

-dumpfullversion
Print the full compiler version---and don't do anything else. The output is always three
numbers separated by dots, major, minor and patchlevel version.

-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC
itself is being built.)

Machine-Dependent Options
Each target machine supported by GCC can have its own options---for example, to allow you to
compile for a particular processor variant or ABI, or to control optimizations specific to
that machine. By convention, the names of machine-specific options start with -m.

Some configurations of the compiler also support additional target-specific options, usually
for compatibility with other compilers on the same platform.

AArch64 Options

These options are defined for AArch64 implementations:

-mabi=name
Generate code for the specified data model. Permissible values are ilp32 for SysV-like
data model where int, long int and pointers are 32 bits, and lp64 for SysV-like data
model where int is 32 bits, but long int and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32
ABIs are not link-compatible; you must compile your entire program with the same ABI, and
link with a compatible set of libraries.

-mbig-endian
Generate big-endian code. This is the default when GCC is configured for an
aarch64_be-*-* target.

-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the
compiler from using floating-point and Advanced SIMD registers but will not impose any
restrictions on the assembler.

-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for an
aarch64-*-* but not an aarch64_be-*-* target.

-mcmodel=tiny
Generate code for the tiny code model. The program and its statically defined symbols
must be within 1MB of each other. Programs can be statically or dynamically linked.

-mcmodel=small
Generate code for the small code model. The program and its statically defined symbols
must be within 4GB of each other. Programs can be statically or dynamically linked.
This is the default code model.

-mcmodel=large
Generate code for the large code model. This makes no assumptions about addresses and
sizes of sections. Programs can be statically linked only. The -mcmodel=large option is
incompatible with -mabi=ilp32, -fpic and -fPIC.

-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object
boundary as described in the architecture specification.

-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behavior is the default.

-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for
a global canary or sysreg for a canary in an appropriate system register.

With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which system register to use as
base register for reading the canary, and from what offset from that base register. There
is no default register or offset as this is entirely for use within the Linux kernel.

-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS
variables. This is the default.

-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS
variables.

-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option
requires binutils 2.26 or newer.

-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This
involves inserting a NOP instruction between memory instructions and 64-bit integer
multiply-accumulate instructions.

-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This
erratum workaround is made at link time and this will only pass the corresponding flag to
the linker.

-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This option only has an
effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this
reduces precision of reciprocal square root results to about 16 bits for single precision
and to 32 bits for double precision.

-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only has an effect if
-ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces
precision of square root results to about 16 bits for single precision and to 32 bits for
double precision. If enabled, it implies -mlow-precision-recip-sqrt.

-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has an effect if
-ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces
precision of division results to about 16 bits for single precision and to 32 bits for
double precision.

-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track speculative execution through
conditional branches. The tracking state can then be used by the compiler when expanding
calls to "__builtin_speculation_safe_copy" to permit a more efficient code sequence to be
generated.

-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic operations. These
helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used;
if not, they will use the load/store-exclusive instructions that are present in the base
ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set. If
using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics
instructions will be used directly. The same applies when using -mcpu= when the selected
cpu supports the lse feature. This option is on by default.

-march=name
Specify the name of the target architecture and, optionally, one or more feature
modifiers. This option has the form -march=arch{+[no]feature}*.

The table below summarizes the permissible values for arch and the features that they
enable by default:

arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a, +pauth
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8.7-a : Armv8.7-A : armv8.6-a, +ls64
armv8.8-a : Armv8.8-a : armv8.7-a, +mops
armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
armv9.2-a : Armv9.2-A : armv9.1-a, +ls64
armv9.3-a : Armv9.3-A : armv9.2-a, +mops
armv8-r : Armv8-R : armv8-r

The value native is available on native AArch64 GNU/Linux and causes the compiler to pick
the architecture of the host system. This option has no effect if the compiler is unable
to recognize the architecture of the host system,

The permissible values for feature are listed in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature
modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly
code. If -march is specified without either of -mtune or -mcpu also being specified, the
code is tuned to perform well across a range of target processors implementing the target
architecture.

-mtune=name
Specify the name of the target processor for which GCC should tune the performance of the
code. Permissible values for this option are: generic, cortex-a35, cortex-a53,
cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae,
cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c,
ares, exynos-m1, emag, falkor, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2,
neoverse-v1, neoverse-v2, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81,
octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95,
octeontx2f95n, octeontx2f95mm, a64fx, thunderx, thunderxt88, thunderxt88p1, thunderxt81,
tsv110, thunderxt83, thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82, cortex-x1, cortex-x1c,
cortex-x2, cortex-x3, cortex-a510, cortex-a710, cortex-a715, ampere1, ampere1a,
cobalt-100 and native.

The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC
should tune for a big.LITTLE system.

The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a)
implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other
words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit
Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number
of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is
more general than tuning for a specific core like Neoverse V1 but is more specific than
the default tuning described below.

Additionally on native AArch64 GNU/Linux systems the value native tunes performance to
the host system. This option has no effect if the compiler is unable to recognize the
processor of the host system.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well
across a range of target processors.

This option cannot be suffixed by feature modifiers.

-mcpu=name
Specify the name of the target processor, optionally suffixed by one or more feature
modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible
values for cpu are the same as those available for -mtune. The permissible values for
feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu
Feature Modifiers. Where conflicting feature modifiers are specified, the right-most
feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly
code (as if by -march) and to determine the target processor for which to tune for
performance (as if by -mtune). Where this option is used in conjunction with -march or
-mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but
instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector
bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb
generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first
Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb
tunes code in the same way as for -mtune=neoverse-512tvb.

-moverride=string
Override tuning decisions made by the back-end in response to a -mtune= switch. The
syntax, semantics, and accepted values for string in this option are not guaranteed to be
consistent across releases.

This option is only intended to be useful when developing GCC.

-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for
use in debugging the compiler.

-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option literal pools are accessed
using a single instruction and emitted after each function. This limits the maximum size
of functions to 1MB. This is enabled by default for -mcmodel=tiny.

-msign-return-address=scope
Select the function scope on which return address signing will be applied. Permissible
values are none, which disables return address signing, non-leaf, which enables pointer
signing for functions which are not leaf functions, and all, which enables pointer
signing for all functions. The default value is none. This option has been deprecated by
-mbranch-protection.

-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti
Select the branch protection features to use. none is the default and turns off all
types of branch protection. standard turns on all types of branch protection features.
If a feature has additional tuning options, then standard sets it to its standard level.
pac-ret[+leaf] turns on return address signing to its standard level: signing functions
that save the return address to memory (non-leaf functions will practically always do
this) using the a-key. The optional argument leaf can be used to extend the signing to
include leaf functions. The optional argument b-key can be used to sign the functions
with the B-key instead of the A-key. bti turns on branch target identification
mechanism.

-mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS). opts is a comma-
separated list of the following options:

retbr
blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables
all SLS hardening.

-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option only has an effect
when SVE is enabled.

GCC supports two forms of SVE code generation: "vector-length agnostic" output that works
with any size of vector register and "vector-length specific" output that allows GCC to
make assumptions about the vector length when it is useful for optimization reasons. The
possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable
selects vector-length agnostic output. At present -msve-vector-bits=128 also generates
vector-length agnostic output for big-endian targets. All other values generate vector-
length specific code. The behavior of these values may change in future releases and no
value except scalable should be relied on for producing code that is portable across
different hardware SVE vector lengths.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.

-march and -mcpu Feature Modifiers

Feature modifiers used with -march and -mcpu can be any of the following and their inverses
nofeature:

crc Enable CRC extension. This is on by default for -march=armv8.1-a.

crypto
Enable Crypto extension. This also enables Advanced SIMD and floating-point
instructions.

fp Enable floating-point instructions. This is on by default for all possible values for
options -march and -mcpu.

simd
Enable Advanced SIMD instructions. This also enables floating-point instructions. This
is on by default for all possible values for options -march and -mcpu.

sve Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and
floating-point instructions.

lse Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.

rdma
Enable Round Double Multiply Accumulate instructions. This is on by default for
-march=armv8.1-a.

fp16
Enable FP16 extension. This also enables floating-point instructions.

fp16fml
Enable FP16 fmla extension. This also enables FP16 extensions and floating-point
instructions. This option is enabled by default for -march=armv8.4-a. Use of this option
with architectures prior to Armv8.2-A is not supported.

rcpc
Enable the RCpc extension. This enables the use of the LDAPR instructions for load-
acquire atomic semantics, and passes it on to the assembler, enabling inline asm
statements to use instructions from the RCpc extension.

dotprod
Enable the Dot Product extension. This also enables Advanced SIMD instructions.

aes Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD
instructions.

sha2
Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.

sha3
Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD
instructions. Use of this option with architectures prior to Armv8.2-A is not supported.

sm4 Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions.
Use of this option with architectures prior to Armv8.2-A is not supported.

profile
Enable the Statistical Profiling extension. This option is only to enable the extension
at the assembler level and does not affect code generation.

rng Enable the Armv8.5-a Random Number instructions. This option is only to enable the
extension at the assembler level and does not affect code generation.

memtag
Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures
prior to Armv8.5-A is not supported.

sb Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the
extension at the assembler level and does not affect code generation. This option is
enabled by default for -march=armv8.5-a.

ssbs
Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to
enable the extension at the assembler level and does not affect code generation. This
option is enabled by default for -march=armv8.5-a.

predres
Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option
is only to enable the extension at the assembler level and does not affect code
generation. This option is enabled by default for -march=armv8.5-a.

sve2
Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.

sve2-bitperm
Enable SVE2 bitperm instructions. This also enables SVE2 instructions.

sve2-sm4
Enable SVE2 sm4 instructions. This also enables SVE2 instructions.

sve2-aes
Enable SVE2 aes instructions. This also enables SVE2 instructions.

sve2-sha3
Enable SVE2 sha3 instructions. This also enables SVE2 instructions.

tme Enable the Transactional Memory Extension.

i8mm
Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and
floating-point instructions. This option is enabled by default for -march=armv8.6-a.
Use of this option with architectures prior to Armv8.2-A is not supported.

f32mm
Enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE
instructions. Use of this option with architectures prior to Armv8.2-A is not supported.

f64mm
Enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE
instructions. Use of this option with architectures prior to Armv8.2-A is not supported.

bf16
Enable brain half-precision floating-point instructions. This also enables Advanced SIMD
and floating-point instructions. This option is enabled by default for -march=armv8.6-a.
Use of this option with architectures prior to Armv8.2-A is not supported.

ls64
Enable the 64-byte atomic load and store instructions for accelerators. This option is
enabled by default for -march=armv8.7-a.

mops
Enable the instructions to accelerate memory operations like "memcpy", "memmove",
"memset". This option is enabled by default for -march=armv8.8-a

flagm
Enable the Flag Manipulation instructions Extension.

pauth
Enable the Pointer Authentication Extension.

cssc
Enable the Common Short Sequence Compression instructions.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies
nosimd, which implies nocrypto, noaes and nosha2.

Adapteva Epiphany Options

These -m options are defined for Adapteva Epiphany:

-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to run on
hardware variants that lack these registers.

-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction generation. This can
result in increased instruction count, so this may either reduce or increase overall code
size.

-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a
heuristic and is not guaranteed to produce consistent results across releases.

-mcmove
Enable the generation of conditional moves.

-mnops=num
Emit num NOPs before every other generated instruction.

-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub" instruction and test the
flags. This is faster than a software comparison, but can get incorrect results in the
presence of NaNs, or when two different small numbers are compared such that their
difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but
IEEE-compliant, software comparisons.

-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8
means that the eight bytes in the range "sp+0...sp+7" can be used by leaf functions
without stack allocation. Values other than 8 or 16 are untested and unlikely to work.
Note also that this option changes the ABI; compiling a program with a different stack
offset than the libraries have been compiled with generally does not work. This option
can be useful if you want to evaluate if a different stack offset would give you better
code, but to actually use a different stack offset to build working programs, it is
recommended to configure the toolchain with the appropriate --with-stack-offset=num
option.

-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating. The default
is -mround-nearest.

-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset
range of the "b" / "bl" instructions, and therefore load the function address into a
register before performing a (otherwise direct) call. This is the default.

-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of
the "b" / "bl" instructions, so use these instructions for direct calls. The default is
-mlong-calls.

-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to
function addresses for which -mlong-calls semantics are in effect.

-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines the floating-point
mode that is provided and expected at function call and return time. Making this mode
match the mode you predominantly need at function start can make your programs smaller
and faster by avoiding unnecessary mode switches.

mode can be set to one the following values:

caller
Any mode at function entry is valid, and retained or restored when the function
returns, and when it calls other functions. This mode is useful for compiling
libraries or other compilation units you might want to incorporate into different
programs with different prevailing FPU modes, and the convenience of being able to
use a single object file outweighs the size and speed overhead for any extra mode
switching that might be needed, compared with what would be needed with a more
specific choice of prevailing FPU mode.

truncate
This is the mode used for floating-point calculations with truncating (i.e. round
towards zero) rounding mode. That includes conversion from floating point to
integer.

round-nearest
This is the mode used for floating-point calculations with round-to-nearest-or-even
rounding mode.

int This is the mode used to perform integer calculations in the FPU, e.g. integer
multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller

-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation
of post-increment addresses, and generation of post-modify addresses. The defaults are
msplit-lohi, -mpost-inc, and -mpost-modify.

-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses
DImode as preferred SIMD mode.

-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8.
Note that this is an ABI change, even though many library function interfaces are
unaffected if they don't use SIMD vector modes in places that affect size and/or
alignment of relevant types.

-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this can give better
register allocation, but so far the reverse seems to be generally the case.

-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative constants
and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use
of that register as a fixed register, and none, which means that no register is used for
this purpose. The default is -m1reg-none.

AMD GCN Options

These options are defined specifically for the AMD GCN port.

-march=gpu
-mtune=gpu
Set architecture type or tuning for gpu. Supported values for gpu are

fiji
Compile for GCN3 Fiji devices (gfx803).

gfx900
Compile for GCN5 Vega 10 devices (gfx900).

gfx906
Compile for GCN5 Vega 20 devices (gfx906).

gfx908
Compile for CDNA1 Instinct MI100 series devices (gfx908).

gfx90a
Compile for CDNA2 Instinct MI200 series devices (gfx90a).

-msram-ecc=on
-msram-ecc=off
-msram-ecc=any
Compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or
either mode. This feature can be enabled per-process on some devices. The compiled code
must match the device mode. The default is any, for devices that support it.

-mstack-size=bytes
Specify how many bytes of stack space will be requested for each GPU thread (wave-front).
Beware that there may be many threads and limited memory available. The size of the
stack allocation may also have an impact on run-time performance. The default is 32KB
when using OpenACC or OpenMP, and 1MB otherwise.

-mxnack
Compile binaries suitable for devices with the XNACK feature enabled. Some devices
always require XNACK and some allow the user to configure XNACK. The compiled code must
match the device mode. The default is -mno-xnack. At present this option is a
placeholder for support that is not yet implemented.

ARC Options

The following options control the architecture variant for which code is being compiled:

-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the default unless
-mcpu=ARC601 or -mcpu=ARCEM is in effect.

-mjli-always
Force to call a function using jli_s instruction. This option is valid only for ARCv2
architecture.

-mcpu=cpu
Set architecture type, register usage, and instruction scheduling parameters for cpu.
There are also shortcut alias options available for backward compatibility and
convenience. Supported values for cpu are

arc600
Compile for ARC600. Aliases: -mA6, -mARC600.

arc601
Compile for ARC601. Alias: -mARC601.

arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured
with --with-cpu=arc700.

arcem
Compile for ARC EM.

archs
Compile for ARC HS.

em Compile for ARC EM CPU with no hardware extensions.

em4 Compile for ARC EM4 CPU.

em4_dmips
Compile for ARC EM4 DMIPS CPU.

em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.

em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist
instructions.

hs Compile for ARC HS CPU with no hardware extensions except the atomic instructions.

hs34
Compile for ARC HS34 CPU.

hs38
Compile for ARC HS38 CPU.

hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.

hs4x
Compile for ARC HS4x CPU.

hs4xd
Compile for ARC HS4xD CPU.

hs4x_rel31
Compile for ARC HS4x CPU release 3.10a.

arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.

arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.

arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.

arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.

arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.

arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.

nps400
Compile for ARC 700 on NPS400 chip.

em_mini
Compile for ARC EM minimalist configuration featuring reduced register set.

-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact implementation.

-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast implementation.

-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux registers.

-mea
Generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and
"sat16" are supported. Only valid for -mcpu=ARC700.

-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This option is deprecated.

-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.

-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.

-mnorm
Generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.

-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact implementation.

-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast implementation.

-msimd
Enable generation of ARC SIMD instructions via target-specific builtins. Only valid for
-mcpu=ARC700.

-msoft-float
This option ignored; it is provided for compatibility purposes only. Software floating-
point code is emitted by default, and this default can overridden by FPX options; -mspfp,
-mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or
-mdpfp-fast for double precision.

-mswap
Generate "swap" instructions.

-matomic
This enables use of the locked load/store conditional extension to implement atomic
memory built-in functions. Not available for ARC 6xx or ARC EM cores.

-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.

-mcode-density
Enable code density instructions for ARC EM. This option is on by default for ARC HS.

-mll64
Enable double load/store operations for ARC HS cores.

-mtp-regno=regno
Specify thread pointer register number.

-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can specify the option using
either a string or numeric value for multo. wlh1 is the default value. The recognized
values are:

0
none
No multiplier available.

1
w 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw"
and "mpyuw".

2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".

3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".

4
wlh3
Two 16x16 multipliers, blocking, sequential. The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".

5
wlh4
One 16x16 multiplier, blocking, sequential. The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".

6
wlh5
One 32x4 multiplier, blocking, sequential. The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".

7
plus_dmpy
ARC HS SIMD support.

8
plus_macd
ARC HS SIMD support.

9
plus_qmacw
ARC HS SIMD support.

This option is only available for ARCv2 cores.

-mfpu=fpu
Enables support for specific floating-point hardware extensions for ARCv2 cores.
Supported values for fpu are:

fpus
Enables support for single-precision floating-point hardware extensions.

fpud
Enables support for double-precision floating-point hardware extensions. The single-
precision floating-point extension is also enabled. Not available for ARC EM.

fpuda
Enables support for double-precision floating-point hardware extensions using double-
precision assist instructions. The single-precision floating-point extension is also
enabled. This option is only available for ARC EM.

fpuda_div
Enables support for double-precision floating-point hardware extensions using double-
precision assist instructions. The single-precision floating-point, square-root, and
divide extensions are also enabled. This option is only available for ARC EM.

fpuda_fma
Enables support for double-precision floating-point hardware extensions using double-
precision assist instructions. The single-precision floating-point and fused
multiply and add hardware extensions are also enabled. This option is only available
for ARC EM.

fpuda_all
Enables support for double-precision floating-point hardware extensions using double-
precision assist instructions. All single-precision floating-point hardware
extensions are also enabled. This option is only available for ARC EM.

fpus_div
Enables support for single-precision floating-point, square-root and divide hardware
extensions.

fpud_div
Enables support for double-precision floating-point, square-root and divide hardware
extensions. This option includes option fpus_div. Not available for ARC EM.

fpus_fma
Enables support for single-precision floating-point and fused multiply and add
hardware extensions.

fpud_fma
Enables support for double-precision floating-point and fused multiply and add
hardware extensions. This option includes option fpus_fma. Not available for ARC
EM.

fpus_all
Enables support for all single-precision floating-point hardware extensions.

fpud_all
Enables support for all single- and double-precision floating-point hardware
extensions. Not available for ARC EM.

-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor automatically saves/restores on
interrupt entry and exit. register-range is specified as two registers separated by a
dash. The register range always starts with "r0", the upper limit is "fp" register.
blink and lp_count are optional. This option is only valid for ARC EM and ARC HS cores.

-mrgf-banked-regs=number
Specifies the number of registers replicated in second register bank on entry to fast
interrupt. Fast interrupts are interrupts with the highest priority level P0. These
interrupts save only PC and STATUS32 registers to avoid memory transactions during
interrupt entry and exit sequences. Use this option when you are using fast interrupts
in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.

-mlpc-width=width
Specify the width of the "lp_count" register. Valid values for width are 8, 16, 20, 24,
28 and 32 bits. The default width is fixed to 32 bits. If the width is less than 32,
the compiler does not attempt to transform loops in your program to use the zero-delay
loop mechanism unless it is known that the "lp_count" register can hold the required
loop-counter value. Depending on the width specified, the compiler and run-time library
might continue to use the loop mechanism for various needs. This option defines macro
"__ARC_LPC_WIDTH__" with the value of width.

-mrf16
This option instructs the compiler to generate code for a 16-entry register file. This
option defines the "__ARC_RF16__" preprocessor macro.

-mbranch-index
Enable use of "bi" or "bih" instructions to implement jump tables.

The following options are passed through to the assembler, and also define preprocessor macro
symbols.

-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions. Also sets the
preprocessor symbol "__Xdsp_packa". This option is deprecated.

-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets
the preprocessor symbol "__Xdvbf". This option is deprecated.

-mlock
Passed down to the assembler to enable the locked load/store conditional extension. Also
sets the preprocessor symbol "__Xlock".

-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16". This
option is deprecated.

-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24". This
option is deprecated.

-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp counter extension
instruction. Also sets the preprocessor symbol "__Xrtsc". This option is deprecated.

-mswape
Passed down to the assembler to enable the swap byte ordering extension instruction.
Also sets the preprocessor symbol "__Xswape".

-mtelephony
Passed down to the assembler to enable dual- and single-operand instructions for
telephony. Also sets the preprocessor symbol "__Xtelephony". This option is deprecated.

-mxy
Passed down to the assembler to enable the XY memory extension. Also sets the
preprocessor symbol "__Xxy".

The following options control how the assembly code is annotated:

-misize
Annotate assembler instructions with estimated addresses.

-mannotate-align
Explain what alignment considerations lead to the decision to make an instruction short
or long.

The following options are passed through to the linker:

-marclinux
Passed through to the linker, to specify use of the "arclinux" emulation. This option is
enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets when profiling is not requested.

-marclinux_prof
Passed through to the linker, to specify use of the "arclinux_prof" emulation. This
option is enabled by default in tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is requested.

The following options control the semantics of generated code:

-mlong-calls
Generate calls as register indirect calls, thus providing access to the full 32-bit
address range.

-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is the offset available for
an unconditional branch-and-link instruction. Conditional execution of function calls is
suppressed, to allow use of the 25-bit range, rather than the 21-bit range with
conditional branch-and-link. This is the default for tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc" targets.

-G num
Put definitions of externally-visible data in a small data section if that data is no
bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8
when we have double load/store operations.

-mno-sdata
Do not generate sdata references. This is the default for tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc" targets.

-mvolatile-cache
Use ordinarily cached memory accesses for volatile references. This is the default.

-mno-volatile-cache
Enable cache bypass for volatile references.

The following options fine tune code generation:

-malign-call
Does nothing. Preserved for backward compatibility.

-mauto-modify-reg
Enable the use of pre/post modify with register displacement.

-mbbit-peephole
Enable bbit peephole2.

-mno-brcc
This option disables a target-specific pass in arc_reorg to generate compare-and-branch
("brcc") instructions. It has no effect on generation of these instructions driven by
the combiner pass.

-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening. This is the default
for -Os.

-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os, and only available for
ARCv1 cores. This option is deprecated.

-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional execution instructions.

Due to delay slot scheduling and interactions between operand numbers, literal sizes,
instruction lengths, and the support for conditional execution, the target-independent
pass to generate conditional execution is often lacking, so the ARC port has kept a
special pass around that tries to find more conditional execution generation
opportunities after register allocation, branch shortening, and delay slot scheduling
have been done. This pass generally, but not always, improves performance and code size,
at the cost of extra compilation time, which is why there is an option to switch it off.
If you have a problem with call instructions exceeding their allowable offset range
because they are conditionalized, you should consider using -mmedium-calls instead.

-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.

-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. This option
is deprecated.

-mindexed-loads
Enable the use of indexed loads. This can be problematic because some optimizers then
assume that indexed stores exist, which is not the case.

-mlra
Enable Local Register Allocation. This is still experimental for ARC, so by default the
compiler uses standard reload (i.e. -mno-lra).

-mlra-priority-none
Don't indicate any priority for target registers.

-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.

-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.

-mmillicode
When optimizing for size (using -Os), prologues and epilogues that have to save or
restore a large number of registers are often shortened by using call to a special
function in libgcc; this is referred to as a millicode call. As these calls can pose
performance issues, and/or cause linking issues when linking in a nonstandard way, this
option is provided to turn on or off millicode call generation.

-mcode-density-frame
This option enable the compiler to emit "enter" and "leave" instructions. These
instructions are only valid for CPUs with code-density feature.

-mmixed-code
Does nothing. Preserved for backward compatibility.

-mq-class
Ths option is deprecated. Enable q instruction alternatives. This is the default for
-Os.

-mRcq
Does nothing. Preserved for backward compatibility.

-mRcw
Does nothing. Preserved for backward compatibility.

-msize-level=level
Fine-tune size optimization with regards to instruction lengths and alignment. The
recognized values for level are:

0 No size optimization. This level is deprecated and treated like 1.

1 Short instructions are used opportunistically.

2 In addition, alignment of loops and of code after barriers are dropped.

3 In addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set
is equivalent to level 1.

-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any implied by -mcpu=.

Supported values for cpu are

ARC600
Tune for ARC600 CPU.

ARC601
Tune for ARC601 CPU.

ARC700
Tune for ARC700 CPU with standard multiplier block.

ARC700-xmac
Tune for ARC700 CPU with XMAC block.

ARC725D
Tune for ARC725D CPU.

ARC750D
Tune for ARC750D CPU.

core3
Tune for ARCv2 core3 type CPU. This option enable usage of "dbnz" instruction.

release31a
Tune for ARC4x release 3.10a.

-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to a normal instruction.

-munalign-prob-threshold=probability
Does nothing. Preserved for backward compatibility.

The following options are maintained for backward compatibility, but are now deprecated and
will be removed in a future release:

-margonaut
Obsolete FPX.

-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now deprecated. Big-endian
code is supported by configuring GCC to build "arceb-elf32" and "arceb-linux-uclibc"
targets, for which big endian is the default.

-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is now deprecated. Little-
endian code is supported by configuring GCC to build "arc-elf32" and "arc-linux-uclibc"
targets, for which little endian is the default.

-mbarrel_shifter
Replaced by -mbarrel-shifter.

-mdpfp_compact
Replaced by -mdpfp-compact.

-mdpfp_fast
Replaced by -mdpfp-fast.

-mdsp_packa
Replaced by -mdsp-packa.

-mEA
Replaced by -mea.

-mmac_24
Replaced by -mmac-24.

-mmac_d16
Replaced by -mmac-d16.

-mspfp_compact
Replaced by -mspfp-compact.

-mspfp_fast
Replaced by -mspfp-fast.

-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601,
ARC700 and ARC700-xmac respectively.

-multcost=num
Replaced by -mmultcost.

ARM Options

These -m options are defined for the ARM port:

-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs,
aapcs-linux and iwmmxt.

-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all
functions, even if this is not strictly necessary for correct execution of the code.
Specifying -fomit-frame-pointer with this option causes the stack frames not to be
generated for leaf functions. The default is -mno-apcs-frame. This option is
deprecated.

-mapcs
This is a synonym for -mapcs-frame and is deprecated.

-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets. Without
this option, on pre-v5 architectures, the two instruction sets cannot be reliably used
inside one program. The default is -mno-thumb-interwork, since slightly larger code is
generated when -mthumb-interwork is specified. In AAPCS configurations this option is
meaningless.

-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or the merging of those
instruction with the instructions in the function's body. This means that all functions
start with a recognizable set of instructions (or in fact one of a choice from a small
set of different function prologues), and this information can be used to locate the
start of functions inside an executable piece of code. The default is -msched-prolog.

-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and
hard.

Specifying soft causes GCC to generate output containing library calls for floating-point
operations. softfp allows the generation of code using hardware floating-point
instructions, but still uses the soft-float calling conventions. hard allows generation
of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and
soft-float ABIs are not link-compatible; you must compile your entire program with the
same ABI, and link with a compatible set of libraries.

-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all
standard configurations.

-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code
for a little-endian processor.

-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32 formats. The option has no
effect for little-endian images and is ignored. The default is dependent on the selected
target architecture. For ARMv6 and later architectures the default is BE8, for older
architectures the default is BE32. BE32 format has been deprecated by ARM.

-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC uses this name to determine
what kind of instructions it can emit when generating assembly code. This option can be
used in conjunction with or instead of the -mcpu= option.

Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2,
armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a,
armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m,
armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, armv9-a, iwmmxt and iwmmxt2.

Additionally, the following architectures, which lack support for the Thumb execution
state, are recognized but support is deprecated: armv4.

Many of the architectures support extensions. These can be added by appending +extension
to the architecture name. Extension options are processed in order and capabilities
accumulate. An extension will also enable any necessary base extensions upon which it
depends. For example, the +crypto extension will always enable the +simd extension. The
exception to the additive construction is for extensions that are prefixed with +no...:
these extensions disable the specified option and any other extensions that may depend on
the presence of that extension.

For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4
since the +simd option is entirely disabled by the +nofp option that follows it.

Most extension names are generically named, but have an effect that is dependent upon the
architecture to which it is applied. For example, the +simd option can be applied to
both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced
SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.

The table below lists the supported extensions for each architecture. Architectures not
mentioned do not support any extensions.

armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an
alias for this extension.

+nofp
Disable the floating-point instructions.

armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.

+fp The VFPv3 floating-point instructions, with 16 double-precision registers. The
extension +vfpv3-d16 can be used as an alias for this extension. Note that
floating-point is not supported by the base ARMv7-M architecture, but is
compatible with both the ARMv7-A and ARMv7-R architectures.

+nofp
Disable the floating-point instructions.

armv7-a
+mp The multiprocessing extension.

+sec
The security extension.

+fp The VFPv3 floating-point instructions, with 16 double-precision registers. The
extension +vfpv3-d16 can be used as an alias for this extension.

+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The
extensions +neon and +neon-vfpv3 can be used as aliases for this extension.

+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision registers.

+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision registers and the
half-precision floating-point conversion operations.

+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision registers and the
half-precision floating-point conversion operations.

+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision registers.

+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision registers.

+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the
half-precision floating-point conversion operations.

+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.

+nosimd
Disable the Advanced SIMD instructions (does not disable floating point).

+nofp
Disable the floating-point and Advanced SIMD instructions.

armv7ve
The extended version of the ARMv7-A architecture with support for virtualization.

+fp The VFPv4 floating-point instructions, with 16 double-precision registers. The
extension +vfpv4-d16 can be used as an alias for this extension.

+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The
extension +neon-vfpv4 can be used as an alias for this extension.

+vfpv3-d16
The VFPv3 floating-point instructions, with 16 double-precision registers.

+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision registers.

+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision registers and the
half-precision floating-point conversion operations.

+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision registers and the
half-precision floating-point conversion operations.

+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision registers.

+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision registers.

+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The
extension +neon-vfpv3 can be used as an alias for this extension.

+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the
half-precision floating-point conversion operations.

+nosimd
Disable the Advanced SIMD instructions (does not disable floating point).

+nofp
Disable the floating-point and Advanced SIMD instructions.

armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.

+simd
The ARMv8-A Advanced SIMD and floating-point instructions.

+crypto
The cryptographic instructions.

+nocrypto
Disable the cryptographic instructions.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

+sb Speculation Barrier Instruction.

+predres
Execution and Data Prediction Restriction Instructions.

armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.

+crypto
The cryptographic instructions. This also enables the Advanced SIMD and
floating-point instructions.

+nocrypto
Disable the cryptographic instructions.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

+sb Speculation Barrier Instruction.

+predres
Execution and Data Prediction Restriction Instructions.

armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing instructions. This also
enables the Advanced SIMD and floating-point instructions.

+fp16fml
The half-precision floating-point fmla extension. This also enables the half-
precision floating-point extension and Advanced SIMD and floating-point
instructions.

+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.

+crypto
The cryptographic instructions. This also enables the Advanced SIMD and
floating-point instructions.

+dotprod
Enable the Dot Product extension. This also enables Advanced SIMD instructions.

+nocrypto
Disable the cryptographic extension.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

+sb Speculation Barrier Instruction.

+predres
Execution and Data Prediction Restriction Instructions.

+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and
floating-point instructions.

+bf16
Brain half-precision floating-point instructions. This also enables Advanced
SIMD and floating-point instructions.

armv8.4-a
+fp16
The half-precision floating-point data processing instructions. This also
enables the Advanced SIMD and floating-point instructions as well as the Dot
Product extension and the half-precision floating-point fmla extension.

+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot
Product extension.

+crypto
The cryptographic instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product extension.

+nocrypto
Disable the cryptographic extension.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

+sb Speculation Barrier Instruction.

+predres
Execution and Data Prediction Restriction Instructions.

+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and
floating-point instructions.

+bf16
Brain half-precision floating-point instructions. This also enables Advanced
SIMD and floating-point instructions.

armv8.5-a
+fp16
The half-precision floating-point data processing instructions. This also
enables the Advanced SIMD and floating-point instructions as well as the Dot
Product extension and the half-precision floating-point fmla extension.

+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot
Product extension.

+crypto
The cryptographic instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product extension.

+nocrypto
Disable the cryptographic extension.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and
floating-point instructions.

+bf16
Brain half-precision floating-point instructions. This also enables Advanced
SIMD and floating-point instructions.

armv8.6-a
+fp16
The half-precision floating-point data processing instructions. This also
enables the Advanced SIMD and floating-point instructions as well as the Dot
Product extension and the half-precision floating-point fmla extension.

+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot
Product extension.

+crypto
The cryptographic instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product extension.

+nocrypto
Disable the cryptographic extension.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and
floating-point instructions.

+bf16
Brain half-precision floating-point instructions. This also enables Advanced
SIMD and floating-point instructions.

armv7-r
+fp.sp
The single-precision VFPv3 floating-point instructions. The extension +vfpv3xd
can be used as an alias for this extension.

+fp The VFPv3 floating-point instructions with 16 double-precision registers. The
extension +vfpv3-d16 can be used as an alias for this extension.

+vfpv3xd-d16-fp16
The single-precision VFPv3 floating-point instructions with 16 double-precision
registers and the half-precision floating-point conversion operations.

+vfpv3-d16-fp16
The VFPv3 floating-point instructions with 16 double-precision registers and the
half-precision floating-point conversion operations.

+nofp
Disable the floating-point extension.

+idiv
The ARM-state integer division instructions.

+noidiv
Disable the ARM-state integer division extension.

armv7e-m
+fp The single-precision VFPv4 floating-point instructions.

+fpv5
The single-precision FPv5 floating-point instructions.

+fp.dp
The single- and double-precision FPv5 floating-point instructions.

+nofp
Disable the floating-point extensions.

armv8.1-m.main
+dsp
The DSP instructions.

+mve
The M-Profile Vector Extension (MVE) integer instructions.

+mve.fp
The M-Profile Vector Extension (MVE) integer and single precision floating-point
instructions.

+fp The single-precision floating-point instructions.

+fp.dp
The single- and double-precision floating-point instructions.

+nofp
Disable the floating-point extension.

+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to
the numbers given in the options in the range 0 to 7.

+pacbti
Enable the Pointer Authentication and Branch Target Identification Extension.

armv8-m.main
+dsp
The DSP instructions.

+nodsp
Disable the DSP extension.

+fp The single-precision floating-point instructions.

+fp.dp
The single- and double-precision floating-point instructions.

+nofp
Disable the floating-point extension.

+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to
the numbers given in the options in the range 0 to 7.

armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.

+fp.sp
The single-precision FPv5 floating-point instructions.

+simd
The ARMv8-A Advanced SIMD and floating-point instructions.

+crypto
The cryptographic instructions.

+nocrypto
Disable the cryptographic instructions.

+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.

-march=native causes the compiler to auto-detect the architecture of the build computer.
At present, this feature is only supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the option has no effect.

-mtune=name
This option specifies the name of the target ARM processor for which GCC should tune the
performance of the code. For some ARM implementations better performance can be obtained
by using this option. Permissible names are: arm7tdmi, arm7tdmi-s, arm710t, arm720t,
arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9,
arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s,
arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s,
generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15,
cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72,
cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae,
cortex-a78c, cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8,
cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4,
cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m55, cortex-m85, cortex-x1,
cortex-x1c, cortex-m1.small-multiply, cortex-m0.small-multiply,
cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2,
neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626,
fa726te, star-mc1, xgene1.

Additionally, this option can specify that GCC should tune the performance of the code
for a big.LITTLE system. Permissible names are: cortex-a15.cortex-a7,
cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55,
cortex-a76.cortex-a55.

-mtune=generic-arch specifies that GCC should tune the performance for a blend of
processors within architecture arch. The aim is to generate code that run well on the
current most popular processors, balancing between optimizations that benefit some CPUs
in the range, and avoiding performance pitfalls of other CPUs. The effects of this
option may change in future GCC versions as CPU models come and go.

-mtune permits the same extension options as -mcpu, but the extension options do not
affect the tuning of the generated code.

-mtune=native causes the compiler to auto-detect the CPU of the build computer. At
present, this feature is only supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the option has no effect.

-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this name to derive the
name of the target ARM architecture (as if specified by -march) and the ARM processor
type for which to tune for performance (as if specified by -mtune). Where this option is
used in conjunction with -march or -mtune, those options take precedence over the
appropriate part of this option.

Many of the supported CPUs implement optional architectural extensions. Where this is so
the architectural extensions are normally enabled by default. If implementations that
lack the extension exist, then the extension syntax can be used to disable those
extensions that have been omitted. For floating-point and Advanced SIMD (Neon)
instructions, the settings of the options -mfloat-abi and -mfpu must also be considered:
floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set
to soft; and any setting of -mfpu other than auto will override the available floating-
point and SIMD extension instructions.

For example, cortex-a9 can be found in three major configurations: integer only, with
just a floating-point unit or with floating-point and Advanced SIMD. The default is to
enable all the instructions, but the extensions +nosimd and +nofp can be used to disable
just the SIMD or both the SIMD and floating-point instructions respectively.

Permissible names for this option are the same as those for -mtune.

The following extension options are common to the listed CPUs:

+nodsp
Disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m55 and cortex-m85.
Also disable the M-Profile Vector Extension (MVE) integer and single precision
floating-point instructions on cortex-m55 and cortex-m85.

+nopacbti
Disable the Pointer Authentication and Branch Target Identification Extension on
cortex-m85.

+nomve
Disable the M-Profile Vector Extension (MVE) integer and single precision floating-
point instructions on cortex-m55 and cortex-m85.

+nomve.fp
Disable the M-Profile Vector Extension (MVE) single precision floating-point
instructions on cortex-m55 and cortex-m85.

+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the
numbers given in the options in the range 0 to 7 on cortex-m55.

+nofp
Disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s,
arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8,
cortex-m4, cortex-m7, cortex-m33, cortex-m35p cortex-m4, cortex-m7, cortex-m33,
cortex-m35p, cortex-m55 and cortex-m85. Disables the floating-point and SIMD
instructions on generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a32, cortex-a35, cortex-a53 and cortex-a55.

+nofp.dp
Disables the double-precision component of the floating-point instructions on
cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.

+nosimd
Disables the SIMD (but not floating-point) instructions on generic-armv7-a,
cortex-a5, cortex-a7 and cortex-a9.

+crypto
Enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53,
cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53 and cortex-a75.cortex-a55.

Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision
registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3,
vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4.
The meanings are the same as for the extensions to -march=armv7-a.

-mcpu=generic-arch is also permissible, and is equivalent to -march=arch
-mtune=generic-arch. See -mtune for more information.

-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At
present, this feature is only supported on GNU/Linux, and not all architectures are
recognized. If the auto-detect is unsuccessful the option has no effect.

-mfpu=name
This specifies what floating-point hardware (or hardware emulation) is available on the
target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16,
vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16,
fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for
vfpv2.

The setting auto is the default and is special. It causes the compiler to select the
floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon),
note that floating-point operations are not generated by GCC's auto-vectorization pass
unless -funsafe-math-optimizations is also specified. This is because NEON hardware does
not fully implement the IEEE 754 standard for floating-point arithmetic (in particular
denormal values are treated as zero), so the use of NEON instructions may lead to a loss
of precision.

You can also set the fpu name at function level by using the target("fpu=") function
attributes or pragmas.

-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type. Permissible names
are none, ieee, and alternative; the default is none, in which case the "__fp16" type is
not defined.

-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple of the number of bits
set by this option. Permissible values are 8, 32 and 64. The default value varies for
different toolchains. For the COFF targeted toolchain the default value is 8. A value
of 64 is only allowed if the underlying ABI supports it.

Specifying a larger number can produce faster, more efficient code, but can also increase
the size of the program. Different values are potentially incompatible. Code compiled
with one value cannot necessarily expect to work with code or libraries compiled with
another value, if they exchange information using structures or unions.

This option is deprecated.

-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It is
executed if the function tries to return.

-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function
into a register and then performing a subroutine call on this register. This switch is
needed if the target function lies outside of the 64-megabyte addressing range of the
offset-based version of subroutine call instruction.

Even if this switch is enabled, not all function calls are turned into long calls. The
heuristic is that static functions, functions that have the "short_call" attribute,
functions that are inside the scope of a "#pragma no_long_calls" directive, and functions
whose definitions have already been compiled within the current compilation unit are not
turned into long calls. The exceptions to this rule are that weak function definitions,
functions with the "long_call" attribute or the "section" attribute, and functions that
are within the scope of a "#pragma long_calls" directive are always turned into long
calls.

This feature is not enabled by default. Specifying -mno-long-calls restores the default
behavior, as does placing the function calls within the scope of a "#pragma
long_calls_off" directive. Note these switches have no effect on how the compiler
generates code to handle function calls via function pointers.

-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the
prologue for each function. The runtime system is responsible for initializing this
register with an appropriate value before execution begins.

-mpic-register=reg
Specify the register to be used for PIC addressing. For standard PIC base case, the
default is any suitable register determined by compiler. For single PIC base case, the
default is R9 if target is EABI based or stack-checking is enabled, otherwise the default
is R10.

-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link
time. This permits using PC-relative addressing operations to access data known to be in
the data segment. For non-VxWorks RTP targets, this option is enabled by default. When
disabled on such targets, it will enable -msingle-pic-base by default.

-mpoke-function-name
Write the name of each function into the text section, directly preceding the function
prologue. The generated code is similar to this:

t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4

When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0".
If the trace function then looks at location "pc - 12" and the top 8 bits are set, then
we know that there is a function name embedded immediately preceding this location and
has length "((pc[-3]) & 0xff000000)".

-mthumb
-marm
Select between generating code that executes in ARM and Thumb states. The default for
most configurations is to generate code that executes in ARM state, but the default can
be changed by configuring GCC with the --with-mode=state configure option.

You can also override the ARM and Thumb mode for each function by using the
target("thumb") and target("arm") function attributes or pragmas.

-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is provided for regression
testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in
compiling code.

-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all
non-leaf functions. (A leaf function is one that does not call any other functions.)
The default is -mno-tpcs-frame.

-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all
leaf functions. (A leaf function is one that does not call any other functions.) The
default is -mno-apcs-leaf-frame.

-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set
header which switches to Thumb mode before executing the rest of the function. This
allows these functions to be called from non-interworking code. This option is not valid
in AAPCS configurations because interworking is enabled by default.

-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly
regardless of whether the target code has been compiled for interworking or not. There
is a small overhead in the cost of executing a function pointer if this option is
enabled. This option is not valid in AAPCS configurations because interworking is
enabled by default.

-mtp=name
Specify the access model for the thread local storage pointer. The valid models are
soft, which generates calls to "__aeabi_read_tp", cp15, which fetches the thread pointer
from "cp15" directly (supported in the arm6k architecture), and auto, which uses the best
available method for the selected processor. The default setting is auto.

-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two dialects are
supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting
local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme,
which provides better performance for shared libraries. The GNU descriptor scheme is
compatible with the original scheme, but does require new assembler, linker and library
support. Initial and local exec TLS models are unaffected by this option and always use
the original scheme.

-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is
enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this
restriction, and when -fpic or -fPIC is specified. This option conflicts with
-mslow-flash-data.

-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping
destination and base registers are used. This option avoids generating these
instructions. This option is enabled by default when -mcpu=cortex-m3 is specified.

-mfix-cortex-a57-aes-1742098
-mno-fix-cortex-a57-aes-1742098
-mfix-cortex-a72-aes-1655431
-mno-fix-cortex-a72-aes-1655431
Enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the
AES cryptographic instructions. This option is enabled by default when either
-mcpu=cortex-a57 or -mcpu=cortex-a72 is specified.

-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that
are not 16- or 32- bit aligned. By default unaligned access is disabled for all
pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other
architectures. If unaligned access is not enabled then words in packed data structures
are accessed a byte at a time.

The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to
either true or false, depending upon the setting of this option. If unaligned access is
enabled then the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined.

-mneon-for-64bits
This option is deprecated and has no effect.

-mslow-flash-data
Assume loading data from flash is slower than fetching instruction. Therefore literal
load is minimized for better performance. This option is only supported when compiling
for ARMv7 M-profile and off by default. It conflicts with -mword-relocations.

-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default is currently off which
implies divided syntax. This option has no impact on Thumb2. However, this may change in
future releases of GCC. Divided syntax should be considered deprecated.

-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only
contain a single 16-bit instruction from a select set of instructions. This option is on
by default for ARMv8-A Thumb mode.

-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is an option used only
for regression testing of the compiler and not intended for ordinary use in compiling
code. This option is disabled by default.

-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for
use in debugging the compiler.

-mpure-code
Do not allow constant data to be placed in code sections. Additionally, when compiling
for ELF object format give all text sections the ELF processor-specific section attribute
"SHF_ARM_PURECODE". This option is only available when generating non-pic code for
M-profile targets.

-mcmse
Generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development
Tools Engineering Specification", which can be found on
<https://developer.arm.com/documentation/ecm0359818/latest/>.

-mfix-cmse-cve-2021-35465
Mitigate against a potential security issue with the "VLLDM" instruction in some
M-profile devices when using CMSE (CVE-2021-365465). This option is enabled by default
when the option -mcpu= is used with "cortex-m33", "cortex-m35p", "cortex-m55",
"cortex-m85" or "star-mc1". The option -mno-fix-cmse-cve-2021-35465 can be used to
disable the mitigation.

-mstack-protector-guard=guard
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for
a global canary or tls for a canary accessible via the TLS register. The option
-mstack-protector-guard-offset= is for use with -fstack-protector-guard=tls and not for
use in user-land code.

-mfdpic
-mno-fdpic
Select the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to
functions. When the compiler is configured for "arm-*-uclinuxfdpiceabi" targets, this
option is on by default and implies -fPIE if none of the PIC/PIE-related options is
provided. On other targets, it only enables the FDPIC-specific code generation features,
and the user should explicitly provide the PIC/PIE-related options as needed.

Note that static linking is not supported because it would still involve the dynamic
linker when the program self-relocates. If such behavior is acceptable, use -static and
-Wl,-dynamic-linker options.

The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using
the same ("arm-*-uclinuxfdpiceabi") toolchain as the one used to build the userland
programs.

-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]
Enable branch protection features (armv8.1-m.main only). none generate code without
branch protection or return address signing. standard[+leaf] generate code with all
branch protection features enabled at their standard level. pac-ret[+leaf] generate code
with return address signing set to its standard level, which is to sign all functions
that save the return address to memory. leaf When return address signing is enabled,
also sign leaf functions even if they do not write the return address to memory. +bti
Add landing-pad instructions at the permitted targets of indirect branch instructions.

If the +pacbti architecture extension is not enabled, then all branch protection and
return address signing operations are constrained to use only the instructions defined in
the architectural-NOP space. The generated code will remain backwards-compatible with
earlier versions of the architecture, but the additional security can be enabled at run
time on processors that support the PACBTI extension.

Branch target enforcement using BTI can only be enabled at runtime if all code in the
application has been compiled with at least -mbranch-protection=bti.

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code without branch protection or return address signing.

AVR Options

These options are defined for AVR implementations:

-mmcu=mcu
Specify the AVR instruction set architecture (ISA) or device type. The default for this
option is "avr2".

The following AVR devices and ISAs are supported. Note: A complete device support
consists of startup code "crtmcu.o", a device header "avr/io*.h", a device library
"libmcu.a" and a device-specs ("https://gcc.gnu.org/wiki/avr-gcc#spec-files") file
"specs-mcu". Only the latter is provided by the compiler according the supported "mcu"s
below. The rest is supported by AVR-LibC ("https://www.nongnu.org/avr-libc/"), or by
means of "atpack" ("https://gcc.gnu.org/wiki/avr-gcc#atpack") files from the hardware
manufacturer.

"avr2"
"Classic" devices with up to 8 KiB of program memory. mcu = "attiny22", "attiny26",
"at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433",
"at90s4434", "at90c8534", "at90s8515", "at90s8535".

"avr25"
"Classic" devices with up to 8 KiB of program memory and with the "MOVW" instruction.
mcu = "attiny13", "attiny13a", "attiny24", "attiny24a", "attiny25", "attiny261",
"attiny261a", "attiny2313", "attiny2313a", "attiny43u", "attiny44", "attiny44a",
"attiny45", "attiny48", "attiny441", "attiny461", "attiny461a", "attiny4313",
"attiny84", "attiny84a", "attiny85", "attiny87", "attiny88", "attiny828",
"attiny841", "attiny861", "attiny861a", "ata5272", "ata6616c", "at86rf401".

"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = "at76c711",
"at43usb355".

"avr31"
"Classic" devices with 128 KiB of program memory. mcu = "atmega103", "at43usb320".

"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program memory and with the "MOVW"
instruction. mcu = "attiny167", "attiny1634", "atmega8u2", "atmega16u2",
"atmega32u2", "ata5505", "ata6617c", "ata664251", "at90usb82", "at90usb162".

"avr4"
"Enhanced" devices with up to 8 KiB of program memory. mcu = "atmega48",
"atmega48a", "atmega48p", "atmega48pa", "atmega48pb", "atmega8", "atmega8a",
"atmega8hva", "atmega88", "atmega88a", "atmega88p", "atmega88pa", "atmega88pb",
"atmega8515", "atmega8535", "ata5795", "ata6285", "ata6286", "ata6289", "ata6612c",
"at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".

"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = "atmega16",
"atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb",
"atmega16m1", "atmega16u4", "atmega161", "atmega162", "atmega163", "atmega164a",
"atmega164p", "atmega164pa", "atmega165", "atmega165a", "atmega165p", "atmega165pa",
"atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega168pb", "atmega169",
"atmega169a", "atmega169p", "atmega169pa", "atmega32", "atmega32a", "atmega32c1",
"atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6",
"atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega324pb", "atmega325",
"atmega325a", "atmega325p", "atmega325pa", "atmega328", "atmega328p", "atmega328pb",
"atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3250", "atmega3250a",
"atmega3250p", "atmega3250pa", "atmega3290", "atmega3290a", "atmega3290p",
"atmega3290pa", "atmega406", "atmega64", "atmega64a", "atmega64c1", "atmega64hve",
"atmega64hve2", "atmega64m1", "atmega64rfr2", "atmega640", "atmega644", "atmega644a",
"atmega644p", "atmega644pa", "atmega644rfr2", "atmega645", "atmega645a",
"atmega645p", "atmega649", "atmega649a", "atmega649p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega6490", "atmega6490a", "atmega6490p", "ata5790", "ata5790n",
"ata5791", "ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210", "ata8510",
"ata5787", "ata5835", "ata5700m322", "ata5702m322", "at90pwm161", "at90pwm216",
"at90pwm316", "at90can32", "at90can64", "at90scr100", "at90usb646", "at90usb647",
"at94k", "m3000".

"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu = "atmega128", "atmega128a",
"atmega128rfa1", "atmega128rfr2", "atmega1280", "atmega1281", "atmega1284",
"atmega1284p", "atmega1284rfr2", "at90can128", "at90usb1286", "at90usb1287".

"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory.
mcu = "atmega256rfr2", "atmega2560", "atmega2561", "atmega2564rfr2".

"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu =
"atxmega8e5", "atxmega16a4", "atxmega16a4u", "atxmega16c4", "atxmega16d4",
"atxmega16e5", "atxmega32a4", "atxmega32a4u", "atxmega32c3", "atxmega32c4",
"atxmega32d3", "atxmega32d4", "atxmega32e5", "avr64da28", "avr64da32", "avr64da48",
"avr64da64", "avr64db28", "avr64db32", "avr64db48", "avr64db64", "avr64dd14",
"avr64dd20", "avr64dd28", "avr64dd32", "avr64du28", "avr64du32", "avr64ea28",
"avr64ea32", "avr64ea48".

"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with
program memory visible in the RAM address space. mcu = "attiny202", "attiny204",
"attiny212", "attiny214", "attiny402", "attiny404", "attiny406", "attiny412",
"attiny414", "attiny416", "attiny416auto", "attiny417", "attiny424", "attiny426",
"attiny427", "attiny804", "attiny806", "attiny807", "attiny814", "attiny816",
"attiny817", "attiny824", "attiny826", "attiny827", "attiny1604", "attiny1606",
"attiny1607", "attiny1614", "attiny1616", "attiny1617", "attiny1624", "attiny1626",
"attiny1627", "attiny3214", "attiny3216", "attiny3217", "attiny3224", "attiny3226",
"attiny3227", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208",
"atmega3209", "atmega4808", "atmega4809", "avr16dd14", "avr16dd20", "avr16dd28",
"avr16dd32", "avr16du14", "avr16du20", "avr16du28", "avr16du32", "avr16ea28",
"avr16ea32", "avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28", "avr16eb32",
"avr32da28", "avr32da32", "avr32da48", "avr32db28", "avr32db32", "avr32db48",
"avr32dd14", "avr32dd20", "avr32dd28", "avr32dd32", "avr32du14", "avr32du20",
"avr32du28", "avr32du32", "avr32ea28", "avr32ea32", "avr32ea48".

"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu =
"atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3",
"atxmega64c3", "atxmega64d3", "atxmega64d4", "avr128da28", "avr128da32",
"avr128da48", "avr128da64", "avr128db28", "avr128db32", "avr128db48", "avr128db64".

"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more
than 64 KiB of RAM. mcu = "atxmega64a1", "atxmega64a1u".

"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory. mcu = "atxmega128a3",
"atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3",
"atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3",
"atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3",
"atxmega256d3", "atxmega384c3", "atxmega384d3".

"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM.
mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u".

"avrtiny"
"TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu = "attiny4",
"attiny5", "attiny9", "attiny10", "attiny102", "attiny104", "attiny20", "attiny40".

"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler only.
mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".

-mabsdata
Assume that all data in static storage can be accessed by LDS / STS instructions. This
option has only an effect on reduced Tiny devices like ATtiny40. See also the "absdata"
AVR Variable Attributes,variable attribute.

-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for
outgoing function arguments once in function prologue/epilogue. Without this option,
outgoing arguments are pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR so that
accumulating the stack space might lead to smaller executables because arguments need not
be removed from the stack after such a function call.

This option can lead to reduced code size for functions that perform several calls to
functions that get their arguments on the stack like calls to printf-like functions.

-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost. Reasonable values for
cost are small, non-negative integers. The default branch cost is 0.

-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code
size is smaller.

-mdouble=bits
-mlong-double=bits
Set the size (in bits) of the "double" or "long double" type, respectively. Possible
values for bits are 32 and 64. Whether or not a specific value for bits is allowed
depends on the "--with-double=" and "--with-long-double=" configure options
("https://gcc.gnu.org/install/configure.html#avr"), and the same applies for the default
values of the options.

-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo instruction supported by
GNU Binutils. If this option is on, the feature can still be disabled for individual
ISRs by means of the AVR Function Attributes,,"no_gccisr" function attribute. This
feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize
Options}), and if GNU Binutils support PR21683 ("https://sourceware.org/PR21683").

-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1
byte, an "int" is 1 byte, a "long" is 2 bytes, and "long long" is 4 bytes. Please note
that this option does not conform to the C standards, but it results in smaller code
size.

-mmain-is-OS_task
Do not save registers in "main". The effect is the same like attaching attribute AVR
Function Attributes,,"OS_task" to "main". It is activated per default if optimization is
on.

-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size is smaller.

-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP"
instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the
assembler's command line and the --relax option to the linker's command line.

Jump relaxing is performed by the linker because jump offsets are not known before code
is located. Therefore, the assembler code generated by the compiler is the same, but the
instructions in the executable may differ from instructions in the assembler code.

Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and
linker stubs below.

-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that "X" is only
used in indirect, post-increment or pre-decrement addressing.

Without this option, the "X" register may be used in the same way as "Y" or "Z" which
then is emulated by additional instructions. For example, loading a value with "X+const"
addressing with a small non-negative "const < 64" to a register Rn is performed as

adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const

-mtiny-stack
Only change the lower 8 bits of the stack pointer.

-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for fractional fixed-point
types.

-nodevicelib
Don't link against AVR-LibC's device specific library "lib<mcu>.a".

-nodevicespecs
Don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user
takes responsibility for supplying the sub-processes like compiler proper, assembler and
linker with appropriate command line options. This means that the user has to supply her
private device specs file by means of -specs=path-to-specs-file. There is no more need
for option -mmcu=mcu.

This option can also serve as a replacement for the older way of specifying custom
device-specs files that needed -B some-path to point to a directory which contains a
folder named "device-specs" which contains a specs file named "specs-mcu", where mcu was
specified by -mmcu=mcu.

-Waddr-space-convert
Warn about conversions between address spaces in the case where the resulting address
space is not contained in the incoming address space.

-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.

"EIND" and Devices with More Than 128 Ki Bytes of Flash

Pointers in the implementation are 16 bits wide. The address of a function or label is
represented as word address so that indirect jumps and calls can target any code address in
the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory
space, there is a special function register called "EIND" that serves as most significant
part of the target address when "EICALL" or "EIJMP" instructions are used.

Indirect jumps and calls on these devices are handled as follows by the compiler and are
subject to some limitations:

* The compiler never sets "EIND".

* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions or might read "EIND"
directly in order to emulate an indirect call/jump by means of a "RET" instruction.

* The compiler assumes that "EIND" never changes during the startup code or during the
application. In particular, "EIND" is not saved/restored in function or interrupt service
routine prologue/epilogue.

* For indirect calls to functions and computed goto, the linker generates stubs. Stubs are
jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a
stub. The stub contains a direct jump to the desired address.

* Linker relaxation must be turned on so that the linker generates the stubs correctly in
all situations. See the compiler option -mrelax and the linker option --relax. There are
corner cases where the linker is supposed to generate stubs but aborts without relaxation
and without a helpful error message.

* The default linker script is arranged for code with "EIND = 0". If code is supposed to
work for a setup with "EIND != 0", a custom linker script has to be used in order to
place the sections whose name start with ".trampolines" into the segment where "EIND"
points to.

* The startup code from libgcc never sets "EIND". Notice that startup code is a blend of
code from libgcc and AVR-LibC. For the impact of AVR-LibC on "EIND", see the AVR-
LibC user manual ("https://www.nongnu.org/avr-libc/user-manual/").

* It is legitimate for user-specific startup code to set up "EIND" early, for example by
means of initialization code located in section ".init3". Such code runs prior to general
startup code that initializes RAM and calls constructors, but after the bit of startup
code from AVR-LibC that sets "EIND" to the segment where the vector table is located.

#include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

The "__trampolines_start" symbol is defined in the linker script.

* Stubs are generated automatically by the linker if the following two conditions are met:

-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:

LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))

-<The final location of that label is in a code segment>
outside the segment where the stubs are located.

* The compiler emits such "gs" modifiers for code labels in the following situations:

-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.

-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line option.

-<C and C++ constructors/destructors called during startup/shutdown.>
-<If the tools hit a gs() modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:

int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}

Instead, a stub has to be set up, i.e. the function has to be called through a symbol
("func_4" in the example):

int main (void)
{
extern int func_4 (void);

/* Call function at byte address 0x4 */
return func_4();
}

and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, "func_4" can
be defined in the linker script.

Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers

Some AVR devices support memories larger than the 64 KiB range that can be accessed with
16-bit pointers. To access memory locations outside this 64 KiB range, the content of a
"RAMP" register is used as high part of the address: The "X", "Y", "Z" address register is
concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to
get a wide address. Similarly, "RAMPD" is used together with direct addressing.

* The startup code initializes the "RAMP" special function registers with zero.

* If a AVR Named Address Spaces,named address space other than generic or "__flash" is
used, then "RAMPZ" is set as needed before the operation.

* If the device supports RAM larger than 64 KiB and the compiler needs to change "RAMPZ" to
accomplish an operation, "RAMPZ" is reset to zero after the operation.

* If the device comes with a specific "RAMP" register, the ISR prologue/epilogue
saves/restores that SFR and initializes it with zero in case the ISR code might
(implicitly) use it.

* RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline
assembler to read from locations outside the 16-bit address range and change one of the
"RAMP" registers, you must reset it to zero after the access.

AVR Built-in Macros

GCC defines several built-in macros so that the user code can test for the presence or
absence of features. Almost any of the following built-in macros are deduced from device
capabilities and thus triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in
Functions.

"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies the architecture and
depends on the -mmcu=mcu option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",

respectively and

100, 102, 103, 104, 105, 106, 107

for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6",
"avrxmega7", respectively. If mcu specifies a device, this built-in macro is set
accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.

"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects the device's name. For
example, -mmcu=atmega8 defines the built-in macro "__AVR_ATmega8__", -mmcu=attiny261a
defines "__AVR_ATtiny261A__", etc.

The built-in macros' names follow the scheme "__AVR_Device__" where Device is the device
name as from the AVR user manual. The difference between Device in the built-in macro and
device in -mmcu=device is that the latter is always lowercase.

If device is not a device but only a core architecture like avr51, this macro is not
defined.

"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's name. For example, with
-mmcu=atmega8 the macro is defined to "atmega8".

If device is not a device but only a core architecture like avr51, this macro is not
defined.

"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.

"__AVR_HAVE_ADIW__"
The device has the "ADIW" and "SBIW" instructions.

"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.

"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.

"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.

"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-register moves.

"__AVR_HAVE_MUL__"
The device has a hardware multiplier.

"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case for devices with more
than 8 KiB of program memory.

"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with
more than 128 KiB of program memory. This also means that the program counter (PC) is 3
bytes wide.

"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB
of program memory.

"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the
compiler. The definition of these macros is affected by -mtiny-stack.

"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function register or has an
8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu=
and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.

"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register,
respectively.

"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.

"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a
hardware erratum. Skip instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The
second macro is only defined if "__AVR_HAVE_JMP_CALL__" is also set.

"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).

"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers directly like "IN", "OUT",
"SBI", etc. may use a different address as if addressed by an instruction to access RAM
like "LD" or "STS". This offset depends on the device architecture and has to be
subtracted from the RAM address in order to get the respective I/O address.

"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.

"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of "LD*" instructions. The flash
memory is seen in the data address space at an offset of "__AVR_PM_BASE_ADDRESS__". If
this macro is not defined, this feature is not available. If defined, the address space
is linear and there is no need to put ".rodata" into RAM. This is handled by the default
linker description file, and is currently available for "avrtiny" and "avrxmega3". Even
more convenient, there is no need to use address spaces like "__flash" or features like
attribute "progmem" and "pgm_read_*".

"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc
configure option.

"__HAVE_DOUBLE_MULTILIB__"
Defined if -mdouble= acts as a multilib option.

"__HAVE_DOUBLE32__"
"__HAVE_DOUBLE64__"
Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is
specified by option -mdouble=.

"__DEFAULT_DOUBLE__"
The size in bits of "double" if -mdouble= is not set. To test the layout of "double" in
a program, use the built-in macro "__SIZEOF_DOUBLE__".

"__HAVE_LONG_DOUBLE32__"
"__HAVE_LONG_DOUBLE64__"
"__HAVE_LONG_DOUBLE_MULTILIB__"
"__DEFAULT_LONG_DOUBLE__"
Same as above, but for "long double" instead of "double".

"__WITH_DOUBLE_COMPARISON__"
Reflects the "--with-double-comparison={tristate|bool|libf7}" configure option
("https://gcc.gnu.org/install/configure.html#avr") and is defined to 2 or 3.

"__WITH_LIBF7_LIBGCC__"
"__WITH_LIBF7_MATH__"
"__WITH_LIBF7_MATH_SYMBOLS__"
Reflects the "--with-libf7={libgcc|math|math-symbols}" configure option
("https://gcc.gnu.org/install/configure.html#avr").

AVR Internal Options

The following options are used internally by the compiler and to communicate between device
specs files and the compiler proper. You don't need to set these options by hand, in
particular they are not optimization options. Using these options in the wrong way may lead
to sub-optimal or wrong code. They are documented for completeness, and in order to get a
better understanding of device specs ("https://gcc.gnu.org/wiki/avr-gcc#spec-files") files.

-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB. This determines which
"__flashN" address spaces are available.

-mrmw
Assume that the device supports the Read-Modify-Write instructions "XCH", "LAC", "LAS"
and "LAT".

-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program memory. This option is used
for multilib generation and selection for the devices from architecture "avrxmega3".

-mskip-bug
Generate code without skips ("CPSE", "SBRS", "SBRC", "SBIS", "SBIC") over 32-bit
instructions.

-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the
stack pointer is zero. This option is used by the compiler to select and build multilibs
for architectures "avr2" and "avr25". These architectures mix devices with and without
"SPH".

Blackfin Options

-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512,
bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533,
bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m,
bf547m, bf548m, bf549m, bf561, bf592.

The optional sirevision specifies the silicon revision of the target Blackfin processor.
Any workarounds available for the targeted silicon revision are enabled. If sirevision
is none, no workarounds are enabled. If sirevision is any, all workarounds for the
targeted processor are enabled. The "__SILICON_REVISION__" macro is defined to two
hexadecimal digits representing the major and minor numbers in the silicon revision. If
sirevision is none, the "__SILICON_REVISION__" is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff. If this optional sirevision is not used,
GCC assumes the latest known silicon revision of the targeted Blackfin processor.

GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this
option causes the hardware BSP provided by libgloss to be linked in if -msim is not
given.

Without this option, bf532 is used as the processor by default.

Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is
defined.

-msim
Specifies that the program will be run on the simulator. This causes the simulator BSP
provided by libgloss to be linked in. This option has effect only for bfin-elf
toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.

-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the
instructions to save, set up and restore frame pointers and makes an extra register
available in leaf functions.

-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not contain speculative
loads after jump instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS"
is defined.

-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.

-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not contain CSYNC or
SSYNC instructions too soon after conditional branches. If this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is defined.

-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon
after a conditional branch.

-mlow64k
When enabled, the compiler is free to take advantage of the knowledge that the entire
program fits into the low 64k of memory.

-mno-low64k
Assume that the program is arbitrarily large. This is the default.

-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the uClinux
kernel.

-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for
execute in place and shared libraries in an environment without virtual memory
management. This option implies -fPIC. With a bfin-elf target, this option implies
-msim.

-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the
default.

-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but assumes that
this library or executable won't link against any other ID shared libraries. That allows
the compiler to use faster code for jumps and calls.

-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared libraries.
Slower code is generated for jump and call insns.

-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled.
Specifying a value of 0 generates more compact code; specifying other values forces the
allocation of that number to the current library but is no more space- or time-efficient
than omitting this option.

-msep-data
Generate code that allows the data segment to be located in a different area of memory
from the text segment. This allows for execute in place in an environment without
virtual memory management by eliminating relocations against the text section.

-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the
default.

-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function
into a register and then performing a subroutine call on this register. This switch is
needed if the target function lies outside of the 24-bit addressing range of the offset-
based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default
behavior. Note these switches have no effect on how the compiler generates code to
handle function calls via function pointers.

-mfast-fp
Link with the fast floating-point library. This library relaxes some of the IEEE
floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the
interest of performance.

-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind
locally. It has no effect without -mfdpic.

-mmulticore
Build a standalone application for multicore Blackfin processors. This option causes
proper start files and link scripts supporting multicore to be used, and defines the
macro "__BFIN_MULTICORE". It can only be used with -mcpu=bf561[-sirevision].

This option can be used with -mcorea or -mcoreb, which selects the one-application-per-
core programming model. Without -mcorea or -mcoreb, the single-application/dual-core
programming model is used. In this model, the main function of Core B should be named as
"coreb_main".

If this option is not used, the single-core application programming model is used.

-mcorea
Build a standalone application for Core A of BF561 when using the one-application-per-
core programming model. Proper start files and link scripts are used to support Core A,
and the macro "__BFIN_COREA" is defined. This option can only be used in conjunction
with -mmulticore.

-mcoreb
Build a standalone application for Core B of BF561 when using the one-application-per-
core programming model. Proper start files and link scripts are used to support Core B,
and the macro "__BFIN_COREB" is defined. When this option is used, "coreb_main" should be
used instead of "main". This option can only be used in conjunction with -mmulticore.

-msdram
Build a standalone application for SDRAM. Proper start files and link scripts are used to
put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader
should initialize SDRAM before loading the application.

-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly
workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for
standalone applications the default is off.

C6X Options

-march=name
This specifies the name of the target architecture. GCC uses this name to determine what
kind of instructions it can emit when generating assembly code. Permissible names are:
c62x, c64x, c64x+, c67x, c67x+, c674x.

-mbig-endian
Generate code for a big-endian target.

-mlittle-endian
Generate code for a little-endian target. This is the default.

-msim
Choose startup files and linker script suitable for the simulator.

-msdata=default
Put small global and static data in the ".neardata" section, which is pointed to by
register "B14". Put small uninitialized global and static data in the ".bss" section,
which is adjacent to the ".neardata" section. Put small read-only data into the
".rodata" section. The corresponding sections used for large pieces of data are
".fardata", ".far" and ".const".

-msdata=all
Put all data, not just small objects, into the sections reserved for small data, and use
addressing relative to the "B14" register to access them.

-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access
all data. Put all initialized global and static data in the ".fardata" section, and all
uninitialized data in the ".far" section. Put all constant data into the ".const"
section.

CRIS Options

These options are defined specifically for the CRIS ports.

-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3,
v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0.

-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the
ABI and the set of available instructions. The choices for architecture-type are the
same as for -march=architecture-type.

-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.

-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8
respectively.

-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies.
This option is disabled by default.

-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option
also has the effect of turning off the #NO_APP formatted-code indicator to the assembler
at the beginning of the assembly file.

-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test
instructions before use of condition codes.

-mno-side-effects
Do not emit instructions with side effects in addressing modes other than post-increment.

-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for the stack frame,
individual data and constants to be aligned for the maximum single data access size for
the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such
as structure layout are not affected by these options.

-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for
stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The
default is 32-bit alignment.

-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the
stack frame are omitted and no return instructions or return sequences are generated in
the code. Use this option only together with visual inspection of the compiled code: no
warnings or errors are generated when call-saved registers must be saved, or storage for
local variables needs to be allocated.

-melf
Legacy no-op option.

-sim
This option arranges to link with input-output functions from a simulator library. Code,
initialized data and zero-initialized data are allocated consecutively.

-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-
initialized data at 0x80000000.

C-SKY Options

GCC supports these options when compiling for C-SKY V2 processors.

-march=arch
Specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803,
ck807, and ck810. The default is ck810.

-mcpu=cpu
Specify the C-SKY target processor. Valid values for cpu are: ck801, ck801t, ck802,
ck802t, ck802j, ck803, ck803h, ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh,
ck803et, ck803eht, ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1,
ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1, ck803etr1, ck803ehtr1,
ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1, ck803efhtr1, ck803s, ck803st, ck803se,
ck803sf, ck803sef, ck803seft, ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef,
ck810eft, ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and ck810ftv.

-mbig-endian
-EB
-mlittle-endian
-EL Select big- or little-endian code. The default is little-endian.

-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and
hard.

-mhard-float
-msoft-float
Select hardware or software floating-point implementations. The default is soft float.

-mdouble-float
-mno-double-float
When -mhard-float is in effect, enable generation of double-precision float instructions.
This is the default except when compiling for CK803.

-mfdivdu
-mno-fdivdu
When -mhard-float is in effect, enable generation of "frecipd", "fsqrtd", and "fdivd"
instructions. This is the default except when compiling for CK803.

-mfpu=fpu
Select the floating-point processor. This option can only be used with -mhard-float.
Values for fpu are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2
(-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu).

-melrw
-mno-elrw
Enable the extended "lrw" instruction. This option defaults to on for CK801 and off
otherwise.

-mistack
-mno-istack
Enable interrupt stack instructions; the default is off.

The -mistack option is required to handle the "interrupt" and "isr" function attributes.

-mmp
Enable multiprocessor instructions; the default is off.

-mcp
Enable coprocessor instructions; the default is off.

-mcache
Enable coprocessor instructions; the default is off.

-msecurity
Enable C-SKY security instructions; the default is off.

-mtrust
Enable C-SKY trust instructions; the default is off.

-mdsp
-medsp
-mvdsp
Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these
options default to off.

-mdiv
-mno-div
Generate divide instructions. Default is off.

-msmart
-mno-smart
Generate code for Smart Mode, using only registers numbered 0-7 to allow use of 16-bit
instructions. This option is ignored for CK801 where this is the required behavior, and
it defaults to on for CK802. For other targets, the default is off.

-mhigh-registers
-mno-high-registers
Generate code using the high registers numbered 16-31. This option is not supported on
CK801, CK802, or CK803, and is enabled by default for other processors.

-manchor
-mno-anchor
Generate code using global anchor symbol addresses.

-mpushpop
-mno-pushpop
Generate code using "push" and "pop" instructions. This option defaults to on.

-mmultiple-stld
-mstm
-mno-multiple-stld
-mno-stm
Generate code using "stm" and "ldm" instructions. This option isn't supported on CK801
but is enabled by default on other processors.

-mconstpool
-mno-constpool
Create constant pools in the compiler instead of deferring it to the assembler. This
option is the default and required for correct code generation on CK801 and CK802, and is
optional on other processors.

-mstack-size
-mno-stack-size
Emit ".stack_size" directives for each function in the assembly output. This option
defaults to off.

-mccrt
-mno-ccrt
Generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to
off.

-mbranch-cost=n
Set the branch costs to roughly "n" instructions. The default is 1.

-msched-prolog
-mno-sched-prolog
Permit scheduling of function prologue and epilogue sequences. Using this option can
result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that
cannot be debugged or backtraced. It is disabled by default.

-msim
Links the library libsemi.a which is in compatible with simulator. Applicable to ELF
compiler only.

Darwin Options

These options are defined for all architectures running the Darwin operating system.

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the
single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat"
files if multiple -arch options are used; it does so by running the compiler or linker
multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags
that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL
option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler,
as, only permits instructions to be used that are valid for the subtype of the file it is
generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for
shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared
library with a less restrictive subtype than its input files (for instance, trying to put a
ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the
executable the most restrictive subtype of any of its input files.

-Fdir
Add the framework directory dir to the head of the list of directories to be searched for
header files. These directories are interleaved with those specified by -I options and
are scanned in a left-to-right order.

A framework directory is a directory with frameworks in it. A framework is a directory
with a Headers and/or PrivateHeaders directory contained directly in it that ends in
.framework. The name of a framework is the name of this directory excluding the
.framework. Headers associated with the framework are found in one of those two
directories, with Headers being searched first. A subframework is a framework directory
that is in a framework's Frameworks directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework, or in a sibling
subframework header. Two subframeworks are siblings if they occur in the same framework.
A subframework should not have the same name as a framework; a warning is issued if this
is violated. Currently a subframework cannot have subframeworks; in the future, the
mechanism may be extended to support this. The standard frameworks can be found in
/System/Library/Frameworks and /Library/Frameworks. An example include looks like
"#include <Framework/header.h>", where Framework denotes the name of the framework and
header.h is found in the PrivateHeaders or Headers directory.

-iframeworkdir
Like -F except the directory is a treated as a system directory. The main difference
between this -iframework and -F is that with -iframework the compiler does not warn about
constructs contained within header files found via dir. This option is valid only for
the C family of languages.

-gused
Emit debugging information for symbols that are used. For stabs debugging format, this
enables -feliminate-unused-debug-symbols. This is by default ON.

-gfull
Emit debugging information for all symbols and types.

-fconstant-cfstrings
The -fconstant-cfstrings is an alias for -mconstant-cfstrings.

-mconstant-cfstrings
When the NeXT runtime is being used (the default on these systems), override any
-fconstant-string-class setting and cause "@"..."" literals to be laid out as constant
CoreFoundation strings.

-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical
values supported for version include 12, 10.12, and 10.5.8.

If the compiler was built to use the system's headers by default, then the default for
this option is the system version on which the compiler is running, otherwise the default
is to make choices that are compatible with as many systems and code bases as possible.

-mkernel
Enable kernel development mode. The -mkernel option sets -static, -fno-common,
-fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak
and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float,
-fno-builtin and -mlong-branch for PowerPC targets.

-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1". By default sizeof(bool) is 4
when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has
no effect on x86.

Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary
compatible with code generated without that switch. Using this switch may require
recompiling all other modules in a program, including system libraries. Use this switch
to conform to a non-default data model.

-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to
dynamically load .o files into already-running programs. -findirect-data and
-ffix-and-continue are provided for backwards compatibility.

-all_load
Loads all members of static archive libraries. See man ld(1) for more information.

-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.

-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined
references when the file is loaded or launched.

-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.

-bundle_loader executable
This option specifies the executable that will load the build output file being linked.
See man ld(1) for more information.

-dynamiclib
When passed this option, GCC produces a dynamic library instead of an executable when
linking, using the Darwin libtool command.

-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one controlled by the
-mcpu or -march option.

-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them
in detail.

DEC Alpha Options

These -m options are defined for the DEC Alpha implementations:

-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations.
When -msoft-float is specified, functions in libgcc.a are used to perform floating-point
operations. Unless they are replaced by routines that emulate the floating-point
operations, or compiled in such a way as to call such emulations routines, these routines
issue floating-point operations. If you are compiling for an Alpha without floating-
point operations, you must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are required to have
floating-point registers.

-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs
implies -msoft-float. If the floating-point register set is not used, floating-point
operands are passed in integer registers as if they were integers and floating-point
results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any
function with a floating-point argument or return value called by code compiled with
-mno-fp-regs must also be compiled with that option.

A typical use of this option is building a kernel that does not use, and hence need not
save and restore, any floating-point registers.

-mieee
The Alpha architecture implements floating-point hardware optimized for maximum
performance. It is mostly compliant with the IEEE floating-point standard. However, for
full compliance, software assistance is required. This option generates code fully IEEE-
compliant code except that the inexact-flag is not maintained (see below). If this
option is turned on, the preprocessor macro "_IEEE_FP" is defined during compilation.
The resulting code is less efficient but is able to correctly support denormalized
numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other
Alpha compilers call this option -ieee_with_no_inexact.

DEBIAN SPECIFIC: This option is on by default for alpha-linux-gnu, unless
-ffinite-math-only (which is part of the -ffast-math set) is specified, because the
software functions in the GNU libc math libraries generate denormalized numbers, NaNs,
and infs (all of which will cause a programs to SIGFPE when it attempts to use the
results without -mieee).

-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag.
Turning on this option causes the generated code to implement fully-compliant IEEE math.
In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro. On some
Alpha implementations the resulting code may execute significantly slower than the code
generated by default. Since there is very little code that depends on the inexact-flag,
you should normally not specify this option. Other Alpha compilers call this option
-ieee_with_inexact.

-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha
compilers call this option -fptm trap-mode. The trap mode can be set to one of four
values:

n This is the default (normal) setting. The only traps that are enabled are the ones
that cannot be disabled in software (e.g., division by zero trap).

u In addition to the traps enabled by n, underflow traps are enabled as well.

su Like u, but the instructions are marked to be safe for software completion (see Alpha
architecture manual for details).

sui Like su, but inexact traps are enabled as well.

-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-
mode. The rounding-mode can be one of:

n Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest
machine number or towards the even machine number in case of a tie.

m Round towards minus infinity.

c Chopped rounding mode. Floating-point numbers are rounded towards zero.

d Dynamic rounding mode. A field in the floating-point control register (fpcr, see
Alpha architecture reference manual) controls the rounding mode in effect. The C
library initializes this register for rounding towards plus infinity. Thus, unless
your program modifies the fpcr, d corresponds to round towards plus infinity.

-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means without
software assistance it is impossible to recover from a floating trap and program
execution normally needs to be terminated. GCC can generate code that can assist
operating system trap handlers in determining the exact location that caused a floating-
point trap. Depending on the requirements of an application, different levels of
precisions can be selected:

p Program precision. This option is the default and means a trap handler can only
identify which program caused a floating-point exception.

f Function precision. The trap handler can determine the function that caused a
floating-point exception.

i Instruction precision. The trap handler can determine the exact instruction that
caused a floating-point exception.

Other Alpha compilers provide the equivalent options called -scope_safe and
-resumption_safe.

-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option
unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function
prologue of the generated assembly file.

-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from
smaller constants in two or three instructions. If it cannot, it outputs the constant as
a literal and generates code to load it from the data segment at run time.

Use this option to require GCC to construct all integer constants using code, even if it
takes more instructions (the maximum is six).

You typically use this option to build a shared library dynamic loader. Itself a shared
library, it must relocate itself in memory before it can find the variables and constants
in its own data segment.

-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX
instruction sets. The default is to use the instruction sets supported by the CPU type
specified via -mcpu= option or that of the CPU on which GCC was built if none is
specified.

-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of
IEEE single and double precision.

-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via
assembler macros. Use of these macros does not allow optimal instruction scheduling.
GNU binutils as of version 2.12 supports a new syntax that allows the compiler to
explicitly mark which relocations should apply to which instructions. This option is
mostly useful for debugging, as GCC detects the capabilities of the assembler when it is
built and sets the default accordingly.

-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations.
When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data
area (the ".sdata" and ".sbss" sections) and are accessed via 16-bit relocations off of
the $gp register. This limits the size of the small data area to 64KB, but allows the
variables to be directly accessed via a single instruction.

The default is -mlarge-data. With this option the data area is limited to just below
2GB. Programs that require more than 2GB of data must use "malloc" or "mmap" to allocate
the data in the heap instead of in the program's data segment.

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies
-mlarge-data.

-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or
shared library) fits in 4MB, and is thus reachable with a branch instruction. When
-msmall-data is used, the compiler can assume that all local symbols share the same $gp
value, and thus reduce the number of instructions required for a function call from 4 to
1.

The default is -mlarge-text.

-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu_type.
You can specify either the EV style name or the corresponding chip number. GCC supports
scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the
default values for the instruction set from the processor you specify. If you do not
specify a processor type, GCC defaults to the processor on which the compiler was built.

Supported values for cpu_type are

ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.

ev5
21164
Schedules as an EV5 and has no instruction set extensions.

ev56
21164a
Schedules as an EV5 and supports the BWX extension.

pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.

ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.

ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

Native toolchains also support the value native, which selects the best architecture
option for the host processor. -mcpu=native has no effect if GCC does not recognize the
processor.

-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The
instruction set is not changed.

Native toolchains also support the value native, which selects the best architecture
option for the host processor. -mtune=native has no effect if GCC does not recognize the
processor.

-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the
application. This number is highly dependent on the memory access patterns used by the
application and the size of the external cache on the machine.

Valid options for time are

number
A decimal number representing clock cycles.

L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5
hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as
well as to main memory. Note that L3 is only valid for EV5.

eBPF Options

-mframe-limit=bytes
This specifies the hard limit for frame sizes, in bytes. Currently, the value that can
be specified should be less than or equal to 32767. Defaults to whatever limit is
imposed by the version of the Linux kernel targeted.

-mkernel=version
This specifies the minimum version of the kernel that will run the compiled program. GCC
uses this version to determine which instructions to use, what kernel helpers to allow,
etc. Currently, version can be one of 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9,
4.10, 4.11, 4.12, 4.13, 4.14, 4.15, 4.16, 4.17, 4.18, 4.19, 4.20, 5.0, 5.1, 5.2, latest
and native.

-mbig-endian
Generate code for a big-endian target.

-mlittle-endian
Generate code for a little-endian target. This is the default.

-mjmpext
Enable generation of extra conditional-branch instructions. Enabled for CPU v2 and
above.

-mjmp32
Enable 32-bit jump instructions. Enabled for CPU v3 and above.

-malu32
Enable 32-bit ALU instructions. Enabled for CPU v3 and above.

-mcpu=version
This specifies which version of the eBPF ISA to target. Newer versions may not be
supported by all kernels. The default is v3.

Supported values for version are:

v1 The first stable eBPF ISA with no special features or extensions.

v2 Supports the jump extensions, as in -mjmpext.

v3 All features of v2, plus:

-<32-bit jump operations, as in -mjmp32>
-<32-bit ALU operations, as in -malu32>
-mco-re
Enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by
-gbtf.

-mno-co-re
Disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled
by default when generating BTF debug information for the BPF target.

-mxbpf
Generate code for an expanded version of BPF, which relaxes some of the restrictions
imposed by the BPF architecture:

-<Save and restore callee-saved registers at function entry and>
exit, respectively.

FR30 Options

These options are defined specifically for the FR30 port.

-msmall-model
Use the small address space model. This can produce smaller code, but it does assume
that all symbolic values and addresses fit into a 20-bit range.

-mno-lsim
Assume that runtime support has been provided and so there is no need to include the
simulator library (libsim.a) on the linker command line.

FT32 Options

These options are defined specifically for the FT32 port.

-msim
Specifies that the program will be run on the simulator. This causes an alternate
runtime startup and library to be linked. You must not use this option when generating
programs that will run on real hardware; you must provide your own runtime library for
whatever I/O functions are needed.

-mlra
Enable Local Register Allocation. This is still experimental for FT32, so by default the
compiler uses standard reload.

-mnodiv
Do not use div and mod instructions.

-mft32b
Enable use of the extended instructions of the FT32B processor.

-mcompress
Compress all code using the Ft32B code compression scheme.

-mnopm
Do not generate code that reads program memory.

FRV Options

-mgpr-32
Only use the first 32 general-purpose registers.

-mgpr-64
Use all 64 general-purpose registers.

-mfpr-32
Use only the first 32 floating-point registers.

-mfpr-64
Use all 64 floating-point registers.

-mhard-float
Use hardware instructions for floating-point operations.

-msoft-float
Use library routines for floating-point operations.

-malloc-cc
Dynamically allocate condition code registers.

-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".

-mdword
Change ABI to use double word insns.

-mno-dword
Do not use double word instructions.

-mdouble
Use floating-point double instructions.

-mno-double
Do not use floating-point double instructions.

-mmedia
Use media instructions.

-mno-media
Do not use media instructions.

-mmuladd
Use multiply and add/subtract instructions.

-mno-muladd
Do not use multiply and add/subtract instructions.

-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions.
Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes
GOT entries and small data are within a 12-bit range from the GOT base address; with
-fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this
option implies -msim.

-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind
locally. It has no effect without -mfdpic. It's enabled by default if optimizing for
speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization
option such as -O3 or above is present in the command line.

-mTLS
Assume a large TLS segment when generating thread-local code.

-mtls
Do not assume a large TLS segment when generating thread-local code.

-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in
read-only sections. It's enabled by default, except for -fpic or -fpie: even though it
may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC
or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols,
and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to
be a win. If it is not, -mno-gprel-ro can be used to disable it.

-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as
by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.

-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is
allocated. This option is enabled by default and can be disabled with -mno-linked-fp.

-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This
allows the functions to be placed anywhere within the 32-bit address space.

-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet.
This option only has an effect when VLIW packing is enabled. It doesn't create new
packets; it merely adds NOPs to existing ones.

-mlibrary-pic
Generate position-independent EABI code.

-macc-4
Use only the first four media accumulator registers.

-macc-8
Use all eight media accumulator registers.

-mpack
Pack VLIW instructions.

-mno-pack
Do not pack VLIW instructions.

-mno-eflags
Do not mark ABI switches in e_flags.

-mcond-move
Enable the use of conditional-move instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mno-cond-move
Disable the use of conditional-move instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mscc
Enable the use of conditional set instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mno-scc
Disable the use of conditional set instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mcond-exec
Enable the use of conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mno-cond-exec
Disable the use of conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mnested-cond-exec
Enable nested conditional execution optimizations (default).

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-mno-nested-cond-exec
Disable nested conditional execution optimizations.

This switch is mainly for debugging the compiler and will likely be removed in a future
version.

-moptimize-membar
This switch removes redundant "membar" instructions from the compiler-generated code. It
is enabled by default.

-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the
generated code.

-mtomcat-stats
Cause gas to print out tomcat statistics.

-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550,
tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

GNU/Linux Options

These -m options are defined for GNU/Linux targets:

-mglibc
Use the GNU C library. This is the default except on *-*-linux-*uclibc*,
*-*-linux-*musl* and *-*-linux-*android* targets.

-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.

-mmusl
Use the musl C library. This is the default on *-*-linux-*musl* targets.

-mbionic
Use Bionic C library. This is the default on *-*-linux-*android* targets.

-mandroid
Compile code compatible with Android platform. This is the default on
*-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by
default. When linking, this option makes the GCC driver pass Android-specific options to
the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be
defined.

-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default.

-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the
linker.

H8/300 Options

These -m options are defined for the H8/300 implementations:

-mrelax
Shorten some address references at link time, when possible; uses the linker option
-relax.

-mh Generate code for the H8/300H.

-ms Generate code for the H8S.

-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used
either with -mh or -ms.

-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.

-mexr
Extended registers are stored on stack before execution of function with monitor
attribute. Default option is -mexr. This option is valid only for H8S targets.

-mno-exr
Extended registers are not stored on stack before execution of function with monitor
attribute. Default option is -mno-exr. This option is valid only for H8S targets.

-mint32
Make "int" data 32 bits by default.

-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for
the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300
causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.

HPPA Options

These -m options are defined for the HPPA family of computers:

-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0
for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine the proper architecture option for
your machine. Code compiled for lower numbered architectures runs on higher numbered
architectures, but not the other way around.

-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

-matomic-libcalls
Generate libcalls for atomic loads and stores when sync libcalls are disabled. This
option is enabled by default. It only affects the generation of atomic libcalls by the
HPPA backend.

Both the sync and libatomic libcall implementations use locking. As a result, processor
stores are not atomic with respect to other atomic operations. Processor loads up to
DImode are atomic with respect to other atomic operations provided they are implemented
as a single access.

The PA-RISC architecture does not support any atomic operations in hardware except for
the "ldcw" instruction. Thus, all atomic support is implemented using sync and atomic
libcalls. Sync libcall support is in libgcc.a. Atomic libcall support is in libatomic.

This option generates "__atomic_exchange" calls for atomic stores. It also provides
special handling for atomic DImode accesses on 32-bit targets.

-mbig-switch
Does nothing. Preserved for backward compatibility.

-mcaller-copies
The caller copies function arguments passed by hidden reference. This option should be
used with care as it is not compatible with the default 32-bit runtime. However, only
aggregates larger than eight bytes are passed by hidden reference and the option provides
better compatibility with OpenMP.

-mcoherent-ldcw
Use ldcw/ldcd coherent cache-control hint.

-mdisable-fpregs
Disable floating-point registers. Equivalent to "-msoft-float".

-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure
problems when compiling MIG generated code under MACH.

-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit
code that performs faster indirect calls.

This option does not work in the presence of shared libraries or nested functions.

-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is
one that the register allocator cannot use. This is useful when compiling kernel code.
A register range is specified as two registers separated by a dash. Multiple register
ranges can be specified separated by a comma.

-mgas
Enable the use of assembler directives only GAS understands.

-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld when building a shared
library. It is the default when GCC is configured, explicitly or implicitly, with the
GNU linker. This option does not affect which ld is called; it only changes what
parameters are passed to that ld. The ld that is called is determined by the --with-ld
configure option, GCC's program search path, and finally by the user's PATH. The linker
used by GCC can be printed using which `gcc -print-prog-name=ld`. This option is only
available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.

-mhp-ld
Use options specific to HP ld. This passes -b to ld when building a shared library and
passes +Accept TypeMismatch to ld on all links. It is the default when GCC is
configured, explicitly or implicitly, with the HP linker. This option does not affect
which ld is called; it only changes what parameters are passed to that ld. The ld that
is called is determined by the --with-ld configure option, GCC's program search path, and
finally by the user's PATH. The linker used by GCC can be printed using which `gcc
-print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e.
configured with hppa*64*-*-hpux*.

-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging
impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give
bogus error messages when linking some programs.

-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to
reach linker generated stubs. The default is to generate long calls only when the
distance from the call site to the beginning of the function or translation unit, as the
case may be, exceeds a predefined limit set by the branch type being used. The limits
for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X
architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections
option, or when using the -mgas and -mno-portable-runtime options together under HP-UX
with the SOM linker.

It is normally not desirable to use this option as it degrades performance. However, it
may be useful in large applications, particularly when partial linking is used to build
the application.

The types of long calls used depends on the capabilities of the assembler and linker, and
the type of code being generated. The impact on systems that support long absolute
calls, and long pic symbol-difference or pc-relative calls should be relatively small.
However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10
linker. This is equivalent to the +k option to the HP compilers.

-mjump-in-delay
This option is ignored and provided for compatibility purposes only.

-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to
generate faster indirect calls and use unscaled index address modes.

Such code is suitable for level 0 PA systems and kernels.

-mordered
Assume memory references are ordered and barriers are not needed.

-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.

-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices
for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models
on an HP-UX system to determine the proper scheduling option for your machine. The
default scheduling is 8000.

-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates
the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These
options are available under HP-UX and HI-UX.

-msoft-float
Generate output containing library calls for floating point. Warning: the requisite
libraries are not available for all HPPA targets. Normally the facilities of the
machine's usual C compiler are used, but this cannot be done directly in cross-
compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation.

-msoft-float changes the calling convention in the output file; therefore, it is only
useful if you compile all of a program with this option. In particular, you need to
compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to
work.

-msoft-mult
Use software integer multiplication.

This disables the use of the "xmpyu" instruction.

-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The
choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is
available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The
default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-
UX 11.11 and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional
predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o.
-munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED",
"_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.

It is important to note that this option changes the interfaces for various library
routines. It also affects the operational behavior of the C library. Thus, extreme care
is needed in using this option.

Library code that is intended to operate with more than one UNIX standard must test, set
and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software
doesn't provide this capability.

-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is
specified on HP-UX 10 and later.

-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't
an archive version of libdld.sl. Thus, when the -static option is specified, special
link options are needed to resolve this dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl
when the -static option is specified. This causes the resulting binary to be dynamic.
On the 64-bit port, the linkers generate dynamic binaries by default in any case. The
-nolibdld option can be used to prevent the GCC driver from adding these link options.

-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets
flags for both the preprocessor and linker.

IA-64 Options

These are the -m options defined for the Intel IA-64 architecture.

-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.

-mlittle-endian
Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.

-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.

-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.

-mno-pic
Generate code that does not use a global pointer register. The result is not position
independent code, and violates the IA-64 ABI.

-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm statements.

-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers. This may
make assembler output more readable.

-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful
for working around optimizer bugs.

-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when
compiling kernel code.

-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when
compiling firmware code.

-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the minimum latency
algorithm.

-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the maximum throughput
algorithm.

-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.

-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.

-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput
algorithm.

-mno-inline-int-divide
Do not generate inline code for divides of integer values.

-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.

-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.

-mno-inline-sqrt
Do not generate inline code for "sqrt".

-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract
instructions. The default is to use these instructions.

-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line number debugging info. This may
be useful when not using the GNU assembler.

-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that
triggered the stop bit. This can improve instruction scheduling, but does not always do
so.

-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.

-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1,
merced, itanium2, and mckinley.

-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long
and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer
to 64 bits. These are HP-UX specific flags.

-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This results in generation of
"ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The
default setting is disabled.

-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This results in generation of
"ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The
default setting is enabled.

-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only during
region scheduling (i.e. before reload). This results in generation of the "ld.s"
instructions and the corresponding check instructions "chk.s". The default setting is
disabled.

-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data
speculative loads before reload. This is effective only with -msched-br-data-spec
enabled. The default setting is enabled.

-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data
speculative loads after reload. This is effective only with -msched-ar-data-spec
enabled. The default setting is enabled.

-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control
speculative loads. This is effective only with -msched-control-spec enabled. The
default setting is enabled.

-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule only if there are no
other choices at the moment. This makes the use of the data speculation much more
conservative. The default setting is disabled.

-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule only if there are no
other choices at the moment. This makes the use of the control speculation much more
conservative. The default setting is disabled.

-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during computation of the
instructions priorities. This makes the use of the speculation a bit more conservative.
The default setting is disabled.

-msched-spec-ldc
Use a simple data speculation check. This option is on by default.

-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.

-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.

-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when
placed into the same instruction group. This option is disabled by default.

-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled
by default.

-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower priority to
subsequent memory insns attempting to schedule in the same instruction group. Frequently
useful to prevent cache bank conflicts. The default value is 1.

-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than
that number in an instruction group. Otherwise, the limit is "soft", meaning that non-
memory operations are preferred when the limit is reached, but memory operations may
still be scheduled.

LM32 Options

These -m options are defined for the LatticeMico32 architecture:

-mbarrel-shift-enabled
Enable barrel-shift instructions.

-mdivide-enabled
Enable divide and modulus instructions.

-mmultiply-enabled
Enable multiply instructions.

-msign-extend-enabled
Enable sign extend instructions.

-muser-enabled
Enable user-defined instructions.

LoongArch Options

These command-line options are defined for LoongArch targets:

-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type,
which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows
GCC to generate code that may not run at all on processors other than the one indicated.
Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

native
This selects the CPU to generate code for at compilation time by determining the
processor type of the compiling machine. Using -march=native enables all instruction
subsets supported by the local machine (hence the result might not run on different
machines). Using -mtune=native produces code optimized for the local machine under
the constraints of the selected instruction set.

loongarch64
A generic CPU with 64-bit extensions.

la464
LoongArch LA464 CPU with LBT, LSX, LASX, LVZ.

-mtune=cpu-type
Optimize the output for the given processor, specified by microarchitecture name.

-mabi=base-abi-type
Generate code for the specified calling convention. base-abi-type can be one of:

lp64d
Uses 64-bit general purpose registers and 32/64-bit floating-point registers for
parameter passing. Data model is LP64, where int is 32 bits, while long int and
pointers are 64 bits.

lp64f
Uses 64-bit general purpose registers and 32-bit floating-point registers for
parameter passing. Data model is LP64, where int is 32 bits, while long int and
pointers are 64 bits.

lp64s
Uses 64-bit general purpose registers and no floating-point registers for parameter
passing. Data model is LP64, where int is 32 bits, while long int and pointers are
64 bits.

-mfpu=fpu-type
Generate code for the specified FPU type, which can be one of:

64 Allow the use of hardware floating-point instructions for 32-bit and 64-bit
operations.

32 Allow the use of hardware floating-point instructions for 32-bit operations.

none
0 Prevent the use of hardware floating-point instructions.

-msoft-float
Force -mfpu=none and prevents the use of floating-point registers for parameter passing.
This option may change the target ABI.

-msingle-float
Force -mfpu=32 and allow the use of 32-bit floating-point registers for parameter
passing. This option may change the target ABI.

-mdouble-float
Force -mfpu=64 and allow the use of 32/64-bit floating-point registers for parameter
passing. This option may change the target ABI.

-mbranch-cost=n
Set the cost of branches to roughly n instructions.

-mcheck-zero-division
-mno-check-zero-divison
Trap (do not trap) on integer division by zero. The default is -mcheck-zero-division for
-O0 or -Og, and -mno-check-zero-division for other optimization levels.

-mcond-move-int
-mno-cond-move-int
Conditional moves for integral data in general-purpose registers are enabled (disabled).
The default is -mcond-move-int.

-mcond-move-float
-mno-cond-move-float
Conditional moves for floating-point registers are enabled (disabled). The default is
-mcond-move-float.

-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves. The default is
-mno-memcpy, which allows GCC to inline most constant-sized copies. Setting optimization
level to -Os also forces the use of "memcpy", but -mno-memcpy may override this behavior
if explicitly specified, regardless of the order these options on the command line.

-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object
boundary as described in the architecture specification. The default is
-mno-strict-align.

-msmall-data-limit=number
Put global and static data smaller than number bytes into a special section (on some
targets). The default value is 0.

-mmax-inline-memcpy-size=n
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal
to n bytes. The default value of n is 1024.

-mcmodel=code-model
Set the code model to one of:

tiny-static (Not implemented yet)
tiny (Not implemented yet)
normal
The text segment must be within 128MB addressing space. The data segment must be
within 2GB addressing space.

medium
The text segment and data segment must be within 2GB addressing space.

large (Not implemented yet)
extreme
This mode does not limit the size of the code segment and data segment. The
-mcmodel=extreme option is incompatible with -fplt and -mno-explicit-relocs.

The default code model is "normal".

-mexplicit-relocs
-mno-explicit-relocs
Use or do not use assembler relocation operators when dealing with symbolic addresses.
The alternative is to use assembler macros instead, which may limit instruction
scheduling but allow linker relaxation. The default value for the option is determined
with the assembler capability detected during GCC build-time and the setting of
"-mrelax": "-mexplicit-relocs" if the assembler supports relocation operators but
"-mrelax" is not enabled, "-mno-explicit-relocs" otherwise.

-mdirect-extern-access
-mno-direct-extern-access
Do not use or use GOT to access external symbols. The default is
-mno-direct-extern-access: GOT is used for external symbols with default visibility, but
not used for other external symbols.

With -mdirect-extern-access, GOT is not used and all external symbols are PC-relatively
addressed. It is only suitable for environments where no dynamic link is performed, like
firmwares, OS kernels, executables linked with -static or -static-pie.
-mdirect-extern-access is not compatible with -fPIC or -fpic.

-mrelax
-mno-relax
Take (do not take) advantage of linker relaxations. If -mpass-mrelax-to-as is enabled,
this option is also passed to the assembler. The default is determined during GCC build-
time by detecting corresponding assembler support: -mrelax if the assembler supports both
the -mrelax option and the conditional branch relaxation (it's required or the ".align"
directives and conditional branch instructions in the assembly code outputted by GCC may
be rejected by the assembler because of a relocation overflow), -mno-relax otherwise.

-mpass-mrelax-to-as
-mno-pass-mrelax-to-as
Pass (do not pass) the -mrelax or -mno-relax option to the assembler. The default is
determined during GCC build-time by detecting corresponding assembler support:
-mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as
otherwise. This option is mostly useful for debugging, or interoperation with assemblers
different from the build-time one.

M32C Options

-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny
series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for
the M32C/80 series.

-msim
Specifies that the program will be run on the simulator. This causes an alternate
runtime library to be linked in which supports, for example, file I/O. You must not use
this option when generating programs that will run on real hardware; you must provide
your own runtime library for whatever I/O functions are needed.

-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses during code generation.
These pseudo-registers are used like real registers, so there is a tradeoff between GCC's
ability to fit the code into available registers, and the performance penalty of using
memory instead of registers. Note that all modules in a program must be compiled with
the same value for this option. Because of that, you must not use this option with GCC's
default runtime libraries.

M32R/D Options

These -m options are defined for Renesas M32R/D architectures:

-m32r2
Generate code for the M32R/2.

-m32rx
Generate code for the M32R/X.

-m32r
Generate code for the M32R. This is the default.

-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be
loaded with the "ld24" instruction), and assume all subroutines are reachable with the
"bl" instruction. This is the default.

The addressability of a particular object can be set with the "model" attribute.

-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler generates
"seth/add3" instructions to load their addresses), and assume all subroutines are
reachable with the "bl" instruction.

-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler generates
"seth/add3" instructions to load their addresses), and assume subroutines may not be
reachable with the "bl" instruction (the compiler generates the much slower
"seth/add3/jl" instruction sequence).

-msdata=none
Disable use of the small data area. Variables are put into one of ".data", ".bss", or
".rodata" (unless the "section" attribute has been specified). This is the default.

The small data area consists of sections ".sdata" and ".sbss". Objects may be explicitly
put in the small data area with the "section" attribute using one of these sections.

-msdata=sdata
Put small global and static data in the small data area, but do not generate special code
to reference them.

-msdata=use
Put small global and static data in the small data area, and generate special
instructions to reference them.

-G num
Put global and static objects less than or equal to num bytes into the small data or BSS
sections instead of the normal data or BSS sections. The default value of num is 8. The
-msdata option must be set to one of sdata or use for this option to have any effect.

All modules should be compiled with the same -G num value. Compiling with different
values of num may or may not work; if it doesn't the linker gives an error
message---incorrect code is not generated.

-mdebug
Makes the M32R-specific code in the compiler display some statistics that might help in
debugging programs.

-malign-loops
Align all loops to a 32-byte boundary.

-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.

-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.

-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are preferred over conditional code,
if it is 2, then the opposite applies.

-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers
are between 0 and 15 inclusive.

-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.

-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The
default is _flush_cache, but a function call is only used if a trap is not available.

-mno-flush-func
Indicates that there is no OS function for flushing the cache.

M680x0 Options

These are the -m options defined for M680x0 and ColdFire processors. The default settings
depend on which architecture was selected when the compiler was configured; the defaults for
the most common choices are given below.

-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture.
Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030,
68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA
classification and the permissible values are: isaa, isaaplus, isab and isac.

GCC defines a macro "__mcfarch__" whenever it is generating code for a ColdFire target.
The arch in this macro is one of the -march arguments given above.

When used together, -march and -mtune select code that runs on a family of similar
processors but that is optimized for a particular microarchitecture.

-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000,
68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given
by the table below, which also classifies the CPUs into families:

Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of
-mcpu and -march are rejected.

GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is selected. It also
defines "__mcf_family_family", where the value of family is given by the table above.

-mtune=tune
Tune the code for a particular microarchitecture within the constraints set by -march and
-mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and
cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020,
68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well.
These two options select the same tuning decisions as -m68020-40 and -m68020-60
respectively.

GCC defines the macros "__mcarch" and "__mcarch__" when tuning for 680x0 architecture
arch. It also defines "mcarch" unless either -ansi or a non-GNU -std option is used. If
GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or
-mtune=68020-60, it defines the macros for every architecture in the range.

GCC also defines the macro "__muarch__" when tuning for ColdFire microarchitecture uarch,
where uarch is one of the arguments given above.

-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for
68000-based systems. It is equivalent to -march=68000.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008,
68302, 68306, 68307, 68322, 68328 and 68356.

-m68010
Generate output for a 68010. This is the default when the compiler is configured for
68010-based systems. It is equivalent to -march=68010.

-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for
68020-based systems. It is equivalent to -march=68020.

-m68030
Generate output for a 68030. This is the default when the compiler is configured for
68030-based systems. It is equivalent to -march=68030.

-m68040
Generate output for a 68040. This is the default when the compiler is configured for
68040-based systems. It is equivalent to -march=68040.

This option inhibits the use of 68881/68882 instructions that have to be emulated by
software on the 68040. Use this option if your 68040 does not have code to emulate those
instructions.

-m68060
Generate output for a 68060. This is the default when the compiler is configured for
68060-based systems. It is equivalent to -march=68060.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be
emulated by software on the 68060. Use this option if your 68060 does not have code to
emulate those instructions.

-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for
CPU32-based systems. It is equivalent to -march=cpu32.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330,
68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

-m5200
Generate output for a 520X ColdFire CPU. This is the default when the compiler is
configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated
in favor of that option.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203,
MCF5204 and MCF5206.

-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the
equivalent -mcpu=5206e.

-m528x
Generate output for a member of the ColdFire 528X family. The option is now deprecated
in favor of the equivalent -mcpu=528x.

-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the
equivalent -mcpu=5307.

-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the
equivalent -mcpu=5407.

-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of
hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now
deprecated in favor of that option.

-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in
code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.
The generated code does use the 68881 instructions that are emulated on the 68040.

The option is equivalent to -march=68020 -mtune=68020-40.

-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in
code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.
The generated code does use the 68881 instructions that are emulated on the 68060.

The option is equivalent to -march=68020 -mtune=68020-60.

-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above, and for
ColdFire devices that have an FPU. It defines the macro "__HAVE_68881__" on M680x0
targets and "__mcffpu__" on ColdFire targets.

-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the
default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices
that have no FPU.

-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions. If
-march is used without -mcpu, the default is "on" for ColdFire architectures and "off"
for M680x0 architectures. Otherwise, the default is taken from the target CPU (either
the default CPU, or the one specified by -mcpu). For example, the default is "off" for
-mcpu=5206 and "on" for -mcpu=5206e.

GCC defines the macro "__mcfhwdiv__" when this option is enabled.

-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters
passed on the stack are also aligned to a 16-bit boundary even on targets whose API
mandates promotion to 32-bit.

-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.

-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply
-mnobitfield.

-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the
default if you use a configuration designed for a 68020.

-mrtd
Use a different function-calling convention, in which functions that take a fixed number
of arguments return with the "rtd" instruction, which pops their arguments while
returning. This saves one instruction in the caller since there is no need to pop the
arguments there.

This calling convention is incompatible with the one normally used on Unix, so you cannot
use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers
of arguments (including "printf"); otherwise incorrect code is generated for calls to
those functions.

In addition, seriously incorrect code results if you call a function with too many
arguments. (Normally, extra arguments are harmlessly ignored.)

The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32
processors, but not by the 68000 or 5200.

The default is -mno-rtd.

-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long
double" variables on a 32-bit boundary (-malign-int) or a 16-bit boundary
(-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs
somewhat faster on processors with 32-bit busses at the expense of more memory.

Warning: if you use the -malign-int switch, GCC aligns structures containing the above
types differently than most published application binary interface specifications for the
m68k.

Use the pc-relative addressing mode of the 68000 directly, instead of using a global
offset table. At present, this option implies -fpic, allowing at most a 16-bit offset
for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this
could be supported for 68020 and higher processors.

-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by the system.

-msep-data
Generate code that allows the data segment to be located in a different area of memory
from the text segment. This allows for execute-in-place in an environment without
virtual memory management. This option implies -fPIC.

-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the
default.

-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for
execute-in-place and shared libraries in an environment without virtual memory
management. This option implies -fPIC.

-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the
default.

-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled.
Specifying a value of 0 generates more compact code; specifying other values forces the
allocation of that number to the current library, but is no more space- or time-efficient
than omitting this option.

-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the
GOT has more than 8192 entries. This code is larger and slower than code generated
without this option. On M680x0 processors, this option is not needed; -fPIC suffices.

GCC normally uses a single instruction to load values from the GOT. While this is
relatively efficient, it only works if the GOT is smaller than about 64k. Anything
larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with
very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4
instructions to fetch the value of a global symbol.

Note that some linkers, including newer versions of the GNU linker, can create multiple
GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot
when compiling a single object file that accesses more than 8192 GOT entries. Very few
do.

These options have no effect unless GCC is generating position-independent code.

-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.

MCore Options

These are the -m options defined for the Motorola M*Core processors.

-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.

-mdiv
-mno-div
Use the divide instruction. (Enabled by default).

-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.

-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.

-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.

-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.

-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.

-mlittle-endian
-mbig-endian
Generate code for a little-endian target.

-m210
-m340
Generate code for the 210 processor.

-mno-lsim
Assume that runtime support has been provided and so omit the simulator library
(libsim.a) from the linker command line.

-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values can increase
the speed of programs that contain functions that need a large amount of stack space, but
they can also trigger a segmentation fault if the stack is extended too much. The
default value is 0x1000.

MicroBlaze Options

-msoft-float
Use software emulation for floating point (default).

-mhard-float
Use hardware floating-point instructions.

-mmemcpy
Do not optimize block moves, use "memcpy".

-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.

-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported values are in the
format vX.YY.Z, where X is a major version, YY is the minor version, and Z is
compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.

-mxl-soft-mul
Use software multiply emulation (default).

-mxl-soft-div
Use software emulation for divides (default).

-mxl-barrel-shift
Use the hardware barrel shifter.

-mxl-pattern-compare
Use pattern compare instructions.

-msmall-divides
Use table lookup optimization for small signed integer divisions.

-mxl-stack-check
This option is deprecated. Use -fstack-check instead.

-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.

-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.

-mxl-float-convert
Use hardware floating-point conversion instructions.

-mxl-float-sqrt
Use hardware floating-point square root instruction.

-mbig-endian
Generate code for a big-endian target.

-mlittle-endian
Generate code for a little-endian target.

-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).

-mxl-mode-app-model
Select application model app-model. Valid models are

executable
normal executable (default), uses startup code crt0.o.

xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug
agent called xmdstub. This uses startup file crt1.o and sets the start address of the
program to 0x800.

bootstrap
for applications that are loaded using a bootloader. This model uses startup file
crt2.o which does not contain a processor reset vector handler. This is suitable for
transferring control on a processor reset to the bootloader rather than the
application.

novectors
for applications that do not require any of the MicroBlaze vectors. This option may
be useful for applications running within a monitoring application. This model uses
crt3.o as a startup file.

Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.

-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link
time. This allows data to be referenced by offset from start of text address instead of
GOT since PC-relative addressing is not supported.

MIPS Options

-EB Generate big-endian code.

-EL Generate little-endian code. This is the default for mips*el-*-* configurations.

-march=arch
Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name
of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32,
mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and
mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc,
5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1,
34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500,
interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc,
m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600,
r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000,
rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130,
vr4300, vr5000, vr5400, vr5500, xlr and xlp. The special value from-abi selects the most
compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3
for 64-bit ABIs).

The native Linux/GNU toolchain also supports the value native, which selects the best
architecture option for the host processor. -march=native has no effect if GCC does not
recognize the processor.

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k).
Prefixes are optional, and vr may be written r.

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the
core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as
the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of
3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for
nf2_1 while nx and bfx are accepted as synonyms for nf1_1.

GCC defines two macros based on the value of this option. The first is "_MIPS_ARCH",
which gives the name of target architecture, as a string. The second has the form
"_MIPS_ARCH_foo", where foo is the capitalized value of "_MIPS_ARCH". For example,
-march=r2000 sets "_MIPS_ARCH" to "r2000" and defines the macro "_MIPS_ARCH_R2000".

Note that the "_MIPS_ARCH" macro uses the processor names given above. In other words,
it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the
macro names the resolved architecture (either "mips1" or "mips3"). It names the default
architecture when no -march option is given.

-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are
scheduled, and the perceived cost of arithmetic operations. The list of arch values is
the same as for -march.

When this option is not used, GCC optimizes for the processor specified by -march. By
using -march and -mtune together, it is possible to generate code that runs on a family
of processors, but optimize the code for one particular member of that family.

-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way
as the -march ones described above.

-mips1
Equivalent to -march=mips1.

-mips2
Equivalent to -march=mips2.

-mips3
Equivalent to -march=mips3.

-mips4
Equivalent to -march=mips4.

-mips32
Equivalent to -march=mips32.

-mips32r3
Equivalent to -march=mips32r3.

-mips32r5
Equivalent to -march=mips32r5.

-mips32r6
Equivalent to -march=mips32r6.

-mips64
Equivalent to -march=mips64.

-mips64r2
Equivalent to -march=mips64r2.

-mips64r3
Equivalent to -march=mips64r3.

-mips64r5
Equivalent to -march=mips64r5.

-mips64r6
Equivalent to -march=mips64r6.

-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64
architecture, it makes use of the MIPS16e ASE.

MIPS16 code generation can also be controlled on a per-function basis by means of
"mips16" and "nomips16" attributes.

-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression
testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use
in compiling user code.

-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard (uncompressed) MIPS ISA be link-
compatible with MIPS16 and microMIPS code, and vice versa.

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or
microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed
therefore disables direct jumps unless GCC knows that the target of the jump is not
compressed.

-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate
the microMIPS ASE and are retained for backwards compatibility.

-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code
when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

For information about the O64 ABI, see <https://gcc.gnu.org/projects/mipso64-abi.html>.

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather
than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI
relies on the "mthc1" and "mfhc1" instructions and is therefore only supported for
MIPS32R2, MIPS32R3 and MIPS32R5 processors.

The register assignments for arguments and return values remain the same, but each scalar
value is passed in a single 64-bit register rather than a pair of 32-bit registers. For
example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The
set of call-saved registers also remains the same in that the even-numbered double-
precision registers are saved.

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit
to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The
FPXX extension mandates that all code must execute correctly when run using 32-bit or
64-bit registers. The code can be interlinked with either FP32 or FP64, but not both.
The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered
single-precision registers. This can be used in conjunction with the "FRE" mode of FPUs
in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the
same process without changing FPU modes.

-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic objects.
-mabicalls is the default for SVR4-based systems.

-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can
therefore be linked into shared libraries. This option only affects -mabicalls.

All -mabicalls code has traditionally been position-independent, regardless of options
like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to
use absolute accesses for locally-binding symbols. It can also use shorter GP
initialization sequences and generate direct calls to locally-defined functions. This
mode is selected by -mno-shared.

-mno-shared depends on binutils 2.16 or higher and generates objects that can only be
linked by the GNU linker. However, the option does not affect the ABI of the final
executable; it only affects the ABI of relocatable objects. Using -mno-shared generally
makes executables both smaller and quicker.

-mshared is the default.

-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and copy
relocations. This option only affects -mno-shared -mabicalls. For the n64 ABI, this
option has no effect without -msym32.

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is
-mno-plt otherwise.

-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.

relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. This works with very large
GOTs, although the code is also less efficient, since it takes three instructions to
fetch the value of a global symbol.

Note that some linkers can create multiple GOTs. If you have such a linker, you should
only need to use -mxgot when a single object file accesses more than 64k's worth of GOT
entries. Very few do.

These options have no effect unless GCC is generating position independent code.

-mgp32
Assume that general-purpose registers are 32 bits wide.

-mgp64
Assume that general-purpose registers are 64 bits wide.

-mfp32
Assume that floating-point registers are 32 bits wide.

-mfp64
Assume that floating-point registers are 64 bits wide.

-mfpxx
Do not assume the width of floating-point registers.

-mhard-float
Use floating-point coprocessor instructions.

-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point
calculations using library calls instead.

-mno-float
Equivalent to -msoft-float, but additionally asserts that the program being compiled does
not perform any floating-point operations. This option is presently supported only by
some bare-metal MIPS configurations, where it may select a special set of libraries that
lack all floating-point support (including, for example, the floating-point "printf"
formats). If code compiled with -mno-float accidentally contains floating-point
operations, it is likely to suffer a link-time or run-time failure.

-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.

-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is
the default.

-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point registers for the o32 ABI.
This is the default for processors that are known to support these registers. When using
the o32 FPXX ABI, -mno-odd-spreg is set by default.

-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number (NaN) IEEE 754 floating-
point data with the "abs.fmt" and "neg.fmt" machine instructions.

By default or when -mabs=legacy is used the legacy treatment is selected. In this case
these instructions are considered arithmetic and avoided where correct operation is
required and the input operand might be a NaN. A longer sequence of instructions that
manipulate the sign bit of floating-point datum manually is used instead unless the
-ffinite-math-only option has also been specified.

The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these
instructions are considered non-arithmetic and therefore operating correctly in all
cases, including in particular where the input operand is a NaN. These instructions are
therefore always used for the respective operations.

-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number (NaN) IEEE 754 floating-
point data.

The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are
denoted by the first bit of their trailing significand field being 0, whereas signaling
NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted
by the first bit of their trailing significand field being 1, whereas sNaNs are denoted
by the first bit of their trailing significand field being 0.

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in
functions. When neither option is specified, GCC uses the instructions if the target
architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc
can be useful when compiling for nonstandard ISAs. You can make either option the
default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc
is the default for some configurations; see the installation documentation for details.

-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also defines
"__mips_dsp_rev" to 1.

-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also
defines "__mips_dsp_rev" to 2.

-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.

-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.

-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used
when generating 64-bit code and requires hardware floating-point support to be enabled.

-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.

-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis by means of
"micromips" and "nomicromips" attributes.

-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.

-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.

-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing instructions.

-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.

-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.

-mcrc
-mno-crc
Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.

-mginv
-mno-ginv
Use (do not use) the MIPS Global INValidate (GINV) instructions.

-mloongson-mmi
-mno-loongson-mmi
Use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).

-mloongson-ext
-mno-loongson-ext
Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.

-mloongson-ext2
-mno-loongson-ext2
Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.

-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default
and the way that the pointer size is determined.

-mlong32
Force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported
ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the
others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as
integer registers, whichever is smaller.

-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected
ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it
allows GCC to generate shorter and faster references to symbolic addresses.

-G num
Put definitions of externally-visible data in a small data section if that data is no
bigger than num bytes. GCC can then generate more efficient accesses to the data; see
-mgpopt for details.

The default -G option depends on the configuration.

-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static variables in
C. -mlocal-sdata is the default for all configurations.

If the linker complains that an application is using too much small data, you might want
to try rebuilding the less performance-critical parts with -mno-local-sdata. You might
also want to build large libraries with -mno-local-sdata, so that the libraries leave
more room for the main program.

-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small data section if the
size of that data is within the -G limit. -mextern-sdata is the default for all
configurations.

If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a
variable Var that is no bigger than num bytes, you must make sure that Var is placed in a
small data section. If Var is defined by another module, you must either compile that
module with a high-enough -G setting or attach a "section" attribute to Var's definition.
If Var is common, you must link the application with a high-enough -G setting.

The easiest way of satisfying these restrictions is to compile and link every module with
the same -G option. However, you may wish to build a library that supports several
different small data limits. You can do this by compiling the library with the highest
supported -G setting and additionally using -mno-extern-sdata to stop the library from
making assumptions about externally-defined data.

-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data
section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all
configurations.

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp".
For example, if the code is part of a library that might be used in a boot monitor,
programs that call boot monitor routines pass an unknown value in $gp. (In such
situations, the boot monitor itself is usually compiled with -G0.)

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.

-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the
small data section if possible, otherwise in data. This gives slightly slower code than
the default, but reduces the amount of RAM required when executing, and thus may be
preferred for some embedded systems.

-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only
meaningful in conjunction with -membedded-data.

-mcode-readable=setting
Specify whether GCC may generate code that reads from executable sections. There are
three possible settings:

-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.

-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other
instructions must not do so. This option is useful on 4KSc and 4KSd processors when
the code TLBs have the Read Inhibit bit set. It is also useful on processors that
can be configured to have a dual instruction/data SRAM interface and that, like the
M4K, automatically redirect PC-relative loads to the instruction RAM.

-mcode-readable=no
Instructions must not access executable sections. This option can be useful on
targets that are configured to have a dual instruction/data SRAM interface but that
(unlike the M4K) do not automatically redirect PC-relative loads to the instruction
RAM.

-msplit-addresses
-mno-split-addresses
Enable (disable) use of the %hi() and %lo() assembler relocation operators. This option
has been superseded by -mexplicit-relocs but is retained for backwards compatibility.

-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses.
The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.

-mexplicit-relocs is the default if GCC was configured to use an assembler that supports
relocation operators.

-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.

The default is -mcheck-zero-division.

-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a
break instruction. Using traps results in smaller code, but is only supported on MIPS II
and later. Also, some versions of the Linux kernel have a bug that prevents trap from
generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps
on architectures that support them and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using
--with-divide=breaks. Divide-by-zero checks can be completely disabled using
-mno-check-zero-division.

-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or store instructions to
enable load/store bonding. This option is enabled by default but only takes effect when
the selected architecture is known to support bonding.

-munaligned-access
-mno-unaligned-access
Enable (disable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store
unaligned-access support, by hardware or trap&emulate. So -mno-unaligned-access may be
needed by kernel.

-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves. The default is
-mno-memcpy, which allows GCC to inline most constant-sized copies.

-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is
more efficient but requires the caller and callee to be in the same 256 megabyte segment.

This option has no effect on abicalls code. The default is -mno-long-calls.

-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the
R4650 ISA.

-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer instructions. The default is
-mimadd on architectures that support "madd" and "msub" except for the 74k architecture
where it was found to generate slower code.

-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate instructions, when they
are available. The default is -mfused-madd.

On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product
is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit.
This may be undesirable in some circumstances. On other processors the result is
numerically identical to the equivalent computation using separate multiply, add,
subtract and negate instructions.

-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s
suffix) when assembling them.

-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are
implemented by the assembler rather than by GCC.

-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:

- A double-word or a variable shift may give an incorrect result if executed
immediately after starting an integer division.

- A double-word or a variable shift may give an incorrect result if executed while an
integer multiplication is in progress.

- An integer division may give an incorrect result if started in a delay slot of a
taken branch or a jump.

-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:

- A double-word or a variable shift may give an incorrect result if executed
immediately after starting an integer division.

-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:

- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may
deadlock on revisions 2.6 and earlier.

This option can only be used if the target architecture supports branch-likely
instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is
the default otherwise.

-mfix-r5900
-mno-fix-r5900
Do not attempt to schedule the preceding instruction into the delay slot of a branch
instruction placed at the end of a short loop of six instructions or fewer and always
schedule a "nop" instruction there instead. The short loop bug under certain conditions
causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The
workaround is implemented by the assembler rather than by GCC.

-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The workarounds are implemented by the
assembler rather than by GCC.

-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:

- "dmultu" does not always produce the correct result.

- "div" and "ddiv" do not always produce the correct result if one of the operands is
negative.

The workarounds for the division errata rely on special functions in libgcc.a. At
present, these functions are only provided by the "mips64vr*-elf" configurations.

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions.
These errata are handled by the assembler, not by GCC itself.

-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the
assembler rather than by GCC, although GCC avoids using "mflo" and "mfhi" if the VR4130
"macc", "macchi", "dmacc" and "dmacchi" instructions are available instead.

-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1
revision 2 "F1" and "F2" floating-point errata.)

-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side effects of speculation
on R10K processors.

In common with many processors, the R10K tries to predict the outcome of a conditional
branch and speculatively executes instructions from the "taken" branch. It later aborts
these instructions if the predicted outcome is wrong. However, on the R10K, even aborted
instructions can have side effects.

This problem only affects kernel stores and, depending on the system, kernel loads. As
an example, a speculatively-executed store may load the target memory into cache and mark
the cache line as dirty, even if the store itself is later aborted. If a DMA operation
writes to the same area of memory before the "dirty" line is flushed, the cached data
overwrites the DMA-ed data. See the R10K processor manual for a full description,
including other potential problems.

One workaround is to insert cache barrier instructions before every memory access that
might be speculatively executed and that might have side effects even if aborted.
-mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It
assumes that aborted accesses to any byte in the following regions does not have side
effects:

1. the memory occupied by the current function's stack frame;

2. the memory occupied by an incoming stack argument;

3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions
are indeed safe.

If the input program contains a function declaration such as:

void foo (void);

then the implementation of "foo" must allow "j foo" and "jal foo" to be executed
speculatively. GCC honors this restriction for functions it compiles itself. It expects
non-GCC functions (such as hand-written assembly code) to do the same.

The option has three forms:

-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed
and that might have side effects even if aborted.

-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that
might have side effects even if aborted.

-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.

-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such
function. If called, the function must take the same arguments as the common
"_flush_func", that is, the address of the memory range for which the cache is being
flushed, the size of the memory range, and the number 3 (to flush both caches). The
default depends on the target GCC was configured for, but commonly is either
"_flush_func" or "__cpu_flush".

mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a
heuristic and is not guaranteed to produce consistent results across releases. A zero
cost redundantly selects the default, which is based on the -mtune setting.

-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the
selected architecture. By default, Branch Likely instructions may be generated if they
are supported by the selected architecture. An exception is for the MIPS32 and MIPS64
architectures and processors that implement those architectures; for those, Branch Likely
instructions are not be generated by default because the MIPS32 and MIPS64 architectures
specifically deprecate their use.

-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated. The default is
-mcompact-branches=optimal.

The -mcompact-branches=never option ensures that compact branch instructions will never
be generated.

The -mcompact-branches=always option ensures that a compact branch instruction will be
generated if available for MIPS Release 6 onwards. If a compact branch instruction is
not available (or pre-R6), a delay slot form of the branch will be used instead.

If it is used for MIPS16/microMIPS targets, it will be just ignored now. The behaviour
for MIPS16/microMIPS may change in future, since they do have some compact branch
instructions.

The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is
available in the current ISA and the delay slot is successfully filled. If the delay
slot is not filled, a compact branch will be chosen if one is available.

-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions are
scheduled for some processors. The default is that FP exceptions are enabled.

For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit
code, then we can use both FP pipes. Otherwise, we can only use one FP pipe.

-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together
if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of
instructions that it thinks should execute in parallel.

This option only has an effect when optimizing for the VR4130. It normally makes code
faster, but at the expense of making it bigger. It is enabled by default at optimization
level -O3.

-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that support it.
The "synci" instructions (if enabled) are generated when "__builtin___clear_cache" is
compiled.

This option defaults to -mno-synci, but the default can be overridden by configuring GCC
with --with-synci.

When compiling code for single processor systems, it is generally safe to use "synci".
However, on many multi-core (SMP) systems, it does not invalidate the instruction caches
on all cores and may lead to undefined behavior.

-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls.
This is only possible if the linker can resolve the destination at link time and if the
destination is within range for a direct call.

-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker
that support the ".reloc" assembly directive and -mexplicit-relocs is in effect. With
-mno-explicit-relocs, this optimization can be performed by the assembler and the linker
alone without help from the compiler.

-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's return
address. When enabled, this option extends the usual "_mcount" interface with a new ra-
address parameter, which has type "intptr_t *" and is passed in register $12. "_mcount"
can then modify the return address by doing both of the following:

* Returning the new address in register $31.

* Storing the new address in "*ra-address", if ra-address is nonnull.

The default is -mno-mcount-ra-address.

-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI,
calling functions will allocate 16 bytes on the stack for the called function to write
out register arguments. When enabled, this optimization will suppress the allocation of
the frame header if it can be determined that it is unused.

This optimization is off by default at all optimization levels.

-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1", "swxc1", "ldxc1", "sdxc1"
instructions. Enabled by default.

-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand "madd.s", "madd.d" and
related instructions. Enabled by default.

MMIX Options

These options are defined for the MMIX:

-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in
registers, no matter the size.

-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE"
epsilon register.

-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called
function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global
registers $231 and up.

-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-
extending load instructions by default, rather than sign-extending ones.

-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor.
With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the
dividend. Both methods are arithmetically valid, the latter being almost exclusively
used.

-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with
the "PREFIX" assembly directive.

-melf
Generate an executable in the ELF format, rather than the default mmo format used by the
mmix simulator.

-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction
indicates a probable branch.

-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address
automatically generates a request (handled by the assembler and the linker) for a
constant to be set up in a global register. The register is used for one or more base
address requests within the range 0 to 255 from the value held in the register. The
generally leads to short and fast code, but the number of different data items that can
be addressed is limited. This means that a program that uses lots of static data may
require -mno-base-addresses.

-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.

MN10300 Options

These -m options are defined for Matsushita MN10300 architectures:

-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors.
This is the default.

-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300
processors.

-mam33
Generate code using features specific to the AM33 processor.

-mno-am33
Do not generate code using features specific to the AM33 processor. This is the default.

-mam33-2
Generate code using features specific to the AM33/2.0 processor.

-mam34
Generate code using features specific to the AM34 processor.

-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions.
This does not change the targeted processor type. The CPU type must be one of mn10300,
am33, am33-2 or am34.

-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both "a0" and
"d0". Otherwise, the pointer is returned only in "a0", and attempts to call such
functions without a prototype result in errors. Note that this option is on by default;
use -mno-return-pointer-on-d0 to disable it.

-mno-crt0
Do not link in the C run-time initialization object file.

-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten
branches, calls and absolute memory addresses. This option only has an effect when used
on the command line for the final link step.

This option makes symbolic debugging impossible.

-mliw
Allow the compiler to generate Long Instruction Word instructions if the target is the
AM33 or later. This is the default. This option defines the preprocessor macro
"__LIW__".

-mno-liw
Do not allow the compiler to generate Long Instruction Word instructions. This option
defines the preprocessor macro "__NO_LIW__".

-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33
or later. This is the default. This option defines the preprocessor macro "__SETLB__".

-mno-setlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the
preprocessor macro "__NO_SETLB__".

Moxie Options

-meb
Generate big-endian code. This is the default for moxie-*-* configurations.

-mel
Generate little-endian code.

-mmul.x
Generate mul.x and umul.x instructions. This is the default for moxiebox-*-*
configurations.

-mno-crt0
Do not link in the C run-time initialization object file.

MSP430 Options

These options are defined for the MSP430:

-masm-hex
Force assembly output to always use hex constants. Normally such constants are signed
decimals, but this option is available for testsuite and/or aesthetic purposes.

-mmcu=
Select the MCU to target. This is used to create a C preprocessor symbol based upon the
MCU name, converted to upper case and pre- and post-fixed with __. This in turn is used
by the msp430.h header file to select an MCU-specific supplementary header file.

The option also sets the ISA to use. If the MCU name is one that is known to only
support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic
MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x
MCU name selects the 430X ISA.

In addition an MCU-specific linker script is added to the linker command line. The
script's name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the
gcc command line defines the C preprocessor symbol "__XXX__" and cause the linker to
search for a script called xxx.ld.

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC.
However, an external devices.csv file can be used to extend device support beyond those
that have been hard-coded.

GCC searches for the devices.csv file using the following methods in the given precedence
order, where the first method takes precendence over the second which takes precedence
over the third.

Include path specified with "-I" and "-L"
devices.csv will be searched for in each of the directories specified by include
paths and linker library search paths.

Path specified by the environment variable MSP430_GCC_INCLUDE_DIR
Define the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the
full path to the directory containing devices.csv, and GCC will search this directory
for devices.csv. If devices.csv is found, this directory will also be registered as
an include path, and linker library path. Header files and linker scripts in this
directory can therefore be used without manually specifying "-I" and "-L" on the
command line.

The msp430-elf{,bare}/include/devices directory
Finally, GCC will examine msp430-elf{,bare}/include/devices from the toolchain root
directory. This directory does not exist in a default installation, but if the user
has created it and copied devices.csv there, then the MCU data will be read. As
above, this directory will also be registered as an include path, and linker library
path.

If none of the above search methods find devices.csv, then the hard-coded MCU data is
used.

-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between the MCU name specified
by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply
support set by the -mhwmult option. It also toggles warnings about unrecognized MCU
names. This option is on by default.

-mcpu=
Specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This
option is deprecated. The -mmcu= option should be used to select the ISA.

-msim
Link to the simulator runtime libraries and linker script. Overrides any scripts that
would be selected by the -mmcu= option.

-mlarge
Use large-model addressing (20-bit pointers, 20-bit "size_t").

-msmall
Use small-model addressing (16-bit pointers, 16-bit "size_t").

-mrelax
This option is passed to the assembler and linker, and allows the linker to perform
certain optimizations that cannot be done until the final link.

mhwmult=
Describes the type of hardware multiply supported by the target. Accepted values are
none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by
early MCUs. 32bit for the 16/32-bit multiply supported by later MCUs and f5series for
the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given.
This tells GCC to deduce the hardware multiply support based upon the MCU name provided
by the -mmcu option. If no -mmcu option is specified or if the MCU name is not
recognized then no hardware multiply support is assumed. "auto" is the default setting.

Hardware multiplies are normally performed by calling a library routine. This saves
space in the generated code. When compiling at -O3 or higher however the hardware
multiplier is invoked inline. This makes for bigger, but faster code.

The hardware multiply routines disable interrupts whilst running and restore the previous
interrupt state when they finish. This makes them safe to use inside interrupt handlers
as well as in normal code.

-minrt
Enable the use of a minimum runtime environment - no static initializers or constructors.
This is intended for memory-constrained devices. The compiler includes special symbols
in some objects that tell the linker and runtime which code fragments are required.

-mtiny-printf
Enable reduced code size "printf" and "puts" library functions. The tiny implementations
of these functions are not reentrant, so must be used with caution in multi-threaded
applications.

Support for streams has been removed and the string to be printed will always be sent to
stdout via the "write" syscall. The string is not buffered before it is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with
--enable-newlib-nano-formatted-io.

-mmax-inline-shift=
This option takes an integer between 0 and 64 inclusive, and sets the maximum number of
inline shift instructions which should be emitted to perform a shift operation by a
constant amount. When this value needs to be exceeded, an mspabi helper function is used
instead. The default value is 4.

This only affects cases where a shift by multiple positions cannot be completed with a
single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this
option is divided by 2 and the resulting value used instead.

-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data that do not have one of
the "lower", "upper", "either" or "section" attributes. Possible values are "lower",
"upper", "either" or "any". The first three behave like the corresponding attribute.
The fourth possible value - "any" - is the default. It leaves placement entirely up to
the linker script and how it assigns the standard sections (".text", ".data", etc) to the
memory regions.

-msilicon-errata=
This option passes on a request to assembler to enable the fixes for the named silicon
errata.

-msilicon-errata-warn=
This option passes on a request to the assembler to enable warning messages when a
silicon errata might need to be applied.

-mwarn-devices-csv
-mno-warn-devices-csv
Warn if devices.csv is not found or there are problem parsing it (default: on).

NDS32 Options

These options are defined for NDS32 implementations:

-mbig-endian
Generate code in big-endian mode.

-mlittle-endian
Generate code in little-endian mode.

-mreduced-regs
Use reduced-set registers for register allocation.

-mfull-regs
Use full-set registers for register allocation.

-mcmov
Generate conditional move instructions.

-mno-cmov
Do not generate conditional move instructions.

-mext-perf
Generate performance extension instructions.

-mno-ext-perf
Do not generate performance extension instructions.

-mext-perf2
Generate performance extension 2 instructions.

-mno-ext-perf2
Do not generate performance extension 2 instructions.

-mext-string
Generate string extension instructions.

-mno-ext-string
Do not generate string extension instructions.

-mv3push
Generate v3 push25/pop25 instructions.

-mno-v3push
Do not generate v3 push25/pop25 instructions.

-m16-bit
Generate 16-bit instructions.

-mno-16-bit
Do not generate 16-bit instructions.

-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or 16.

-mcache-block-size=num
Specify the size of each cache block, which must be a power of 2 between 4 and 512.

-march=arch
Specify the name of the target architecture.

-mcmodel=code-model
Set the code model to one of

small
All the data and read-only data segments must be within 512KB addressing space. The
text segment must be within 16MB addressing space.

medium
The data segment must be within 512KB while the read-only data segment can be within
4GB addressing space. The text segment should be still within 16MB addressing space.

large
All the text and data segments can be within 4GB addressing space.

-mctor-dtor
Enable constructor/destructor feature.

-mrelax
Guide linker to relax instructions.

Nios II Options

These are the options defined for the Altera Nios II processor.

-G num
Put global and static objects less than or equal to num bytes into the small data or BSS
sections instead of the normal data or BSS sections. The default value of num is 8.

-mgpopt=option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The following option names are
recognized:

none
Do not generate GP-relative accesses.

local
Generate GP-relative accesses for small data objects that are not external, weak, or
uninitialized common symbols. Also use GP-relative addressing for objects that have
been explicitly placed in a small data section via a "section" attribute.

global
As for local, but also generate GP-relative accesses for small data objects that are
external, weak, or common. If you use this option, you must ensure that all parts of
your program (including libraries) are compiled with the same -G setting.

data
Generate GP-relative accesses for all data objects in the program. If you use this
option, the entire data and BSS segments of your program must fit in 64K of memory
and you must use an appropriate linker script to allocate them within the addressable
range of the global pointer.

all Generate GP-relative addresses for function pointers as well as data pointers. If
you use this option, the entire text, data, and BSS segments of your program must fit
in 64K of memory and you must use an appropriate linker script to allocate them
within the addressable range of the global pointer.

-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.

The default is -mgpopt except when -fpic or -fPIC is specified to generate position-
independent code. Note that the Nios II ABI does not permit GP-relative accesses from
shared libraries.

You may need to specify -mno-gpopt explicitly when building programs that include large
amounts of small data, including large GOT data sections. In this case, the 16-bit
offset for GP-relative addressing may not be large enough to allow access to the entire
small data section.

-mgprel-sec=regexp
This option specifies additional section names that can be accessed via GP-relative
addressing. It is most useful in conjunction with "section" attributes on variable
declarations and a custom linker script. The regexp is a POSIX Extended Regular
Expression.

This option does not affect the behavior of the -G option, and the specified sections are
in addition to the standard ".sdata" and ".sbss" small-data sections that are recognized
by -mgpopt.

-mr0rel-sec=regexp
This option specifies names of sections that can be accessed via a 16-bit offset from
"r0"; that is, in the low 32K or high 32K of the 32-bit address space. It is most useful
in conjunction with "section" attributes on variable declarations and a custom linker
script. The regexp is a POSIX Extended Regular Expression.

In contrast to the use of GP-relative addressing for small data, zero-based addressing is
never generated by default and there are no conventional section names used in standard
linker scripts for sections in the low or high areas of memory.

-mel
-meb
Generate little-endian (default) or big-endian (experimental) code, respectively.

-march=arch
This specifies the name of the target Nios II architecture. GCC uses this name to
determine what kind of instructions it can emit when generating assembly code.
Permissible names are: r1, r2.

The preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2,
indicating the targeted ISA level.

-mbypass-cache
-mno-bypass-cache
Force all load and store instructions to always bypass cache by using I/O variants of the
instructions. The default is not to bypass the cache.

-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O variants of the load and store
instructions. The default is not to bypass the cache.

-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers. The default is to use the fast
divide at -O3 and above.

-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
Enable or disable emitting "mul", "mulx" and "div" family of instructions by the
compiler. The default is to emit "mul" and not emit "div" and "mulx".

-mbmx
-mno-bmx
-mcdx
-mno-cdx
Enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density)
instructions. Enabling these instructions also requires -march=r2. Since these
instructions are optional extensions to the R2 architecture, the default is not to emit
them.

-mcustom-insn=N
-mno-custom-insn
Each -mcustom-insn=N option enables use of a custom instruction with encoding N when
generating code that uses insn. For example, -mcustom-fadds=253 generates custom
instruction 253 for single-precision floating-point add operations instead of the default
behavior of using a library call.

The following values of insn are supported. Except as otherwise noted, floating-point
operations are expected to be implemented with normal IEEE 754 semantics and correspond
directly to the C operators or the equivalent GCC built-in functions.

Single-precision floating point:

fadds, fsubs, fdivs, fmuls
Binary arithmetic operations.

fnegs
Unary negation.

fabss
Unary absolute value.

fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
Comparison operations.

fmins, fmaxs
Floating-point minimum and maximum. These instructions are only generated if
-ffinite-math-only is specified.

fsqrts
Unary square root operation.

fcoss, fsins, ftans, fatans, fexps, flogs
Floating-point trigonometric and exponential functions. These instructions are only
generated if -funsafe-math-optimizations is also specified.

Double-precision floating point:

faddd, fsubd, fdivd, fmuld
Binary arithmetic operations.

fnegd
Unary negation.

fabsd
Unary absolute value.

fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
Comparison operations.

fmind, fmaxd
Double-precision minimum and maximum. These instructions are only generated if
-ffinite-math-only is specified.

fsqrtd
Unary square root operation.

fcosd, fsind, ftand, fatand, fexpd, flogd
Double-precision trigonometric and exponential functions. These instructions are
only generated if -funsafe-math-optimizations is also specified.

Conversions:

fextsd
Conversion from single precision to double precision.

ftruncds
Conversion from double precision to single precision.

fixsi, fixsu, fixdi, fixdu
Conversion from floating point to signed or unsigned integer types, with truncation
towards zero.

round
Conversion from single-precision floating point to signed integer, rounding to the
nearest integer and ties away from zero. This corresponds to the "__builtin_lroundf"
function when -fno-math-errno is used.

floatis, floatus, floatid, floatud
Conversion from signed or unsigned integer types to floating-point types.

In addition, all of the following transfer instructions for internal registers X and Y
must be provided to use any of the double-precision floating-point instructions. Custom
instructions taking two double-precision source operands expect the first operand in the
64-bit register X. The other operand (or only operand of a unary operation) is given to
the custom arithmetic instruction with the least significant half in source register src1
and the most significant half in src2. A custom instruction that returns a double-
precision result returns the most significant 32 bits in the destination register and the
other half in 32-bit register Y. GCC automatically generates the necessary code
sequences to write register X and/or read register Y when double-precision floating-point
instructions are used.

fwrx
Write src1 into the least significant half of X and src2 into the most significant
half of X.

fwry
Write src1 into Y.

frdxhi, frdxlo
Read the most or least (respectively) significant half of X and store it in dest.

frdy
Read the value of Y and store it into dest.

Note that you can gain more local control over generation of Nios II custom instructions
by using the target("custom-insn=N") and target("no-custom-insn") function attributes or
pragmas.

-mcustom-fpu-cfg=name
This option enables a predefined, named set of custom instruction encodings (see
-mcustom-insn above). Currently, the following sets are defined:

-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -fsingle-precision-constant

-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant

-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244
-mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250
-mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant

-mcustom-fpu-cfg=fph2 is equivalent to: -mcustom-fabss=224 -mcustom-fnegs=225
-mcustom-fcmpnes=226 -mcustom-fcmpeqs=227 -mcustom-fcmpges=228 -mcustom-fcmpgts=229
-mcustom-fcmples=230 -mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233
-mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250 -mcustom-fsqrts=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255

Custom instruction assignments given by individual -mcustom-insn= options override those
given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line.

Note that you can gain more local control over selection of a FPU configuration by using
the target("custom-fpu-cfg=name") function attribute or pragma.

The name fph2 is an abbreviation for Nios II Floating Point Hardware 2 Component. Please
note that the custom instructions enabled by -mcustom-fmins=233 and -mcustom-fmaxs=234
are only generated if -ffinite-math-only is specified. The custom instruction enabled by
-mcustom-round=248 is only generated if -fno-math-errno is specified. In contrast to the
other configurations, -fsingle-precision-constant is not set.

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

-mhal
Link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and
termination code, and is typically used in conjunction with -msys-crt0= to specify the
location of the alternate startup code provided by the HAL BSP.

-msmallc
Link with a limited version of the C library, -lsmallc, rather than Newlib.

-msys-crt0=startfile
startfile is the file name of the startfile (crt0) to use when linking. This option is
only useful in conjunction with -mhal.

-msys-lib=systemlib
systemlib is the library name of the library that provides low-level system calls
required by the C library, e.g. "read" and "write". This option is typically used to
link with a library provided by a HAL BSP.

Nvidia PTX Options

These options are defined for Nvidia PTX:

-m64
Ignored, but preserved for backward compatibility. Only 64-bit ABI is supported.

-march=architecture-string
Generate code for the specified PTX ISA target architecture (e.g. sm_35). Valid
architecture strings are sm_30, sm_35, sm_53, sm_70, sm_75 and sm_80. The default
depends on how the compiler has been configured, see --with-arch.

This option sets the value of the preprocessor macro "__PTX_SM__"; for instance, for
sm_35, it has the value 350.

-misa=architecture-string
Alias of -march=.

-march-map=architecture-string
Select the closest available -march= value that is not more capable. For instance, for
-march-map=sm_50 select -march=sm_35, and for -march-map=sm_53 select -march=sm_53.

-mptx=version-string
Generate code for the specified PTX ISA version (e.g. 7.0). Valid version strings
include 3.1, 6.0, 6.3, and 7.0. The default PTX ISA version is 6.0, unless a higher
version is required for specified PTX ISA target architecture via option -march=.

This option sets the values of the preprocessor macros "__PTX_ISA_VERSION_MAJOR__" and
"__PTX_ISA_VERSION_MINOR__"; for instance, for 3.1 the macros have the values 3 and 1,
respectively.

-mmainkernel
Link in code for a __main kernel. This is for stand-alone instead of offloading
execution.

-moptimize
Apply partitioned execution optimizations. This is the default when any level of
optimization is selected.

-msoft-stack
Generate code that does not use ".local" memory directly for stack storage. Instead, a
per-warp stack pointer is maintained explicitly. This enables variable-length stack
allocation (with variable-length arrays or "alloca"), and when global memory is used for
underlying storage, makes it possible to access automatic variables from other threads,
or with atomic instructions. This code generation variant is used for OpenMP offloading,
but the option is exposed on its own for the purpose of testing the compiler; to generate
code suitable for linking into programs using OpenMP offloading, use option -mgomp.

-muniform-simt
Switch to code generation variant that allows to execute all threads in each warp, while
maintaining memory state and side effects as if only one thread in each warp was active
outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc,
free, vprintf) are conditionally executed (iff current lane index equals the master lane
index), and the register being assigned is copied via a shuffle instruction from the
master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees
itself as the master. Shared memory array "int __nvptx_uni[]" stores all-zeros or all-
ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each
thread can bitwise-and the bitmask at position "tid.y" with current lane index to compute
the master lane index.

-mgomp
Generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt
options, and selects corresponding multilib variant.

OpenRISC Options

These options are defined for OpenRISC:

-mboard=name
Configure a board specific runtime. This will be passed to the linker for newlib board
library linking. The default is "or1ksim".

-mnewlib
This option is ignored; it is for compatibility purposes only. This used to select
linker and preprocessor options for use with newlib.

-msoft-div
-mhard-div
Select software or hardware divide ("l.div", "l.divu") instructions. This default is
hardware divide.

-msoft-mul
-mhard-mul
Select software or hardware multiply ("l.mul", "l.muli") instructions. This default is
hardware multiply.

-msoft-float
-mhard-float
Select software or hardware for floating point operations. The default is software.

-mdouble-float
When -mhard-float is selected, enables generation of double-precision floating point
instructions. By default functions from libgcc are used to perform double-precision
floating point operations.

-munordered-float
When -mhard-float is selected, enables generation of unordered floating point compare and
set flag ("lf.sfun*") instructions. By default functions from libgcc are used to perform
unordered floating point compare and set flag operations.

-mcmov
Enable generation of conditional move ("l.cmov") instructions. By default the equivalent
will be generated using set and branch.

-mror
Enable generation of rotate right ("l.ror") instructions. By default functions from
libgcc are used to perform rotate right operations.

-mrori
Enable generation of rotate right with immediate ("l.rori") instructions. By default
functions from libgcc are used to perform rotate right with immediate operations.

-msext
Enable generation of sign extension ("l.ext*") instructions. By default memory loads are
used to perform sign extension.

-msfimm
Enable generation of compare and set flag with immediate ("l.sf*i") instructions. By
default extra instructions will be generated to store the immediate to a register first.

-mshftimm
Enable generation of shift with immediate ("l.srai", "l.srli", "l.slli") instructions.
By default extra instructions will be generated to store the immediate to a register
first.

-mcmodel=small
Generate OpenRISC code for the small model: The GOT is limited to 64k. This is the
default model.

-mcmodel=large
Generate OpenRISC code for the large model: The GOT may grow up to 4G in size.

PDP-11 Options

These options are defined for the PDP-11:

-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the
PDP-11/40 is not supported.) Implies -m45.

-msoft-float
Do not use hardware floating point.

-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).

-mno-ac0
Return floating-point results in memory. This is the default.

-m40
Generate code for a PDP-11/40. Implies -msoft-float -mno-split.

-m45
Generate code for a PDP-11/45. This is the default.

-m10
Generate code for a PDP-11/10. Implies -msoft-float -mno-split.

-mint16
-mno-int32
Use 16-bit "int". This is the default.

-mint32
-mno-int16
Use 32-bit "int".

-msplit
Target has split instruction and data space. Implies -m45.

-munix-asm
Use Unix assembler syntax.

-mdec-asm
Use DEC assembler syntax.

-mgnu-asm
Use GNU assembler syntax. This is the default.

-mlra
Use the new LRA register allocator. By default, the old "reload" allocator is used.

PowerPC Options

These are listed under

PRU Options

These command-line options are defined for PRU target:

-minrt
Link with a minimum runtime environment, with no support for static initializers and
constructors. Using this option can significantly reduce the size of the final ELF
binary. Beware that the compiler could still generate code with static initializers and
constructors. It is up to the programmer to ensure that the source program will not use
those features.

-mmcu=mcu
Specify the PRU MCU variant to use. Check Newlib for the exact list of supported MCUs.

-mno-relax
Make GCC pass the --no-relax command-line option to the linker instead of the --relax
option.

-mloop
Allow (or do not allow) GCC to use the LOOP instruction.

-mabi=variant
Specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI while
-mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions.
These are the differences:

Function Pointer Size
TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only
32-bit data and code pointers.

Optional Return Value Pointer
Function return values larger than 64 bits are passed by using a hidden pointer as
the first argument of the function. TI ABI, though, mandates that the pointer can be
NULL in case the caller is not using the returned value. GNU always passes and
expects a valid return value pointer.

The current -mabi=ti implementation simply raises a compile error when any of the above
code constructs is detected. As a consequence the standard C library cannot be built and
it is omitted when linking with -mabi=ti.

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The
TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot
adjust them when shortening adjacent LDI32 pseudo instructions.

RISC-V Options

These command-line options are defined for RISC-V targets:

-mbranch-cost=n
Set the cost of branches to roughly n instructions.

-mplt
-mno-plt
When generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The
default is -mplt.

-mabi=ABI-string
Specify integer and floating-point calling convention. ABI-string contains two parts:
the size of integer types and the registers used for floating-point types. For example
-march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining
int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F
registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the
compiler to generate code that uses the F and D extensions but only allows floating-point
values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in
which no floating-point arguments will be passed in registers.

The default for this argument is system dependent, users who want a specific calling
convention should specify one explicitly. The valid calling conventions are: ilp32,
ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to
implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the
ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits
wide. There is also the ilp32e ABI that can only be used with the rv32e architecture.
This ABI is not well specified at present, and is subject to change.

-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and square root instructions. This
requires the F or D extensions for floating-point registers. The default is to use them
if the specified architecture has these instructions.

-mdiv
-mno-div
Do or don't use hardware instructions for integer division. This requires the M
extension. The default is to use them if the specified architecture has these
instructions.

-misa-spec=ISA-spec-string
Specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to
produce code conforming to. The possibilities for ISA-spec-string are:

2.2 Produce code conforming to version 2.2.

20190608
Produce code conforming to version 20190608.

20191213
Produce code conforming to version 20191213.

The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec=
specifying a different default version.

-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case.
Examples include rv64i, rv32g, rv32e, and rv32imaf.

When -march= is not specified, use the setting from -mcpu.

If both -march and -mcpu= are not specified, the default for this argument is system
dependent, users who want a specific architecture extensions should specify one
explicitly.

-mcpu=processor-string
Use architecture of and optimize the output for the given processor, specified by
particular CPU name. Permissible values for this option are: sifive-e20, sifive-e21,
sifive-e24, sifive-e31, sifive-e34, sifive-e76, sifive-s21, sifive-s51, sifive-s54,
sifive-s76, sifive-u54, and sifive-u74.

-mtune=processor-string
Optimize the output for the given processor, specified by microarchitecture or particular
CPU name. Permissible values for this option are: rocket, sifive-3-series,
sifive-5-series, sifive-7-series, thead-c906, size, and all valid options for -mcpu=.

When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both
are not specified.

The size choice is not intended for use by end-users. This is used when -Os is
specified. It overrides the instruction cost info provided by -mtune=, but does not
override the pipeline info. This helps reduce code size while still giving good
performance.

-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128-bits).

Warning: If you use this switch, then you must build all modules with the same value,
including any libraries. This includes the system libraries and startup modules.

-msmall-data-limit=n
Put global and static data smaller than n bytes into a special section (on some targets).

-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue code that uses library function
calls. The default is to use fast inline prologues and epilogues.

-minline-atomics
-mno-inline-atomics
Do or don't use smaller but slower subword atomic emulation code that uses libatomic
function calls. The default is to use fast inline subword atomics that do not require
libatomic.

-mshorten-memrefs
-mno-shorten-memrefs
Do or do not attempt to make more use of compressed load/store instructions by replacing
a load/store of 'base register + large offset' with a new load/store of 'new base + small
offset'. If the new base gets stored in a compressed register, then the new load/store
can be compressed. Currently targets 32-bit integer load/stores only.

-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default is set depending on whether
the processor we are optimizing for supports fast unaligned access or not.

-mcmodel=medlow
Generate code for the medium-low code model. The program and its statically defined
symbols must lie within a single 2 GiB address range and must lie between absolute
addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is
the default code model.

-mcmodel=medany
Generate code for the medium-any code model. The program and its statically defined
symbols must be within any single 2 GiB address range. Programs can be statically or
dynamically linked.

The code generated by the medium-any code model is position-independent, but is not
guaranteed to function correctly when linked into position-independent executables or
libraries.

-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing with symbolic addresses.
The alternative is to use assembler macros instead, which may limit optimization.

-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of instructions required to
materialize symbol addresses. The default is to take advantage of linker relaxations.

-mriscv-attribute
-mno-riscv-attribute
Emit (do not emit) RISC-V attribute to record extra information into ELF objects. This
feature requires at least binutils 2.32.

-mcsr-check
-mno-csr-check
Enables or disables the CSR checking.

-malign-data=type
Control how GCC aligns variables and constants of array, structure, or union types.
Supported values for type are xlen which uses x register width as the alignment value,
and natural which uses natural alignment. xlen is the default.

-mbig-endian
Generate big-endian code. This is the default when GCC is configured for a riscv64be-*-*
or riscv32be-*-* target.

-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for a
riscv64-*-* or riscv32-*-* but not a riscv64be-*-* or riscv32be-*-* target.

With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which register to use as base
register for reading the canary, and from what offset from that base register. There is
no default register or offset as this is entirely for use within the Linux kernel.

RL78 Options

-msim
Links in additional target libraries to support operation within a simulator.

-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division support to be used. The
simplest is "none", which uses software for both multiplication and division. This is
the default. The "g13" value is for the hardware multiply/divide peripheral found on the
RL78/G13 (S2 core) targets. The "g14" value selects the use of the multiplication and
division instructions supported by the RL78/G14 (S3 core) parts. The value "rl78" is an
alias for "g14" and the value "mg10" is an alias for "none".

In addition a C preprocessor macro is defined, based upon the setting of this option.
Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".

-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core, also known as an S3 core
or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead
it uses a hardware peripheral for these operations. The G10 or S1 core does not have
register banks, so it uses a different calling convention.

If this option is set it also selects the type of hardware multiply support to use,
unless this is overridden by an explicit -mmul=none option on the command line. Thus
specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and
specifying -mcpu=g10 disables the use of hardware multiplications altogether.

Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or
-mcpu=rl78 on the command line does change the behavior of the toolchain since it also
enables G14 hardware multiply support. If these options are not specified on the command
line then software multiplication routines will be used even though the code targets the
RL78 core. This is for backwards compatibility with older toolchains which did not have
hardware multiply and divide support.

In addition a C preprocessor macro is defined, based upon the setting of this option.
Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".

-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They are provided for backwards
compatibility.

-mallregs
Allow the compiler to use all of the available registers. By default registers
"r24..r31" are reserved for use in interrupt handlers. With this option enabled these
registers can be used in ordinary functions as well.

-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in
size. The default is -m32bit-doubles.

-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the MDUC registers. This is
only necessary if normal code might use the MDUC registers, for example because it
performs multiplication and division operations. The default is to ignore the MDUC
registers as this makes the interrupt handlers faster. The target option -mg13 needs to
be passed for this to work as this feature is only available on the G13 target (S2 core).
The MDUC registers will only be saved if the interrupt handler performs a multiplication
or division operation or it calls another function.

IBM RS/6000 and PowerPC Options

These -m options are defined for the IBM RS/6000 and PowerPC:

-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on the processor you
are using. The default value of these options is determined when configuring GCC.
Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend
you use the -mcpu=cpu_type option rather than the options listed above.

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture
instructions in the General Purpose group, including floating-point square root.
Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture
instructions in the Graphics group, including floating-point select.

The -mmfcrf option allows GCC to generate the move from condition register field
instruction implemented on the POWER4 processor and other processors that support the
PowerPC V2.01 architecture. The -mpopcntb option allows GCC to generate the popcount and
double-precision FP reciprocal estimate instruction implemented on the POWER5 processor
and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option
allows GCC to generate the popcount instruction implemented on the POWER7 processor and
other processors that support the PowerPC V2.06 architecture. The -mfprnd option allows
GCC to generate the FP round to integer instructions implemented on the POWER5+ processor
and other processors that support the PowerPC V2.03 architecture. The -mcmpb option
allows GCC to generate the compare bytes instruction implemented on the POWER6 processor
and other processors that support the PowerPC V2.05 architecture. The -mhard-dfp option
allows GCC to generate the decimal floating-point instructions implemented on some POWER
processors.

The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are
found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword
quantities. GCC defaults to -mno-powerpc64.

-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling parameters for machine
type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464,
464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750,
801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500,
ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7,
power8, power9, power10, powerpc, powerpc64, powerpc64le, rs64, and native.

-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either
endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine
types, with an appropriate, generic processor model assumed for scheduling purposes.

Specifying native as cpu type detects and selects the architecture option that
corresponds to the host processor of the system performing the compilation. -mcpu=native
has no effect if GCC does not recognize the processor.

The other options specify a specific processor. Code generated under those options runs
best on that processor, and may not run at all on others.

The -mcpu options automatically enable or disable the following options:

-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -mpowerpc64
-mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm
-mpower8-fusion -mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128
-mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protect

The particular options set for any particular CPU varies between compiler versions,
depending on what setting seems to produce optimal code for that CPU; it doesn't
necessarily reflect the actual hardware's capabilities. If you wish to set an individual
option to a particular value, you may specify it after the -mcpu option, like -mcpu=970
-mno-altivec.

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu
option at present because AIX does not have full support for these options. You may
still enable or disable them individually if you're sure it'll work in your environment.

-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the
architecture type or register usage, as -mcpu=cpu_type does. The same values for
cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated
uses the architecture and registers set by -mcpu, but the scheduling parameters set by
-mtune.

-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.

-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to
a total of 4G in size. This is the default for 64-bit Linux.

-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data
and code is only limited by the 64-bit address space.

-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of
built-in functions that allow more direct access to the AltiVec instruction set. You may
also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

When -maltivec is used, the element order for AltiVec intrinsics such as "vec_splat",
"vec_extract", and "vec_insert" match array element order corresponding to the endianness
of the target. That is, element zero identifies the leftmost element in a vector
register when targeting a big-endian platform, and identifies the rightmost element in a
vector register when targeting a little-endian platform.

-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.

-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with
non-executable ".plt" and ".got" sections. This is a PowerPC 32-bit SYSV ABI option.

-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and requires ".plt" and
".got" sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI
option.

-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.

-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable
the use of built-in functions that allow more direct access to the VSX instruction set.

-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow direct access to the
cryptographic instructions that were added in version 2.07 of the PowerPC ISA.

-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow direct access to the
Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the
PowerPC ISA.

-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations adjacent so that the
instructions can be fused together on power8 and later processors.

-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar instructions that were added
in version 2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow
more direct access to the vector instructions.

-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word memory instructions. The
-mquad-memory option requires use of 64-bit mode.

-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory instructions. The
-mquad-memory-atomic option requires use of 64-bit mode.

-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating point and use either
software emulation for IEEE 128-bit floating point or hardware instructions.

The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point
support. The IEEE 128-bit floating point is only supported on Linux.

The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction
set, and disabled on other systems.

If you use the ISA 3.0 instruction set (-mpower9-vector or -mcpu=power9) on a 64-bit
system, the IEEE 128-bit floating point support will also enable the generation of ISA
3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify to
generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE
128-bit floating point will be done with software emulation.

-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.

The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0
instruction set, and disabled on other systems.

-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including
GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates
code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.

-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable
file. The -mfull-toc option is selected by default. In that case, GCC allocates at
least one TOC entry for each unique non-automatic variable reference in your program.
GCC also places floating-point constants in the TOC. However, only 16,384 entries are
available in the TOC.

If you receive a linker error message that saying you have overflowed the available TOC
space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and
-mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point
constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum
of an address and a constant at run time instead of putting that sum into the TOC. You
may specify one or both of these options. Each causes GCC to produce very slightly
slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options,
specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for
every file. When you specify this option, GCC produces code that is slower and larger
but which uses extremely little TOC space. You may wish to use this option only on files
that contain less frequently-executed code.

-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and
the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while
-maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.

-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-
compatible ABI. Pass floating-point arguments to prototyped functions beyond the
register save area (RSA) on the stack in addition to argument FPRs. Do not assume that
most significant double in 128-bit long double value is properly rounded when comparing
values and converting to double. Use XL symbol names for long double support routines.

The AIX calling convention was extended but not initially documented to handle an obscure
K&R C case of calling a function that takes the address of its arguments with fewer
arguments than declared. IBM XL compilers access floating-point arguments that do not
fit in the RSA from the stack when a subroutine is compiled without optimization.
Because always storing floating-point arguments on the stack is inefficient and rarely
needed, this option is not enabled by default and only is necessary when calling
subroutines compiled by IBM XL compilers without optimization.

-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use
message passing with special startup code to enable the application to run. The system
must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file
must be overridden with the -specs= option to specify the appropriate directory location.
The Parallel Environment does not support threads, so the -mpe option and the -pthread
option are incompatible.

-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides
the ABI-defined alignment of larger types, such as floating-point doubles, on their
natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-
specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.

-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software
floating-point emulation is provided if you use the -msoft-float option, and pass the
option to GCC when linking.

-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store
multiple word instructions. These instructions are generated by default on POWER
systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian
PowerPC systems, since those instructions do not work when the processor is in little-
endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in
little-endian mode.

-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the
base register to the address of the calculated memory location. These instructions are
generated by default. If you use -mno-update, there is a small window between the time
that the stack pointer is updated and the address of the previous frame is stored, which
means code that walks the stack frame across interrupts or signals may get corrupted
data.

-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store
instructions. These instructions can incur a performance penalty on Power6 processors in
certain situations, such as when stepping through large arrays that cross a 16M boundary.
This option is enabled by default when targeting Power6 and disabled otherwise.

-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate
instructions. These instructions are generated by default if hardware floating point is
used. The machine-dependent -mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.

-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-accumulate
instructions on the IBM 405, 440, 464 and 476 processors. These instructions are
generated by default when targeting those processors.

-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on the IBM
405, 440, 464 and 476 processors. This instruction is generated by default when
targeting those processors.

-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that
contain bit-fields to be aligned to the base type of the bit-field.

For example, by default a structure containing nothing but 8 "unsigned" bit-fields of
length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using
-mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size.

-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory
references are handled by the system.

-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated to a
different address at run time. A simple embedded PowerPC system loader should relocate
the entire contents of ".got2" and 4-byte locations listed in the ".fixup" section, a
table of 32-bit addresses generated by this option. For this to work, all objects linked
together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code
aligns the stack to an 8-byte boundary.

-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static
executables to be relocated at run time, but -mrelocatable-lib does not use the smaller
stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked
with objects compiled with any combination of the -mrelocatable options.

-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a
pointer to a global area pointing to the addresses used in the program.

-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little-
endian mode. The -mlittle-endian option is the same as -mlittle.

-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big-endian
mode. The -mbig-endian option is the same as -mbig.

-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its
external references are relocatable. The resulting code is suitable for applications,
but not shared libraries.

-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted
instructions during the second scheduling pass. The argument priority takes the value 0,
1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot
restricted instructions.

-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during
instruction scheduling. The argument dependence_type takes one of the following values:

no No dependence is costly.

all All dependences are costly.

true_store_to_load
A true dependence from store to load is costly.

store_to_load
Any dependence from store to load is costly.

number
Any dependence for which the latency is greater than or equal to number is costly.

-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the second scheduling
pass. The argument scheme takes one of the following values:

no Don't insert NOPs.

pad Pad with NOPs any dispatch group that has vacant issue slots, according to the
scheduler's grouping.

regroup_exact
Insert NOPs to force costly dependent insns into separate groups. Insert exactly as
many NOPs as needed to force an insn to a new group, according to the estimated
processor grouping.

number
Insert NOPs to force costly dependent insns into separate groups. Insert number NOPs
to force an insn to a new group.

-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that
adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC
processor supplement. This is the default unless you configured GCC using
powerpc-*-eabiaix.

-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.

-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.

-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.

-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.

-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.

-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.

-mtraceback=traceback_type
Select the type of traceback table. Valid values for traceback_type are full, part, and
no.

-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).

-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).

-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid
values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for
AIX: vec-extabi, vec-default.

-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is not likely to
work if your system defaults to using IEEE extended-precision long double. If you change
the long double type from IEEE extended-precision, the compiler will issue a warning
unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.

-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is not likely to
work if your system defaults to using IBM extended-precision long double. If you change
the long double type from IBM extended-precision, the compiler will issue a warning
unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.

-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian
PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is
likely to fail in spectacular ways.

-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian
PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is
likely to fail in spectacular ways.

-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes
section that specify ABI variations in function parameters or return values.

-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument
functions are properly prototyped. Otherwise, the compiler must insert an instruction
before every non-prototyped call to set or clear bit 6 of the condition code register
("CR") to indicate whether floating-point values are passed in the floating-point
registers in case the function takes variable arguments. With -mprototype, only calls to
prototyped variable argument functions set or clear the bit.

-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that
the standard C libraries are libsim.a and libc.a. This is the default for
powerpc-*-eabisim configurations.

-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the
standard C libraries are libmvme.a and libc.a.

-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the
standard C libraries are libads.a and libc.a.

-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the
standard C libraries are libyk.a and libc.a.

-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks
system.

-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate
that eabi extended relocations are used.

-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded
Applications Binary Interface (EABI), which is a set of modifications to the System V.4
specifications. Selecting -meabi means that the stack is aligned to an 8-byte boundary,
a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata
option can use both "r2" and "r13" to point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned to a 16-byte boundary, no EABI initialization
function is called from "main", and the -msdata option only uses "r13" to point to a
single small data area. The -meabi option is on by default if you configured GCC using
one of the powerpc*-*-eabi* options.

-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and
static data in the ".sdata2" section, which is pointed to by register "r2". Put small
initialized non-"const" global and static data in the ".sdata" section, which is pointed
to by register "r13". Put small uninitialized global and static data in the ".sbss"
section, which is adjacent to the ".sdata" section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb
option.

-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the
".sdata" section, which is pointed to by register "r13". Put small uninitialized global
and static data in the ".sbss" section, which is adjacent to the ".sdata" section. The
-msdata=sysv option is incompatible with the -mrelocatable option.

-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as
-msdata=eabi, otherwise compile code the same as -msdata=sysv.

-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the ".sdata"
section. Put small uninitialized global data in the ".sbss" section. Do not use
register "r13" to address small data however. This is the default behavior unless other
-msdata options are used.

-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the ".data"
section, and all uninitialized data in the ".bss" section.

-mreadonly-in-sdata
Put read-only objects in the ".sdata" section as well. This is the default.

-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal
to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on
64-bit targets. The default value is target-specific.

-mblock-compare-inline-limit=num
Generate non-looping inline code for all block compares (such as calls to "memcmp" or
structure compares) less than or equal to num bytes. If num is 0, all inline expansion
(non-loop and loop) of block compare is disabled. The default value is target-specific.

-mblock-compare-inline-loop-limit=num
Generate an inline expansion using loop code for all block compares that are less than or
equal to num bytes, but greater than the limit for non-loop inline block compare
expansion. If the block length is not constant, at most num bytes will be compared before
"memcmp" is called to compare the remainder of the block. The default value is target-
specific.

-mstring-compare-inline-limit=num
Compare at most num string bytes with inline code. If the difference or end of string is
not found at the end of the inline compare a call to "strcmp" or "strncmp" will take care
of the rest of the comparison. The default is 64 bytes.

-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes
into the small data or BSS sections instead of the normal data or BSS section. By
default, num is 8. The -G num switch is also passed to the linker. All modules should
be compiled with the same -G num value.

-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the
assembly language output using symbolic forms.

-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more expensive calling
sequence is required. This is required for calls farther than 32 megabytes (33,554,432
bytes) from the current location. A short call is generated if the compiler knows the
call cannot be that far away. This setting can be overridden by the "shortcall" function
attribute, or by #pragma longcall(0).

Some linkers are capable of detecting out-of-range calls and generating glue code on the
fly. On these systems, long calls are unnecessary and generate slower code. As of this
writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned
to add this feature to the GNU linker for 32-bit PowerPC systems as well.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate
long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and
PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.

On Darwin/PPC systems, "#pragma longcall" generates "jbsr callee, L42", plus a branch
island (glue code). The two target addresses represent the callee and the branch island.
The Darwin/PPC linker prefers the first address and generates a "bl callee" if the PPC
"bl" instruction reaches the callee directly; otherwise, the linker generates "bl L42" to
call the branch island. The branch island is appended to the body of the calling
function; it computes the full 32-bit address of the callee and jumps to it.

On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every
direct call, and the Darwin linker decides whether to use or discard it.

In the future, GCC may ignore all longcall specifications when the linker is known to
generate glue.

-mpltseq
-mno-pltseq
Implement (do not implement) -fno-plt and long calls using an inline PLT call sequence
that supports lazy linking and long calls to functions in dlopen'd shared libraries.
Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with
newer GNU linkers, and are enabled by default if the support is detected when configuring
GCC, and, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt.
-mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked
together.

-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function
argument. The relocation allows the linker to reliably associate function call with
argument setup instructions for TLS optimization, which in turn allows GCC to better
schedule the sequence.

-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root estimate
instructions with additional Newton-Raphson steps to increase precision instead of doing
a divide or square root and divide for floating-point arguments. You should use the
-ffast-math option when using -mrecip (or at least -funsafe-math-optimizations,
-ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note that while the
throughput of the sequence is generally higher than the throughput of the non-reciprocal
instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the
inverse of 1.0 equals 0.99999994) for reciprocal square roots.

-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-
separated list of options, which may be preceded by a "!" to invert the option:

all Enable all estimate instructions.

default
Enable the default instructions, equivalent to -mrecip.

none
Disable all estimate instructions, equivalent to -mno-recip.

div Enable the reciprocal approximation instructions for both single and double
precision.

divf
Enable the single-precision reciprocal approximation instructions.

divd
Enable the double-precision reciprocal approximation instructions.

rsqrt
Enable the reciprocal square root approximation instructions for both single and
double precision.

rsqrtf
Enable the single-precision reciprocal square root approximation instructions.

rsqrtd
Enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions,
except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the
double-precision reciprocal square root calculations.

-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision
estimates than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or
-mcpu=power8 automatically selects -mrecip-precision. The double-precision square root
estimate instructions are not generated by default on low-precision machines, since they
do not provide an estimate that converges after three steps.

-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The
only type supported at present is mass, which specifies to use IBM's Mathematical
Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external
libraries. GCC currently emits calls to "acosd2", "acosf4", "acoshd2", "acoshf4",
"asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2", "atan2f4", "atand2", "atanf4",
"atanhd2", "atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4", "erfcd2",
"erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2", "expm1f4",
"hypotd2", "hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2",
"sinhf4", "sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when generating
code for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be
enabled. The MASS libraries must be specified at link time.

-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations
option is used to optimize rounding of floating-point values to 64-bit integer and back
to floating point. The "friz" instruction does not return the same value if the
floating-point number is too large to fit in an integer.

-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register ("r11") when calling
through a pointer on AIX and 64-bit Linux systems where a function pointer points to a
3-word descriptor giving the function address, TOC value to be loaded in register "r2",
and static chain value to be loaded in register "r11". The
-mpointers-to-nested-functions is on by default. You cannot call through pointers to
nested functions or pointers to functions compiled in other languages that use the static
chain if you use -mno-pointers-to-nested-functions.

-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in
the function prologue if the function calls through a pointer on AIX and 64-bit Linux
systems. If the TOC value is not saved in the prologue, it is saved just before the call
through the pointer. The -mno-save-toc-indirect option is the default.

-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum alignment of
64 bits, for compatibility with older versions of GCC.

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on
a 128-bit boundary when that structure contained a member requiring 128-bit alignment.
This is corrected in more recent versions of GCC. This option may be used to generate
code that is compatible with functions compiled with older versions of GCC.

The -mno-compat-align-parm option is the default.

-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard. Supported locations are global for
global canary or tls for per-thread canary in the TLS block (the default with GNU libc
version 2.4 or later).

With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which register to use as base
register for reading the canary, and from what offset from that base register. The
default for those is as specified in the relevant ABI.
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a
canary in the TLS block.

-mpcrel
-mno-pcrel
Generate (do not generate) pc-relative addressing. The -mpcrel option requires that the
medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are
enabled.

-mprefixed
-mno-prefixed
Generate (do not generate) addressing modes using prefixed load and store instructions.
The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled.

-mmma
-mno-mma
Generate (do not generate) the MMA instructions. The -mma option requires that the
option -mcpu=power10 (or later) is enabled.

-mrop-protect
-mno-rop-protect
Generate (do not generate) ROP protection instructions when the target processor supports
them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap).

-mprivileged
-mno-privileged
Generate (do not generate) code that will run in privileged state.

-mblock-ops-unaligned-vsx
-mno-block-ops-unaligned-vsx
Generate (do not generate) unaligned vsx loads and stores for inline expansion of
"memcpy" and "memmove".

--param rs6000-vect-unroll-limit=
The vectorizer will check with target information to determine whether it would be
beneficial to unroll the main vectorized loop and by how much. This parameter sets the
upper bound of how much the vectorizer will unroll the main loop. The default value is
four.

RX Options

These command-line options are defined for RX targets:

-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in
size. The default is -m32bit-doubles. Note RX floating-point hardware only works on
32-bit values, which is why the default is -m32bit-doubles.

-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default
is enabled for the RX600 series and disabled for the RX200 series.

Floating-point instructions are only generated for 32-bit floating-point values, however,
so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.

Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled
automatically. This is because the RX FPU instructions are themselves unsafe.

-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are supported, the
generic RX600 and RX200 series hardware and the specific RX610 CPU. The default is
RX600.

The only difference between RX600 and RX610 is that the RX610 does not support the
"MVTIPL" instruction.

The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by
default when this type is selected.

-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is -mlittle-endian-data,
i.e. to store data in the little-endian format.

-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be placed
into the small data area. Using the small data area can lead to smaller and faster code,
but the size of area is limited and it is up to the programmer to ensure that the area
does not overflow. Also when the small data area is used one of the RX's registers
(usually "r13") is reserved for use pointing to this area, so it is no longer available
for use by the compiler. This could result in slower and/or larger code if variables are
pushed onto the stack instead of being held in this register.

Note, common variables (variables that have not been initialized) and constants are not
placed into the small data area as they are assigned to other sections in the output
executable.

The default value is zero, which disables this feature. Note, this feature is not
enabled by default with higher optimization levels (-O2 etc) because of the potentially
detrimental effects of reserving a register. It is up to the programmer to experiment
and discover whether this feature is of benefit to their program. See the description of
the -mpid option for a description of how the actual register to hold the small data area
pointer is chosen.

-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board-specific runtime.

-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's AS100
assembler. This syntax can also be handled by the GAS assembler, but it has some
restrictions so it is not generated by default.

-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a
RX instruction. Although the RX instruction set does allow constants of up to 4 bytes in
length to be used in instructions, a longer value equates to a longer instruction. Thus
in some circumstances it can be beneficial to restrict the size of constants that are
used in instructions. Constants that are too big are instead placed into a constant pool
and referenced via register indirection.

The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants
of any size are allowed.

-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the linker attempts to
reduce the size of a program by finding shorter versions of various instructions.
Disabled by default.

-mint-register=N
Specify the number of registers to reserve for fast interrupt handler functions. The
value N can be between 0 and 4. A value of 1 means that register "r13" is reserved for
the exclusive use of fast interrupt handlers. A value of 2 reserves "r13" and "r12". A
value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13" through
"r10". A value of 0, the default, does not reserve any registers.

-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register.
This is only necessary if normal code might use the accumulator register, for example
because it performs 64-bit multiplications. The default is to ignore the accumulator as
this makes the interrupt handlers faster.

-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access to constant
data is done via an offset from a base address held in a register. This allows the
location of constant data to be determined at run time without requiring the executable
to be relocated, which is a benefit to embedded applications with tight memory
constraints. Data that can be modified is not affected by this option.

Note, using this feature reserves a register, usually "r13", for the constant data base
address. This can result in slower and/or larger code, especially in complicated
functions.

The actual register chosen to hold the constant data base address depends upon whether
the -msmall-data-limit and/or the -mint-register command-line options are enabled.
Starting with register "r13" and proceeding downwards, registers are allocated first to
satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit.
Thus it is possible for the small data area register to be "r8" if both -mint-register=4
and -mpid are specified on the command line.

By default this feature is not enabled. The default can be restored via the -mno-pid
command-line option.

-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one fast interrupt
handler when it is compiling a file. The default is to issue a warning for each extra
fast interrupt handler found, as the RX only supports one such interrupt.

-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation instructions "SMOVF", "SCMPU",
"SMOVB", "SMOVU", "SUNTIL" "SWHILE" and also the "RMPA" instruction. These instructions
may prefetch data, which is not safe to do if accessing an I/O register. (See section
12.2.7 of the RX62N Group User's Manual for more information).

The default is to allow these instructions, but it is not possible for GCC to reliably
detect all circumstances where a string instruction might be used to access an I/O
register, so their use cannot be disabled automatically. Instead it is reliant upon the
programmer to use the -mno-allow-string-insns option if their program accesses I/O space.

When the instructions are enabled GCC defines the C preprocessor symbol
"__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol
"__RX_DISALLOW_STRING_INSNS__".

-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access functions. This option can be used
when code size exceeds the range of "BSR" instructions. Note that -mno-jsr does not mean
to not use "JSR" but instead means that any type of branch may be used.

Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port
when used with the "interrupt" function attribute. This attribute indicates a function
intended to process fast interrupts. GCC ensures that it only uses the registers "r10",
"r11", "r12" and/or "r13" and only provided that the normal use of the corresponding
registers have been restricted via the -ffixed-reg or -mint-register command-line options.

S/390 and zSeries Options

These are the -m options defined for the S/390 and zSeries architecture.

-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-
point operations. When -msoft-float is specified, functions in libgcc.a are used to
perform floating-point operations. When -mhard-float is specified, the compiler
generates IEEE floating-point instructions. This is the default.

-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-
point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to
perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler
generates decimal-floating-point hardware instructions. This is the default for
-march=z9-ec or higher.

-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long
double" type equivalent to the "double" type. This is the default.

-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain pointer into the
callee's stack frame. A backchain may be needed to allow debugging using tools that do
not understand DWARF call frame information. When -mno-packed-stack is in effect, the
backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in
effect, the backchain is placed into the topmost word of the 96/160 byte register save
area.

In general, code compiled with -mbackchain is call-compatible with code compiled with
-mno-backchain; however, use of the backchain for debugging purposes usually requires
that the whole binary is built with -mbackchain. Note that the combination of
-mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux
kernel use -msoft-float.

The default is to not maintain the backchain.

-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the
compiler uses the all fields of the 96/160 byte register save area only for their default
purpose; unused fields still take up stack space. When -mpacked-stack is specified,
register save slots are densely packed at the top of the register save area; unused space
is reused for other purposes, allowing for more efficient use of the available stack
space. However, when -mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register is always saved two
words below the backchain.

As long as the stack frame backchain is not used, code generated with -mpacked-stack is
call-compatible with code generated with -mno-packed-stack. Note that some non-FSF
releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame
backchain at run time, not just for debugging purposes. Such code is not call-compatible
with code compiled with -mpacked-stack. Also, note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use
-msoft-float.

The default is to not use the packed stack layout.

-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls.
This only works reliably if the total executable size does not exceed 64k. The default
is to use the "basr" instruction instead, which does not have this limitation.

-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When
-m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows
GCC in particular to generate 64-bit instructions. For the s390 targets, the default is
-m31, while the s390x targets default to -m64.

-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on
z/Architecture. When -mesa is specified, generate code using the instructions available
on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant
to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to
the GNU/Linux for zSeries ABI, the default is -mzarch.

-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of instructions available with the
transactional execution facility introduced with the IBM zEnterprise EC12 machine
generation S/390 System z Built-in Functions. -mhtm is enabled by default when using
-march=zEC12.

-mvx
-mno-vx
When -mvx is specified, generate code using the instructions available with the vector
extension facility introduced with the IBM z13 machine generation. This option changes
the ABI for some vector type values with regard to alignment and calling conventions. In
case vector type values are being used in an ABI-relevant context a GAS .gnu_attribute
command will be added to mark the resulting binary with the ABI used. -mvx is enabled by
default when using -march=z13.

-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and builtins using instructions
available with the vector extension facility introduced with the IBM z13 machine
generation. This option adds support for vector to be used as a keyword to define vector
type variables and arguments. vector is only available when GNU extensions are enabled.
It will not be expanded when requesting strict standard compliance e.g. with -std=c99.
In addition to the GCC low-level builtins -mzvector enables a set of builtins added for
compatibility with AltiVec-style implementations like Power and Cell. In order to make
use of these builtins the header file vecintrin.h needs to be included. -mzvector is
disabled by default.

-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves.
When -mno-mvcle is specified, use a "mvc" loop instead. This is the default unless
optimizing for size.

-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to
not print debug information.

-march=cpu-type
Generate code that runs on cpu-type, which is the name of a system representing a certain
processor type. Possible values for cpu-type are z900/arch5, z990/arch6, z9-109,
z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13,
z16/arch14, and native.

The default is -march=z900.

Specifying native as cpu type can be used to select the best architecture option for the
host processor. -march=native has no effect if GCC does not recognize the processor.

-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and
the set of available instructions. The list of cpu-type values is the same as for
-march. The default is the value used for -march.

-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in
the operating system. This option is off by default, even when compiling for the TPF OS.

-mtpf-trace-skip
-mno-tpf-trace-skip
Generate code that changes (does not change) the default branch targets enabled by
-mtpf-trace to point to specialized trace routines providing the ability of selectively
skipping function trace entries for the TPF OS. This option is off by default, even when
compiling for the TPF OS and specifying -mtpf-trace.

-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a
compile-time check it doesn't need to be a real problem when the program runs. It is
intended to identify functions that most probably cause a stack overflow. It is useful
to be used in an environment with limited stack size e.g. the linux kernel.

-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is
generally a bad idea with a limited stack size.

-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional instructions in the
function prologue that trigger a trap if the stack size is stack-guard bytes above the
stack-size (remember that the stack on S/390 grows downward). If the stack-guard option
is omitted the smallest power of 2 larger than the frame size of the compiled function is
chosen. These options are intended to be used to help debugging stack overflow problems.
The additionally emitted code causes only little overhead and hence can also be used in
production-like systems without greater performance degradation. The given values have
to be exact powers of 2 and stack-size has to be greater than stack-guard without
exceeding 64k. In order to be efficient the extra code makes the assumption that the
stack starts at an address aligned to the value given by stack-size. The stack-guard
option can only be used in conjunction with stack-size.

-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function prologue is generated for
all functions in the compilation unit. The funtion label is prepended with the given
number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2
* post-halfwords bytes are appended, using the largest NOP like instructions the
architecture allows (maximum 1000000).

If both arguments are zero, hotpatching is disabled.

This option can be overridden for individual functions with the "hotpatch" attribute.

SH Options

These -m options are defined for the SH implementations:

-m1 Generate code for the SH1.

-m2 Generate code for the SH2.

-m2e
Generate code for the SH2e.

-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the
floating-point unit is not used.

-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point
operations are used.

-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision
mode by default.

-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision
mode by default.

-m3 Generate code for the SH3.

-m3e
Generate code for the SH3e.

-m4-nofpu
Generate code for the SH4 without a floating-point unit.

-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision
arithmetic.

-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by
default.

-m4 Generate code for the SH4.

-m4-100
Generate code for SH4-100.

-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit is not used.

-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by
default.

-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision floating-point
operations are used.

-m4-200
Generate code for SH4-200.

-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-point unit is not used.

-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by
default.

-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision floating-point
operations are used.

-m4-300
Generate code for SH4-300.

-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-point unit is not used.

-m4-300-single
Generate code for SH4-300 in such a way that no double-precision floating-point
operations are used.

-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision floating-point
operations are used.

-m4-340
Generate code for SH4-340 (no MMU, no FPU).

-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.

-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit
is not used.

-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating-point
operations are used.

-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode
by default.

-m4a
Generate code for the SH4a.

-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't
generate any DSP instructions at the moment.

-mb Compile code for the processor in big-endian mode.

-ml Compile code for the processor in little-endian mode.

-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and
thus some functions from the standard C library do not work unless you recompile it first
with -mdalign.

-mrelax
Shorten some address references at link time, when possible; uses the linker option
-relax.

-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.

-mbitops
Enable the use of bit manipulation instructions on SH2A.

-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.

-mrenesas
Comply with the calling conventions defined by Renesas.

-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were
available. This option is the default for all targets of the SH toolchain.

-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is given.

-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the handling of
cases where the result of a comparison is unordered. By default -mieee is implicitly
enabled. If -ffinite-math-only is enabled -mno-ieee is implicitly set, which results in
faster floating-point greater-equal and less-equal comparisons. The implicit settings
can be overridden by specifying either -mieee or -mno-ieee.

-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested function
trampolines. This option has no effect if -musermode is in effect and the selected code
generation option (e.g. -m4) does not allow the use of the "icbi" instruction. If the
selected code generation option does not allow the use of the "icbi" instruction, and
-musermode is not in effect, the inlined code manipulates the instruction cache address
array directly with an associative write. This not only requires privileged mode at run
time, but it also fails if the cache line had been mapped via the TLB and has become
unmapped.

-misize
Dump instruction size and location in the assembly code.

-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is
incompatible with the SH ABI.

-matomic-model=model
Sets the model of atomic operations and additional parameters as a comma separated list.
For details on the atomic built-in functions see __atomic Builtins. The following models
and parameters are supported:

none
Disable compiler generated atomic sequences and emit library calls for atomic
operations. This is the default if the target is not "sh*-*-linux*".

soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in
functions. The generated atomic sequences require additional support from the
interrupt/exception handling code of the system and are only suitable for SH3* and
SH4* single-core systems. This option is enabled by default when the target is
"sh*-*-linux*" and SH3* or SH4*. When the target is SH4A, this option also partially
utilizes the hardware atomic instructions "movli.l" and "movco.l" to create more
efficient code, unless strict is specified.

soft-tcb
Generate software atomic sequences that use a variable in the thread control block.
This is a variation of the gUSA sequences which can also be used on SH1* and SH2*
targets. The generated atomic sequences require additional support from the
interrupt/exception handling code of the system and are only suitable for single-core
systems. When using this model, the gbr-offset= parameter has to be specified as
well.

soft-imask
Generate software atomic sequences that temporarily disable interrupts by setting
"SR.IMASK = 1111". This model works only when the program runs in privileged mode
and is only suitable for single-core systems. Additional support from the
interrupt/exception handling code of the system is not required. This model is
enabled by default when the target is "sh*-*-linux*" and SH1* or SH2*.

hard-llcs
Generate hardware atomic sequences using the "movli.l" and "movco.l" instructions
only. This is only available on SH4A and is suitable for multi-core systems. Since
the hardware instructions support only 32 bit atomic variables access to 8 or 16 bit
variables is emulated with 32 bit accesses. Code compiled with this option is also
compatible with other software atomic model interrupt/exception handling systems if
executed on an SH4A system. Additional support from the interrupt/exception handling
code of the system is not required for this model.

gbr-offset=
This parameter specifies the offset in bytes of the variable in the thread control
block structure that should be used by the generated atomic sequences when the soft-
tcb model has been selected. For other models this parameter is ignored. The
specified value must be an integer multiple of four and in the range 0-1020.

strict
This parameter prevents mixed usage of multiple atomic models, even if they are
compatible, and makes the compiler generate atomic sequences of the specified model
only.

-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that depending on the
particular hardware and software configuration this can degrade overall performance due
to the operand cache line flushes that are implied by the "tas.b" instruction. On multi-
core SH4A processors the "tas.b" instruction must be used with caution since it can
result in data corruption for certain cache configurations.

-mprefergot
When generating position-independent code, emit function calls using the Global Offset
Table instead of the Procedure Linkage Table.

-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode code. Specifying -musermode
also implies -mno-inline-ic_invalidate if the inlined code would not work in user mode.
-musermode is the default when the target is "sh*-*-linux*". If the target is SH1* or
SH2* -musermode has no effect, since there is no user mode.

-multcost=number
Set the cost to assume for a multiply insn.

-mdiv=strategy
Set the division strategy to be used for integer division operations. strategy can be
one of:

call-div1
Calls a library function that uses the single-step division instruction "div1" to
perform the operation. Division by zero calculates an unspecified result and does
not trap. This is the default except for SH4, SH2A and SHcompact.

call-fp
Calls a library function that performs the operation in double precision floating
point. Division by zero causes a floating-point exception. This is the default for
SHcompact with FPU. Specifying this for targets that do not have a double precision
FPU defaults to "call-div1".

call-table
Calls a library function that uses a lookup table for small divisors and the "div1"
instruction with case distinction for larger divisors. Division by zero calculates
an unspecified result and does not trap. This is the default for SH4. Specifying
this for targets that do not have dynamic shift instructions defaults to "call-div1".

When a division strategy has not been specified the default strategy is selected based on
the current target. For SH2A the default strategy is to use the "divs" and "divu"
instructions instead of library function calls.

-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around
each call. Generally beneficial for performance and size. Also needed for unwinding to
avoid changing the stack frame around conditional code.

-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only
affects the name used in the call division strategies, and the compiler still expects the
same sets of input/output/clobbered registers as if this option were not present.

-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers make the compiler try
to generate more branch-free code if possible. If not specified the value is selected
depending on the processor type that is being compiled for.

-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch instructions "bt" and
"bf" are fast. If -mzdcbranch is specified, the compiler prefers zero displacement
branch code sequences. This is enabled by default when generating code for SH4 and SH4A.
It can be explicitly disabled by specifying -mno-zdcbranch.

-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs the delay slot with
a "nop" if a suitable instruction cannot be found. By default this option is disabled.
It can be enabled to work around hardware bugs as found in the original SH7055.

-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for sine and cosine
approximations. The option -mfsca must be used in combination with
-funsafe-math-optimizations. It is enabled by default when generating code for SH4A.
Using -mno-fsca disables sine and cosine approximations even if
-funsafe-math-optimizations is in effect.

-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal square root
approximations. The option -mfsrra must be used in combination with
-funsafe-math-optimizations and -ffinite-math-only. It is enabled by default when
generating code for SH4A. Using -mno-fsrra disables reciprocal square root
approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.

-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns.
This can result in faster code on the SH4 processor.

-mfdpic
Generate code using the FDPIC ABI.

Solaris 2 Options

These -m options are supported on Solaris 2:

-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware capabilities generated by the
Solaris assembler. This is only necessary when object files use ISA extensions not
supported by the current machine, but check at runtime whether or not to use them.

-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the
linker when linking a shared object. Using this option, you can link position-dependent
code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable
sections" linker error message. However, the necessary relocations trigger copy-on-
write, and the shared object is not actually shared across processes. Instead of using
-mimpure-text, you should compile all source code with -fpic or -fPIC.

These switches are supported in addition to the above on Solaris 2:

-pthreads
This is a synonym for -pthread.

SPARC Options

These -m options are supported on the SPARC:

-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the
SPARC SVR4 ABI reserves for applications. Like the global register 1, each global
register 2 through 4 is then treated as an allocable register that is clobbered by
function calls. This is the default.

To be fully SVR4 ABI-compliant at the cost of some performance loss, specify
-mno-app-regs. You should compile libraries and system software with this option.

-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses a "flat"
or single register window model. This model is compatible with the regular register
window model. The local registers and the input registers (0--5) are still treated as
"call-saved" registers and are saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions (except
for leaf functions). This is the normal operating mode.

-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.

-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite
libraries are not available for all SPARC targets. Normally the facilities of the
machine's usual C compiler are used, but this cannot be done directly in cross-
compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide
software floating-point support.

-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.

-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-point
instructions. The functions called are those specified in the SPARC ABI. This is the
default.

As of this writing, there are no SPARC implementations that have hardware support for the
quad-word floating-point instructions. They all invoke a trap handler for one of these
instructions, and then the trap handler emulates the effect of the instruction. Because
of the trap handler overhead, this is much slower than calling the ABI library routines.
Thus the -msoft-quad-float option is the default.

-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are
contained in another type, or if they have an absolute address. Otherwise, it assumes
they have 4-byte alignment. Specifying this option avoids some rare compatibility
problems with code generated by other compilers. It is not the default because it
results in a performance loss, especially for floating-point code.

-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant only for the
"casa" instruction emitted for the LEON3 processor. This is the default.

-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment.
This enables the use of pairs of "ldd" and "std" instructions for copies in structure
assignment, in place of twice as many "ld" and "st" pairs. However, the use of this
changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on
targets where the developer acknowledges that their resulting code is not directly in
line with the rules of the ABI.

-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in functions returning
structures or unions to detect size mismatches between the two sides of function calls,
as per the 32-bit ABI.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode.

-mlra
-mno-lra
Enable Local Register Allocation. This is the default for SPARC since GCC 7 so -mno-lra
needs to be passed to get old Reload.

-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine
type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc,
hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet,
tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and
m8.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the
best architecture option for the host processor. -mcpu=native has no effect if GCC does
not recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture
and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations.

v7 cypress, leon3v7

v8 supersparc, hypersparc, leon, leon3, leon5

sparclite
f930, f934, sparclite86x

sparclet
tsc701

v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless configured otherwise), GCC generates code for the V7 variant of the
SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the
Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also
appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only
difference from V7 code is that the compiler emits the integer multiply and integer
divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc,
the compiler additionally optimizes it for the SuperSPARC chip, as used in the
SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC
architecture. This adds the integer multiply, integer divide step and scan ("ffs")
instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler
additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite,
with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu
MB86934 chip, which is the more recent SPARClite with FPU.

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC
architecture. This adds the integer multiply, multiply/accumulate, integer divide step
and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With
-mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This
adds 64-bit integer and floating-point move instructions, 3 additional floating-point
condition code registers and conditional move instructions. With -mcpu=ultrasparc, the
compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With
-mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally
optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally
optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally
optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally
optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally
optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally
optimizes it for Oracle M8 chips.

-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the
instruction set or register set that the option -mcpu=cpu_type does.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful
values are those that select a particular CPU implementation. Those are cypress,
supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701,
ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With
native Solaris and GNU/Linux toolchains, native can also be used.

-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI
is that the global and out registers are considered 64 bits wide. This is enabled by
default on Solaris in 32-bit mode for all SPARC-V9 processors.

-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction
Set extensions. The default is -mno-vis.

-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC
Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that
supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets
-mvis.

-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC
Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that
supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2
and -mvis.

-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC
Visual Instruction Set extensions. The default is -mvis4 when targeting a cpu that
supports such instructions, such as niagara-7 and later. Setting -mvis4 also sets
-mvis3, -mvis2 and -mvis.

-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC
Visual Instruction Set extensions, plus the additional VIS instructions introduced in the
Oracle SPARC Architecture 2017. The default is -mvis4b when targeting a cpu that
supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4,
-mvis3, -mvis2 and -mvis.

-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-
Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that
supports such instructions, such as Niagara-4 and later.

-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add
Floating-point instructions. The default is -mfmaf when targeting a CPU that supports
such instructions, such as Niagara-3 and later.

-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply
Single to Double (FsMULd) instruction. The default is -mfsmuld when targeting a CPU
supporting the architecture versions V8 or V9 with FPU except -mcpu=leon.

-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count
instruction. The default is -mpopc when targeting a CPU that supports such an
instruction, such as Niagara-2 and later.

-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-
Extended-with-Carry instruction. The default is -msubxc when targeting a CPU that
supports such an instruction, such as Niagara-7 and later.

-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor
(which corresponds to erratum #13 of the AT697E processor).

-mfix-ut699
Enable the documented workarounds for the floating-point errata and the data cache
nullify errata of the UT699 processor.

-mfix-ut700
Enable the documented workaround for the back-to-back store errata of the UT699E/UT700
processor.

-mfix-gr712rc
Enable the documented workaround for the back-to-back store errata of the GR712RC
processor.

These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit
environments:

-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long
and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer
to 64 bits.

-mcmodel=which
Set the code model to one of

medlow
The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32
bits of memory. Programs can be statically or dynamically linked.

medmid
The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44
bits of memory, the text and data segments must be less than 2GB in size and the data
segment must be located within 2GB of the text segment.

medany
The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in
memory, the text and data segments must be less than 2GB in size and the data segment
must be located within 2GB of the text segment.

embmedany
The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and
data segments must be less than 2GB in size, both starting anywhere in memory
(determined at link time). The global register %g4 points to the base of the data
segment. Programs are statically linked and PIC is not supported.

-mmemory-model=mem-model
Set the memory model in force on the processor to one of

default
The default memory model for the processor and operating system.

rmo Relaxed Memory Order

pso Partial Store Order

tso Total Store Order

sc Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture
manual, as set in the processor's "PSTATE.MM" field.

-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are
offset by -2047 which must be added back when making stack frame references. This is the
default in 64-bit mode. Otherwise, assume no such offset is present.

Options for System V

These additional options are available on System V Release 4 for compatibility with other
compilers on those systems:

-G Create a shared object. It is recommended that -symbolic or -shared be used instead.

-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler
directive in the output.

-Qn Refrain from adding ".ident" directives to the output file (this is the default).

-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.

-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

V850 Options

These -m options are defined for V850 implementations:

-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the
compiler always loads the function's address into a register, and calls indirect through
the pointer.

-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more
times to copy pointer into the "ep" register, and use the shorter "sld" and "sst"
instructions. The -mep option is on by default if you optimize.

-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and
epilogue of a function. The external functions are slower, but use less code space if
more than one function saves the same number of registers. The -mprolog-function option
is on by default if you optimize.

-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and
-mprolog-function options.

-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that
register "ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes
for byte references).

-msda=n
Put static or global variables whose size is n bytes or less into the small data area
that register "gp" points to. The small data area can hold up to 64 kilobytes.

-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes
of memory.

-mv850
Specify that the target processor is the V850.

-mv850e3v5
Specify that the target processor is the V850E3V5. The preprocessor constant
"__v850e3v5__" is defined if this option is used.

-mv850e2v4
Specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5
option.

-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constant
"__v850e2v3__" is defined if this option is used.

-mv850e2
Specify that the target processor is the V850E2. The preprocessor constant "__v850e2__"
is defined if this option is used.

-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants "__v850e1__"
and "__v850e__" are defined if this option is used.

-mv850es
Specify that the target processor is the V850ES. This is an alias for the -mv850e1
option.

-mv850e
Specify that the target processor is the V850E. The preprocessor constant "__v850e__" is
defined if this option is used.

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are
defined then a default target processor is chosen and the relevant __v850*__ preprocessor
constant is defined.

The preprocessor constants "__v850" and "__v851__" are always defined, regardless of
which processor variant is the target.

-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for the v850e, v850e1,
v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and
disabled by default when the GCC ABI is in use. If "CALLT" instructions are being
generated then the C preprocessor symbol "__V850_CALLT__" is defined.

-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option to the assembler.

-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump instructions.

-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point instructions. This
option is only significant when the target architecture is V850E2V3 or higher. If
hardware floating point instructions are being generated then the C preprocessor symbol
"__FPU_OK__" is defined, otherwise the symbol "__NO_FPU__" is defined.

-mloop
Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled
by default when the e3v5 architecture is selected because its use is still experimental.

-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This is the default. With this
version of the ABI the following rules apply:

* Integer sized structures and unions are returned via a memory pointer rather than a
register.

* Large structures and unions (more than 8 bytes in size) are passed by value.

* Functions are aligned to 16-bit boundaries.

* The -m8byte-align command-line option is supported.

* The -mdisable-callt command-line option is enabled by default. The
-mno-disable-callt command-line option is not supported.

When this version of the ABI is enabled the C preprocessor symbol "__V850_RH850_ABI__" is
defined.

-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this version of the ABI
the following rules apply:

* Integer sized structures and unions are returned in register "r10".

* Large structures and unions (more than 8 bytes in size) are passed by reference.

* Functions are aligned to 32-bit boundaries, unless optimizing for size.

* The -m8byte-align command-line option is not supported.

* The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled the C preprocessor symbol "__V850_GCC_ABI__" is
defined.

-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be aligned on 8-byte boundaries.
The default is to restrict the alignment of all objects to at most 4-bytes. When
-m8byte-align is in effect the C preprocessor symbol "__V850_8BYTE_ALIGN__" is defined.

-mbig-switch
Generate code suitable for big switch tables. Use this option only if the
assembler/linker complain about out of range branches within a switch table.

-mapp-regs
This option causes r2 and r5 to be used in the code generated by the compiler. This
setting is the default.

-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.

VAX Options

These -m options are defined for the VAX:

-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for
the VAX cannot handle across long ranges.

-mgnu
Do output those jump instructions, on the assumption that the GNU assembler is being
used.

-mg Output code for G-format floating-point numbers instead of D-format.

-mlra
-mno-lra
Enable Local Register Allocation. This is still experimental for the VAX, so by default
the compiler uses standard reload.

Visium Options

-mdebug
A program which performs file I/O and is destined to run on an MCM target should be
linked with this option. It causes the libraries libc.a and libdebug.a to be linked.
The program should be run on the target under the control of the GDB remote debugging
stub.

-msim
A program which performs file I/O and is destined to run on the simulator should be
linked with option. This causes libraries libc.a and libsim.a to be linked.

-mfpu
-mhard-float
Generate code containing floating-point instructions. This is the default.

-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.

-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine
type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the
Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The
only difference from GR5 code is that the compiler will generate block move instructions.

-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the
instruction set or register set that the option -mcpu=cpu_type would.

-msv-mode
Generate code for the supervisor mode, where there are no restrictions on the access to
general registers. This is the default.

-muser-mode
Generate code for the user mode, where the access to some general registers is forbidden:
on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only
registers r29 to r31 are affected.

VMS Options

These -m options are defined for the VMS implementations:

-mvms-return-codes
Return VMS condition codes from "main". The default is to return POSIX-style condition
(e.g. error) codes.

-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the
debugger.

-mmalloc64
Default to 64-bit memory allocation routines.

-mpointer-size=size
Set the default size of pointers. Possible options for size are 32 or short for 32 bit
pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers.
The later option disables "pragma pointer_size".

VxWorks Options

The options in this section are defined for all VxWorks targets. Options specific to the
target hardware are listed with the other options for that target.

-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs). This
option switches from the former to the latter. It also defines the preprocessor macro
"__RTP__".

-non-static
Link an RTP executable against shared libraries rather than static libraries. The
options -static and -shared can also be used for RTPs; -static is the default.

-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with
Diab.

-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is
defined for compatibility with Diab.

-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for
compatibility with Diab.

x86 Options

These -m options are defined for the x86 family of computers.

The choices for cpu-type are:

x86-64
A generic CPU with 64-bit extensions.

x86-64-v2
x86-64-v3
x86-64-v4
These choices for cpu-type select the corresponding micro-architecture level from the
x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features
as the x86-64 psABI documents for the particular micro-architecture level.

Since these cpu-type values do not have a corresponding -mtune setting, using -march
with these values enables generic tuning. Specific tuning can be enabled using the
-mtune=other-cpu-type option with an appropriate other-cpu-type value.

i386
Original Intel i386 CPU.

i486
Intel i486 CPU. (No scheduling is implemented for this chip.)

i586
pentium
Intel Pentium CPU with no MMX support.

lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.

pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.

pentiumpro
Intel Pentium Pro CPU.

i686
When used with -march, the Pentium Pro instruction set is used, so the code runs on
all i686 family chips. When used with -mtune, it has the same meaning as generic.

pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set
support.

pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction
set support.

pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and
FXSR instruction set support. Used by Centrino notebooks.

pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.

prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR
instruction set support.

nocona
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3
and FXSR instruction set support.

core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and
FXSR instruction set support.

nehalem
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.

westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.

sandybridge
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.

ivybridge
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C
instruction set support.

haswell
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2,
BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.

broadwell
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set
support.

skylake
Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2,
BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC,
XSAVES and SGX instruction set support.

bonnell
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support.

silvermont
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set
support.

goldmont
Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED,
XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and FSGSBASE instruction set support.

goldmont-plus
Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED,
XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX
instruction set support.

tremont
Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE,
XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE,
MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support.

sierraforest
Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8,
AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support.

grandridge
Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8,
AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support.

knl Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND,
F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AVX512PF,
AVX512ER, AVX512F, AVX512CD and PREFETCHWT1 instruction set support.

knm Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AVX512PF, AVX512ER,
AVX512F, AVX512CD and PREFETCHWT1, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ
instruction set support.

skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD
instruction set support.

cannonlake
Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND,
F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES,
CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support.

icelake-client
Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ,
AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.

icelake-server
Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ,
AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set
support.

cascadelake
Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and
AVX512VNNI instruction set support.

cooperlake
Intel cooperlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VNNI and AVX512BF16 instruction set support.

tigerlake
Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI,
AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG,
RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER
instruction set support.

sapphirerapids
Intel sapphirerapids CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ,
AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE,
AMX-INT8, AVX-VNNI, AVX512-FP16 and AVX512BF16 instruction set support.

alderlake
Intel Alderlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support.

rocketlake
Intel Rocketlake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3 ,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI,
AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID
and AVX512VPOPCNTDQ instruction set support.

graniterapids
Intel graniterapids CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ,
AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE,
AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI instruction set
support.

graniterapids-d
Intel graniterapids D CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C,
AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ,
AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE,
AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI and AMX-COMPLEX
instruction set support.

k6 AMD K6 CPU with MMX instruction set support.

k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.

athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions
support.

athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction
set support.

k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction set support, including
the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE,
SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)

k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set support.

amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This
supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit
instruction set extensions.)

bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This
supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

bdver2
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets
BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

bdver3
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets
BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE,
SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

bdver4
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets
BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction
set extensions.)

znver1
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets
BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES,
PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC,
XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)

znver2
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets
BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO,
AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set
extensions.)

znver3
AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets
BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO,
AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and
64-bit instruction set extensions.)

znver4
AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets
BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO,
AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F,
AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI,
AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction
set extensions.)

btver1
CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This
supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set
extensions.)

btver2
CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes
MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3,
SSE2, SSE, MMX and 64-bit instruction set extensions.

winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set
support.

winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow!
instruction set support.

c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is
implemented for this chip.)

c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)

c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No
scheduling is implemented for this chip.)

samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is
implemented for this chip.)

nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is
implemented for this chip.)

esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No
scheduling is implemented for this chip.)

eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No
scheduling is implemented for this chip.)

eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and
AVX2 instruction set support. (No scheduling is implemented for this chip.)

nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set
support. (No scheduling is implemented for this chip.)

nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set
support. (No scheduling is implemented for this chip.)

nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set
support. (No scheduling is implemented for this chip.)

nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set
support. (No scheduling is implemented for this chip.)

nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1
instruction set support. (No scheduling is implemented for this chip.)

nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1
instruction set support. (No scheduling is implemented for this chip.)

lujiazui
ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2,
F16C, FXSR, RDSEED instruction set support.

geode
AMD Geode embedded processor with MMX and 3DNow! instruction set support.

-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and
the set of available instructions. While picking a specific cpu-type schedules things
appropriately for that particular chip, the compiler does not generate any code that
cannot run on the default machine type unless you use a -march=cpu-type option. For
example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code
that is tuned for Pentium 4 but still runs on i686 machines.

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2
extra choices for cpu-type:

generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know
the CPU on which your code will run, then you should use the corresponding -mtune or
-march option instead of -mtune=generic. But, if you do not know exactly what CPU
users of your application will have, then you should use this option.

As new processors are deployed in the marketplace, the behavior of this option will
change. Therefore, if you upgrade to a newer version of GCC, code generation
controlled by this option will change to reflect the processors that are most common
at the time that version of GCC is released.

There is no -march=generic option because -march indicates the instruction set the
compiler can use, and there is no generic instruction set applicable to all
processors. In contrast, -mtune indicates the processor (or, in this case,
collection of processors) for which the code is optimized.

intel
Produce code optimized for the most current Intel processors, which are Haswell and
Silvermont for this version of GCC. If you know the CPU on which your code will run,
then you should use the corresponding -mtune or -march option instead of
-mtune=intel. But, if you want your application performs better on both Haswell and
Silvermont, then you should use this option.

As new Intel processors are deployed in the marketplace, the behavior of this option
will change. Therefore, if you upgrade to a newer version of GCC, code generation
controlled by this option will change to reflect the most current Intel processors at
the time that version of GCC is released.

There is no -march=intel option because -march indicates the instruction set the
compiler can use, and there is no common instruction set applicable to all
processors. In contrast, -mtune indicates the processor (or, in this case,
collection of processors) for which the code is optimized.

-mcpu=cpu-type
A deprecated synonym for -mtune.

-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The choices for unit are:

387 Use the standard 387 floating-point coprocessor present on the majority of chips and
emulated otherwise. Code compiled with this option runs almost everywhere. The
temporary results are computed in 80-bit precision instead of the precision specified
by the type, resulting in slightly different results compared to most of other chips.
See -ffloat-store for more detailed description.

This is the default choice for non-Darwin x86-32 targets.

sse Use scalar floating-point instructions present in the SSE instruction set. This
instruction set is supported by Pentium III and newer chips, and in the AMD line by
Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction
set supports only single-precision arithmetic, thus the double and extended-precision
arithmetic are still done using 387. A later version, present only in Pentium 4 and
AMD x86-64 chips, supports double-precision arithmetic too.

For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to
enable SSE extensions and make this option effective. For the x86-64 compiler, these
extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoid
the numerical instability problems of 387 code, but may break some existing code that
expects temporaries to be 80 bits.

This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the
default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is
enabled.

sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This effectively doubles the
amount of available registers, and on chips with separate execution units for 387 and
SSE the execution resources too. Use this option with care, as it is still
experimental, because the GCC register allocator does not model separate functional
units well, resulting in unstable performance.

-masm=dialect
Output assembly instructions using selected dialect. Also affects which dialect is used
for basic "asm" and extended "asm". Supported choices (in dialect order) are att or
intel. The default is att. Darwin does not support intel.

-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons. These
correctly handle the case where the result of a comparison is unordered.

-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.

-mno-80387
-msoft-float
Generate output containing library calls for floating point.

Warning: the requisite libraries are not part of GCC. Normally the facilities of the
machine's usual C compiler are used, but this cannot be done directly in cross-
compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation.

On machines where a function returns floating-point results in the 80387 register stack,
some floating-point opcodes may be emitted even if -msoft-float is used.

-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types "float" and "double" in
an FPU register, even if there is no FPU. The idea is that the operating system should
emulate an FPU.

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers
instead.

-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387.
Specify this option to avoid generating those instructions. This option is overridden
when -march indicates that the target CPU always has an FPU and so the instruction does
not need emulation. These instructions are not generated unless you also use the
-funsafe-math-optimizations switch.

-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two-
word boundary or a one-word boundary. Aligning "double" variables on a two-word boundary
produces code that runs somewhat faster on a Pentium at the expense of more memory.

On x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch, structures containing the above types are
aligned differently than the published application binary interface specifications for
the x86-32 and are not binary compatible with structures in code compiled without that
switch.

-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The x86-32 application binary
interface specifies the size to be 96 bits, so -m96bit-long-double is the default in
32-bit mode.

Modern architectures (Pentium and newer) prefer "long double" to be aligned to an 8- or
16-byte boundary. In arrays or structures conforming to the ABI, this is not possible.
So specifying -m128bit-long-double aligns "long double" to a 16-byte boundary by padding
the "long double" with an additional 32-bit zero.

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies
that "long double" is aligned on 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of
80 bits for a "long double".

Warning: if you override the default value for your target ABI, this changes the size of
structures and arrays containing "long double" variables, as well as modifying the
function calling convention for functions taking "long double". Hence they are not
binary-compatible with code compiled without that switch.

-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long
double" type equivalent to the "double" type. This is the default for 32-bit Bionic C
library. A size of 128 bits makes the "long double" type equivalent to the "__float128"
type. This is the default for 64-bit Bionic C library.

-malign-data=type
Control how GCC aligns variables. Supported values for type are compat uses increased
alignment value compatible uses GCC 4.8 and earlier, abi uses alignment value as
specified by the psABI, and cacheline uses increased alignment value to match the cache
line size. compat is the default.

-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger than threshold are placed in the
large data section. This value must be the same across all objects linked into the
binary, and defaults to 65535.

-mrtd
Use a different function-calling convention, in which functions that take a fixed number
of arguments return with the "ret num" instruction, which pops their arguments while
returning. This saves one instruction in the caller since there is no need to pop the
arguments there.

You can specify that an individual function is called with this calling sequence with the
function attribute "stdcall". You can also override the -mrtd option by using the
function attribute "cdecl".

Warning: this calling convention is incompatible with the one normally used on Unix, so
you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers
of arguments (including "printf"); otherwise incorrect code is generated for calls to
those functions.

In addition, seriously incorrect code results if you call a function with too many
arguments. (Normally, extra arguments are harmlessly ignored.)

-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers
are used to pass arguments, and at most 3 registers can be used. You can control this
behavior for a specific function by using the function attribute "regparm".

Warning: if you use this switch, and num is nonzero, then you must build all modules with
the same value, including any libraries. This includes the system libraries and startup
modules.

-msseregparm
Use SSE register passing conventions for float and double arguments and return values.
You can control this behavior for a specific function by using the function attribute
"sseregparm".

Warning: if you use this switch then you must build all modules with the same value,
including any libraries. This includes the system libraries and startup modules.

-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks
to match the ABI of the Sun Studio compilers until version 12. Only use this option if
you need to remain compatible with existing code produced by those previous compiler
versions or older versions of GCC.

-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the
significands of results of floating-point operations are rounded to 24 bits (single
precision); -mpc64 rounds the significands of results of floating-point operations to 53
bits (double precision) and -mpc80 rounds the significands of results of floating-point
operations to 64 bits (extended double precision), which is the default. When this
option is used, floating-point operations in higher precisions are not available to the
programmer without setting the FPU control word explicitly.

Setting the rounding of floating-point operations to less than the default 80 bits can
speed some programs by 2% or more. Note that some mathematical libraries assume that
extended-precision (80-bit) floating-point operations are enabled by default; routines in
such libraries could suffer significant loss of accuracy, typically through so-called
"catastrophic cancellation", when this option is used to set the precision to less than
extended precision.

-mdaz-ftz
The flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used
to control floating-point calculations.SSE and AVX instructions including scalar and
vector instructions could benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is
specified. Don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is specified, -mdaz-ftz
will set FTZ/DAZ flags even with -shared.

-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign option generates an alternate
prologue and epilogue that realigns the run-time stack if necessary. This supports
mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte
stack alignment for SSE compatibility. See also the attribute "force_align_arg_pointer",
applicable to individual functions.

-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).

Warning: When generating code for the x86-64 architecture with SSE extensions disabled,
-mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte
boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and
intended to be used in controlled environment where stack space is important limitation.
This option leads to wrong code when functions compiled with 16 byte stack alignment
(such as functions from a standard library) are called with misaligned stack. In this
case, SSE instructions may lead to misaligned memory access traps. In addition, variable
arguments are handled incorrectly for 16 byte aligned objects (including x87 long double
and __int128), leading to wrong results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This includes the system
libraries and startup modules.

-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If
-mincoming-stack-boundary is not specified, the one specified by
-mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, "double" and "long double" values should be aligned to an
8-byte boundary (see -malign-double) or suffer significant run time performance
penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type "__m128" may not
work properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as
aligned as that required by any value stored on the stack. Further, every function must
be generated such that it keeps the stack aligned. Thus calling a function compiled with
a higher preferred stack boundary from a function compiled with a lower preferred stack
boundary most likely misaligns the stack. It is recommended that libraries that use
callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases code size.
Code that is sensitive to stack space usage, such as embedded systems and operating
system kernels, may want to reduce the preferred alignment to
-mpreferred-stack-boundary=2.

-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mavx512bf16
-mavx512fp16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-menqcmd
-muintr
-mtsxldtrk
-mavx512vpopcntdq
-mavx512vp2intersect
-mavx5124fmaps
-mavx512vnni
-mavxvnni
-mavx5124vnniw
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwidekl
-mavxifma
-mavxvnniint8
-mavxneconvert
-mcmpccxadd
-mamx-fp16
-mprefetchi
-mraoint
-mamx-complex
These switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4,
SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL,
AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE,
PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1,
RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR,
XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI,
VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD,
AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, SERIALIZE, UINTR, HRESET,
AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8,
AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX or CLDEMOTE extended
instruction sets. Each has a corresponding -mno- option to disable use of these
instructions.

These extensions are also available as built-in functions: see x86 Built-in Functions,
for details of the functions enabled and disabled by these switches.

To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to
387 instructions), see -mfpmath=sse.

GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX
instructions or AVX equivalence for all SSEx instructions when needed.

These options enable GCC to use these extended instructions in generated code, even
without -mfpmath=sse. Applications that perform run-time CPU detection must compile
separate files for each supported architecture, using the appropriate flags. In
particular, the file containing the CPU detection code should be compiled without these
options.

-mdump-tune-features
This option instructs GCC to dump the names of the x86 performance tuning features and
default settings. The names can be used in -mtune-ctrl=feature-list.

-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code generation features. feature-
list is a comma separated list of feature names. See also -mdump-tune-features. When
specified, the feature is turned on if it is not preceded with ^, otherwise, it is turned
off. -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may
lead to code paths not covered by testing and can potentially result in compiler ICEs or
runtime errors.

-mno-default
This option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-
list and -mdump-tune-features.

-mcld
This option instructs GCC to emit a "cld" instruction in the prologue of functions that
use string instructions. String instructions depend on the DF flag to select between
autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared
on function entry, some operating systems violate this specification by not clearing the
DF flag in their exception dispatchers. The exception handler can be invoked with the DF
flag set, which leads to wrong direction mode when string instructions are used. This
option can be enabled by default on 32-bit x86 targets by configuring GCC with the
--enable-cld configure option. Generation of "cld" instructions can be suppressed with
the -mno-cld compiler option in this case.

-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control
flow out of the function to minimize the AVX to SSE transition penalty as well as remove
unnecessary "zeroupper" intrinsics.

-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX
instructions in the auto-vectorizer.

-mprefer-vector-width=opt
This option instructs GCC to use opt-bit vector width in instructions instead of default
on the selected platform.

-mmove-max=bits
This option instructs GCC to set the maximum number of bits can be moved from memory to
memory efficiently to bits. The valid bits are 128, 256 and 512.

-mstore-max=bits
This option instructs GCC to set the maximum number of bits can be stored to memory
efficiently to bits. The valid bits are 128, 256 and 512.

none
No extra limitations applied to GCC other than defined by the selected platform.

128 Prefer 128-bit vector width for instructions.

256 Prefer 256-bit vector width for instructions.

512 Prefer 512-bit vector width for instructions.

-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions in 64-bit code to implement
compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for
atomic updates of data structures exceeding one machine word in size. The compiler uses
this instruction to implement __sync Builtins. However, for __atomic Builtins operating
on 128-bit integers, a library call is always used.

-msahf
This option enables generation of "SAHF" instructions in 64-bit code. Early Intel
Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in
December 2005, lacked the "LAHF" and "SAHF" instructions which are supported by AMD64.
These are load and store instructions, respectively, for certain status flags. In 64-bit
mode, the "SAHF" instruction is used to optimize "fmod", "drem", and "remainder" built-in
functions; see Other Builtins for details.

-mmovbe
This option enables use of the "movbe" instruction to implement "__builtin_bswap32" and
"__builtin_bswap64".

-mshstk
The -mshstk option enables shadow stack built-in functions from x86 Control-flow
Enforcement Technology (CET).

-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi",
"__builtin_ia32_crc32hi", "__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to
generate the "crc32" machine instruction.

-mmwait
This option enables built-in functions "__builtin_ia32_monitor", and
"__builtin_ia32_mwait" to generate the "monitor" and "mwait" machine instructions.

-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized
variants "RCPPS" and "RSQRTPS") with an additional Newton-Raphson step to increase
precision instead of "DIVSS" and "SQRTSS" (and their vectorized variants) for single-
precision floating-point arguments. These instructions are generated only when
-funsafe-math-optimizations is enabled together with -ffinite-math-only and
-fno-trapping-math. Note that while the throughput of the sequence is higher than the
throughput of the non-reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).

Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or "RSQRTPS") already
with -ffast-math (or the above option combination), and doesn't need -mrecip.

Also note that GCC emits the above sequence with additional Newton-Raphson step for
vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the
above option combination), and doesn't need -mrecip.

-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-
separated list of options, which may be preceded by a ! to invert the option:

all Enable all estimate instructions.

default
Enable the default instructions, equivalent to -mrecip.

none
Disable all estimate instructions, equivalent to -mno-recip.

div Enable the approximation for scalar division.

vec-div
Enable the approximation for vectorized division.

sqrt
Enable the approximation for scalar square root.

vec-sqrt
Enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except
for square root.

-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library.
Supported values for type are svml for the Intel short vector math library and acml for
the AMD math core library. To use this option, both -ftree-vectorize and
-funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible
library must be specified at link time.

GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldPow2",
"vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2",
"vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2",
"vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4",
"vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4",
"vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding function type
when -mveclibabi=svml is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp",
"__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf",
"__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding
function type when -mveclibabi=acml is used.

-mabi=name
Generate code for the specified calling convention. Permissible values are sysv for the
ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to
use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other
systems. You can control this behavior for specific functions by using the function
attributes "ms_abi" and "sysv_abi".

-mforce-indirect-call
Force all calls to functions to be indirect. This is useful when using Intel Processor
Trace where it generates more precise timing information for function calls.

-mmanual-endbr
Insert ENDBR instruction at function entry only via the "cf_check" function attribute.
This is useful when used with the option -fcf-protection=branch to control ENDBR
insertion at the function entry.

-mcet-switch
By default, CET instrumentation is turned off on switch statements that use a jump table
and indirect branch track is disabled. Since jump tables are stored in read-only memory,
this does not result in a direct loss of hardening. But if the jump table index is
attacker-controlled, the indirect jump may not be constrained by CET. This option turns
on CET instrumentation to enable indirect branch track for switch statements with jump
tables which leads to the jump targets reachable via any indirect jumps.

-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI
function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for
saving and restoring these registers is emitted inline, resulting in fairly lengthy
prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that
use stubs in the static portion of libgcc to perform these saves and restores, thus
reducing function size at the cost of a few extra instructions.

-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is
the conservative default; gnu2 is more efficient, but it may add compile- and run-time
requirements that cannot be satisfied on all systems.

-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually
equally fast as method using SUB/MOV operations and is enabled by default. In some cases
disabling it may improve performance because of improved scheduling and reduced
dependencies.

-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments is computed in
the function prologue. This is faster on most modern CPUs because of reduced
dependencies, improved scheduling and reduced stack usage when the preferred stack
boundary is not equal to 2. The drawback is a notable increase in code size. This
switch implies -mno-push-args.

-mthreads
Support thread-safe exception handling on MinGW. Programs that rely on thread-safe
exception handling must compile and link all code with the -mthreads option. When
compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper
library -lmingwthrd which cleans up per-thread exception-handling data.

-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native Microsoft Windows compiler.

If "packed" is used on a structure, or if bit-fields are used, it may be that the
Microsoft ABI lays out the structure differently than the way GCC normally does.
Particularly when moving packed data between functions compiled with GCC and the native
Microsoft compiler (either via function call or as data in a file), it may be necessary
to access either format.

This option is enabled by default for Microsoft Windows targets. This behavior can also
be controlled locally by use of variable or type attributes. For more information, see
x86 Variable Attributes and x86 Type Attributes.

The Microsoft structure layout algorithm is fairly simple with the exception of the bit-
field packing. The padding and alignment of members of structures and whether a bit-
field can straddle a storage-unit boundary are determine by these rules:

1. Structure members are stored sequentially in the order in which they are
declared: the first member has the lowest memory address and the last member the
highest.

2. Every data object has an alignment requirement. The alignment requirement
for all data except structures, unions, and arrays is either the size of the object
or the current packing size (specified with either the "aligned" attribute or the
"pack" pragma), whichever is less. For structures, unions, and arrays, the alignment
requirement is the largest alignment requirement of its members. Every object is
allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation
unit if the integral types are the same size and if the next bit-field fits into the
current allocation unit without crossing the boundary imposed by the common alignment
requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

1. If a zero-length bit-field is inserted between two bit-fields that
are normally coalesced, the bit-fields are not coalesced.

For example:

struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;

The size of "t1" is 8 bytes with the zero-length bit-field. If the zero-length bit-
field were removed, "t1"'s size would be 4 bytes.

2. If a zero-length bit-field is inserted after a bit-field, "foo", and the
alignment of the zero-length bit-field is greater than the member that follows it,
"bar", "bar" is aligned as the type of the zero-length bit-field.

For example:

struct
{
char foo : 4;
short : 0;
char bar;
} t2;

struct
{
char foo : 4;
short : 0;
double bar;
} t3;

For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size
of "t2" is 4. For "t3", the zero-length bit-field does not affect the alignment of
"bar" or, as a result, the size of the structure.

Taking this into account, it is important to note the following:

1. If a zero-length bit-field follows a normal bit-field, the type of the
zero-length bit-field may affect the alignment of the structure as whole. For
example, "t2" has a size of 4 bytes, since the zero-length bit-field follows a
normal bit-field, and is of type short.

2. Even if a zero-length bit-field is not followed by a normal bit-field, it may
still affect the alignment of the structure:

struct
{
char foo : 6;
long : 0;
} t4;

Here, "t4" takes up 4 bytes.

3. Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;

Here, "t5" takes up 2 bytes.

-mno-align-stringops
Do not align the destination of inlined string operations. This switch reduces code size
and improves performance in case the destination is already aligned, but GCC doesn't know
about it.

-minline-all-stringops
By default GCC inlines string operations only when the destination is known to be aligned
to least a 4-byte boundary. This enables more inlining and increases code size, but may
improve performance of code that depends on fast "memcpy" and "memset" for short lengths.
The option enables inline expansion of "strlen" for all pointer alignments.

-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for small
blocks and a library call for large blocks.

-mstringop-strategy=alg
Override the internal decision heuristic for the particular algorithm to use for inlining
string operations. The allowed values for alg are:

rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.

byte_loop
loop
unrolled_loop
Expand into an inline loop.

libcall
Always use a library call.

-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if "__builtin_memcpy" should be
inlined and what inline algorithm to use when the expected size of the copy operation is
known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is
specified in -mstringop-strategy, max_size specifies the max byte size with which inline
algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of
the triplets in the list must be specified in increasing order. The minimal byte size
for alg is 0 for the first triplet and "max_size + 1" of the preceding range.

-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is to control
"__builtin_memset" expansion.

-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the
instructions to save, set up, and restore frame pointers and makes an extra register
available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame
pointer for leaf functions, which might make debugging harder.

-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register
(%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added.
Whether or not this is valid depends on the operating system, and whether it maps the
segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on.

-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The option
-mavx turns this on by default.

-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call before the prologue. Note:
On x86 architectures the attribute "ms_hook_prologue" isn't possible at the moment for
-mfentry and -pg.

-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section that contains pointers to
each profiling call. This is useful for automatically patching and out calls.

-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This
is useful when they should be patched in later dynamically. This is likely only useful
together with -mrecord-mcount.

-minstrument-return=type
Instrument function exit in -pg -mfentry instrumented functions with call to specified
function. This only instruments true returns ending with ret, but not sibling calls
ending with jump. Valid types are none to not instrument, call to generate a call to
__return__, or nop5 to generate a 5 byte nop.

-mrecord-return
-mno-record-return
Generate a __return_loc section pointing to all return instrumentation code.

-mfentry-name=name
Set name of __fentry__ symbol called at function entry for -pg -mfentry functions.

-mfentry-section=name
Set name of section to record -mrecord-mcount calls (default __mcount_loc).

-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE extensions disabled,
-mskip-rax-setup can be used to skip setting up RAX register when there are no variable
arguments passed in vector registers.

Warning: Since RAX register is used to avoid unnecessarily saving vector registers on
stack when passing variable arguments, the impacts of this option are callees may waste
some stack space, misbehave or jump to a random location. GCC 4.4 or newer don't have
those issues, regardless the RAX register value.

-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than
32-bit/64-bit integer divide. This option generates a run-time check. If both dividend
and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead
of 32-bit/64-bit integer divide.

-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.

-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for
global canary or tls for per-thread canary in the TLS block (the default). This option
has effect only when -fstack-protector or -fstack-protector-all is specified.

With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or
%gs) to use as base register for reading the canary, and from what offset from that base
register. The default for those is as specified in the relevant ABI.

-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This prevents the compiler
from using floating-point, vector, mask and bound registers.

-mrelax-cmpxchg-loop
When emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins lacking a
native instruction, optimize for the highly contended case by issuing an atomic load
before the "CMPXCHG" instruction, and using the "PAUSE" instruction to save CPU power
when restarting the loop.

-mindirect-branch=choice
Convert indirect call and jump with choice. The default is keep, which keeps indirect
call and jump unmodified. thunk converts indirect call and jump to call and return
thunk. thunk-inline converts indirect call and jump to inlined call and return thunk.
thunk-extern converts indirect call and jump to external call and return thunk provided
in a separate object file. You can control this behavior for a specific function by
using the function attribute "indirect_branch".

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and
-mindirect-branch=thunk-extern since the thunk function may not be reachable in the large
code model.

Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since
the external thunk can be made to enable control-flow check.

-mfunction-return=choice
Convert function return with choice. The default is keep, which keeps function return
unmodified. thunk converts function return to call and return thunk. thunk-inline
converts function return to inlined call and return thunk. thunk-extern converts
function return to external call and return thunk provided in a separate object file.
You can control this behavior for a specific function by using the function attribute
"function_return".

Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since
the external thunk can be made to enable control-flow check.

Note that -mcmodel=large is incompatible with -mfunction-return=thunk and
-mfunction-return=thunk-extern since the thunk function may not be reachable in the large
code model.

-mindirect-branch-register
Force indirect call and jump via register.

-mharden-sls=choice
Generate code to mitigate against straight line speculation (SLS) with choice. The
default is none which disables all SLS hardening. return enables SLS hardening for
function returns. indirect-jmp enables SLS hardening for indirect jumps. all enables
all SLS hardening.

-mindirect-branch-cs-prefix
Add CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers so
that the call and jmp instruction length is 6 bytes to allow them to be replaced with
lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time.

These -m switches are supported in addition to the above on x86-64 processors in 64-bit
environments.

-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets "int",
"long", and pointer types to 32 bits, and generates code that runs in 32-bit mode.

The -m64 option sets "int" to 32 bits and "long" and pointer types to 64 bits, and
generates code for the x86-64 architecture. For Darwin only the -m64 option also turns
off the -fno-pic and -mdynamic-no-pic options.

The -mx32 option sets "int", "long", and pointer types to 32 bits, and generates code for
the x86-64 architecture.

The -m16 option is the same as -m32, except for that it outputs the ".code16gcc" assembly
directive at the beginning of the assembly output so that the binary can run in 16-bit
mode.

The -miamcu option generates code which conforms to Intel MCU psABI. It requires the
-m32 option to be turned on.

-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the
x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not
modified by signal or interrupt handlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag -mno-red-zone disables this red zone.

-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the
lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or
dynamically linked. This is the default code model.

-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the
address space. This model has to be used for Linux kernel code.

-mcmodel=medium
Generate code for the medium model: the program is linked in the lower 2 GB of the
address space. Small symbols are also placed there. Symbols with sizes larger than
-mlarge-data-threshold are put into large data or BSS sections and can be located above
2GB. Programs can be statically or dynamically linked.

-mcmodel=large
Generate code for the large model. This model makes no assumptions about addresses and
sizes of sections.

-maddress-mode=long
Generate code for long address mode. This is only supported for 64-bit and x32
environments. It is the default address mode for 64-bit environments.

-maddress-mode=short
Generate code for short address mode. This is only supported for 32-bit and x32
environments. It is the default address mode for 32-bit and x32 environments.

-mneeded
-mno-needed
Emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-
architecture ISA level required to execute the binary.

-mno-direct-extern-access
Without -fpic nor -fPIC, always use the GOT pointer to access external symbols. With
-fpic or -fPIC, treat access to protected symbols as local symbols. The default is
-mdirect-extern-access.

Warning: shared libraries compiled with -mno-direct-extern-access and executable compiled
with -mdirect-extern-access may not be binary compatible if protected symbols are used in
shared libraries and executable.

-munroll-only-small-loops
Controls conservative small loop unrolling. It is default enabled by O2, and unrolls loop
with less than 4 insns by 1 time. Explicit -f[no-]unroll-[all-]loops would disable this
flag to avoid any unintended unrolling behavior that user does not want.

-mlam=choice
LAM(linear-address masking) allows special bits in the pointer to be used for metadata.
The default is none. With u48, pointer bits in positions 62:48 can be used for metadata;
With u57, pointer bits in positions 62:57 can be used for metadata.

x86 Windows Options

These additional options are available for Microsoft Windows targets:

-mconsole
This option specifies that a console application is to be generated, by instructing the
linker to set the PE header subsystem type required for console applications. This
option is available for Cygwin and MinGW targets and is enabled by default on those
targets.

-mdll
This option is available for Cygwin and MinGW targets. It specifies that a DLL---a
dynamic link library---is to be generated, enabling the selection of the required runtime
startup object and entry point.

-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the "dllimport"
attribute should be ignored.

-mthreads
This option is available for MinGW targets. It specifies that MinGW-specific thread
support is to be used.

-municode
This option is available for MinGW-w64 targets. It causes the "UNICODE" preprocessor
macro to be predefined, and chooses Unicode-capable runtime startup code.

-mwin32
This option is available for Cygwin and MinGW targets. It specifies that the typical
Microsoft Windows predefined macros are to be set in the pre-processor, but does not
influence the choice of runtime library/startup code.

-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI
application is to be generated by instructing the linker to set the PE header subsystem
type appropriately.

-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable flag for the
stack used by nested functions isn't set. This is necessary for binaries running in
kernel mode of Microsoft Windows, as there the User32 API, which is used to set
executable privileges, isn't available.

-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It specifies that relocated-data
in read-only section is put into the ".data" section. This is a necessary for older
runtimes not supporting modification of ".rdata" sections for pseudo-relocation.

-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the GNU
extension to the PE file format that permits the correct alignment of COMMON variables
should be used when generating code. It is enabled by default if GCC detects that the
target assembler found during configuration supports the feature.

See also under x86 Options for standard options.

Xstormy16 Options

These options are defined for Xstormy16:

-msim
Choose startup files and linker script suitable for the simulator.

Xtensa Options

These options are supported for Xtensa targets:

-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The
"CONST16" instruction is currently not a standard option from Tensilica. When enabled,
"CONST16" instructions are always used in place of the standard "L32R" instructions. The
use of "CONST16" is enabled by default only if the "L32R" instruction is not available.

-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the
floating-point option. This has no effect if the floating-point option is not also
enabled. Disabling fused multiply/add and multiply/subtract instructions forces the
compiler to use separate instructions for the multiply and add/subtract operations. This
may be desirable in some cases where strict IEEE 754-compliant results are required: the
fused multiply add/subtract instructions do not round the intermediate result, thereby
producing results with more bits of precision than specified by the IEEE standard.
Disabling fused multiply add/subtract instructions also ensures that the program output
is not sensitive to the compiler's ability to combine multiply and add/subtract
operations.

-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory
references to guarantee sequential consistency. The default is -mserialize-volatile.
Use -mno-serialize-volatile to omit the "MEMW" instructions.

-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent
code (PIC), this option disables PIC for compiling kernel code.

-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The default is
-mno-text-section-literals, which places literals in a separate section in the output
file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows
the linker to combine literal pools from separate object files to remove redundant
literals and improve code size. With -mtext-section-literals, the literals are
interspersed in the text section in order to keep them as close as possible to their
references. This may be necessary for large assembly files. Literals for each function
are placed right before that function.

-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The default is -mno-auto-litpools,
which places literals in a separate section in the output file unless
-mtext-section-literals is used. With -mauto-litpools the literals are interspersed in
the text section by the assembler. Compiler does not produce explicit ".literal"
directives and loads literals into registers with "MOVI" instructions instead of "L32R"
to let the assembler do relaxation and place literals as necessary. This option allows
assembler to create several literal pools per function and assemble very big functions,
which may not be possible with -mtext-section-literals.

-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align
instructions to reduce branch penalties at the expense of some code density. The
assembler attempts to widen density instructions to align branch targets and the
instructions following call instructions. If there are not enough preceding safe density
instructions to align a target, no widening is performed. The default is -mtarget-align.
These options do not affect the treatment of auto-aligned instructions like "LOOP", which
the assembler always aligns, either by widening density instructions or by inserting NOP
instructions.

-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to
indirect calls unless it can determine that the target of a direct call is in the range
allowed by the call instruction. This translation typically occurs for calls to
functions in other source files. Specifically, the assembler translates a direct "CALL"
instruction into an "L32R" followed by a "CALLX" instruction. The default is
-mno-longcalls. This option should be used in programs where the call target can
potentially be out of range. This option is implemented in the assembler, not the
compiler, so the assembly code generated by GCC still shows direct call
instructions---look at the disassembled object code to see the actual instructions. Note
that the assembler uses an indirect call for every cross-file call, not just those that
really are out of range.

-mabi=name
Generate code for the specified ABI. Permissible values are: call0, windowed. Default
ABI is chosen by the Xtensa core configuration.

-mabi=call0
When this option is enabled function parameters are passed in registers "a2" through
"a7", registers "a12" through "a15" are caller-saved, and register "a15" may be used as a
frame pointer. When this version of the ABI is enabled the C preprocessor symbol
"__XTENSA_CALL0_ABI__" is defined.

-mabi=windowed
When this option is enabled function parameters are passed in registers "a10" through
"a15", and called function rotates register window by 8 registers on entry so that its
arguments are found in registers "a2" through "a7". Register "a7" may be used as a frame
pointer. Register window is rotated 8 registers back upon return. When this version of
the ABI is enabled the C preprocessor symbol "__XTENSA_WINDOWED_ABI__" is defined.

-mextra-l32r-costs=n
Specify an extra cost of instruction RAM/ROM access for "L32R" instructions, in clock
cycles. This affects, when optimizing for speed, whether loading a constant from literal
pool using "L32R" or synthesizing the constant from a small one with a couple of
arithmetic instructions. The default value is 0.

zSeries Options

These are listed under

ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of
them work by specifying directories or prefixes to use when searching for various kinds of
files. Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as -B, -I and -L. These
take precedence over places specified using environment variables, which in turn take
precedence over those specified by the configuration of GCC.

LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information which
allows GCC to work with different national conventions. GCC inspects the locale
categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale
categories can be set to any value supported by your installation. A typical value is
en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

The LC_CTYPE environment variable specifies character classification. GCC uses it to
determine the character boundaries in a string; this is needed for some multibyte
encodings that contain quote and escape characters that are otherwise interpreted as a
string end or escape.

The LC_MESSAGES environment variable specifies the language to use in diagnostic
messages.

If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and
LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC defaults to traditional C
English behavior.

TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses
temporary files to hold the output of one stage of compilation which is to be used as
input to the next stage: for example, the output of the preprocessor, which is the input
to the compiler proper.

GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler
driver. See the documentation of this option for more details.

GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms
executed by the compiler. No slash is added when this prefix is combined with the name
of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use
based on the pathname it is invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the
usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the
installed compiler. In many cases prefix is the value of "prefix" when you ran the
configure script.

Other prefixes specified with -B take precedence over this prefix.

This prefix is also used for finding files such as crt0.o that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search
for header files. For each of the standard directories whose name normally begins with
/usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an alternate directory
name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard
directory /usr/local/lib/bar. If a standard directory begins with the configured prefix
then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files.

COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC
tries the directories thus specified when searching for subprograms, if it cannot find
the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When
configured as a native compiler, GCC tries the directories thus specified when searching
for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using
GCC also uses these directories when searching for ordinary libraries for the -l option
(but directories specified with -L come first).

LANG
This variable is used to pass locale information to the compiler. One way in which this
information is used is to determine the character set to be used when character literals,
string literals and comments are parsed in C and C++. When the compiler is configured to
allow multibyte characters, the following values for LANG are recognized:

C-JIS
Recognize JIS characters.

C-SJIS
Recognize SJIS characters.

C-EUCJP
Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and
"mbtowc" as defined by the default locale to recognize and translate multibyte
characters.

GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional
text will be emitted to stderr when fix-it hints are emitted.
-fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over
this environment variable.

fixits-v1
Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In
particular, columns are expressed as a count of bytes, starting at byte 1 for the
initial column.

fixits-v2
As "fixits-v1", but columns are expressed as display columns, as per
-fdiagnostics-column-unit=display.

Some additional environment variables affect the behavior of the preprocessor.

CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much
like PATH, in which to look for header files. The special character, "PATH_SEPARATOR",
is target-dependent and determined at GCC build time. For Microsoft Windows-based
targets it is a semicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after
any paths given with -I options on the command line. This environment variable is used
regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language
indicated. Each specifies a list of directories to be searched as if specified with
-isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current
working directory. Empty elements can appear at the beginning or end of a path. For
instance, if the value of CPATH is ":/special/include", that has the same effect as
-I. -I/special/include.

DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on
the non-system header files processed by the compiler. System header files are ignored
in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules
are written to that file, guessing the target name from the source file name. Or the
value can have the form file target, in which case the rules are written to file file
using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and
-MF, with an optional -MT switch too.

SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header
files are not ignored, so it implies -M rather than -MM. However, the dependence on the
main input file is omitted.

SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement
of the current date and time in the "__DATE__" and "__TIME__" macros, so that the
embedded timestamps become reproducible.

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds
(excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to
the output of "date +%s" on GNU/Linux and other systems that support the %s extension in
the "date" command.

The value should be a known timestamp such as the last modification time of the source or
package and it should be set by the build process.

BUGS
For instructions on reporting bugs, see <file:///usr/share/doc/gcc-13/README.Bugs>.

FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to
work. On multi-libbed systems, gcc -shared must select the correct support libraries to
link against. Failing to supply the correct flags may lead to subtle defects. Supplying
them in cases where they are not necessary is innocuous. -shared suppresses the addition
of startup code to alter the floating-point environment as done with -ffast-math, -Ofast
or -funsafe-math-optimizations on some targets.

SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries
for gcc, cpp, as, ld, binutils and gdb.

AUTHOR
See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for
contributors to GCC.

Permission is granted to copy, distribute and/or modify this document under the terms of the
GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with the Invariant Sections being "GNU General Public License" and
"Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover
Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.

gcc-13 2024-09-04 GCC(1)

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