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B::Concise(3pm) Perl Programmers Reference Guide B::Concise(3pm)
NAME
B::Concise - Walk Perl syntax tree, printing concise info about ops
SYNOPSIS
perl -MO=Concise[,OPTIONS] foo.pl
use B::Concise qw(set_style add_callback);
DESCRIPTION
This compiler backend prints the internal OPs of a Perl program’s syntax tree in
one of several space-efficient text formats suitable for debugging the inner work-
ings of perl or other compiler backends. It can print OPs in the order they appear
in the OP tree, in the order they will execute, or in a text approximation to their
tree structure, and the format of the information displyed is customizable. Its
function is similar to that of perl’s -Dx debugging flag or the B::Terse module,
but it is more sophisticated and flexible.
EXAMPLE
Here’s is a short example of output (aka ’rendering’), using the default formatting
conventions :
% perl -MO=Concise -e ’$a = $b + 42’
8 <@> leave[1 ref] vKP/REFC ->(end)
1 <0> enter ->2
2 <;> nextstate(main 1 -e:1) v ->3
7 <2> sassign vKS/2 ->8
5 <2> add[t1] sK/2 ->6
- <1> ex-rv2sv sK/1 ->4
3 <$> gvsv(*b) s ->4
4 <$> const(IV 42) s ->5
- <1> ex-rv2sv sKRM*/1 ->7
6 <$> gvsv(*a) s ->7
Each line corresponds to an opcode. Null ops appear as "ex-opname", where opname is
the op that has been optimized away by perl.
The number on the first row indicates the op’s sequence number. It’s given in base
36 by default.
The symbol between angle brackets indicates the op’s type : for example, <2> is a
BINOP, <@> a LISTOP, etc. (see "OP class abbreviations").
The opname may be followed by op-specific information in parentheses (e.g.
"gvsv(*b)"), and by targ information in brackets (e.g. "leave[t1]").
Next come the op flags. The common flags are listed below ("OP flags abbrevia-
tions"). The private flags follow, separated by a slash. For example, "vKP/REFC"
means that the leave op has public flags OPf_WANT_VOID, OPf_KIDS, and OPf_PARENS,
and the private flag OPpREFCOUNTED.
Finally an arrow points to the sequence number of the next op.
OPTIONS
Arguments that don’t start with a hyphen are taken to be the names of subroutines
to print the OPs of; if no such functions are specified, the main body of the pro-
gram (outside any subroutines, and not including use’d or require’d files) is
printed. Passing "BEGIN", "CHECK", "INIT", or "END" will cause all of the corre-
sponding special blocks to be printed.
Options affect how things are rendered (ie printed). They’re presented here by
their visual effect, 1st being strongest. They’re grouped according to how they
interrelate; within each group the options are mutually exclusive (unless otherwise
stated).
Options for Opcode Ordering
These options control the ’vertical display’ of opcodes. The display ’order’ is
also called ’mode’ elsewhere in this document.
-basic
Print OPs in the order they appear in the OP tree (a preorder traversal, start-
ing at the root). The indentation of each OP shows its level in the tree. This
mode is the default, so the flag is included simply for completeness.
-exec
Print OPs in the order they would normally execute (for the majority of con-
structs this is a postorder traversal of the tree, ending at the root). In most
cases the OP that usually follows a given OP will appear directly below it;
alternate paths are shown by indentation. In cases like loops when control
jumps out of a linear path, a ’goto’ line is generated.
-tree
Print OPs in a text approximation of a tree, with the root of the tree at the
left and ’left-to-right’ order of children transformed into ’top-to-bottom’.
Because this mode grows both to the right and down, it isn’t suitable for large
programs (unless you have a very wide terminal).
Options for Line-Style
These options select the line-style (or just style) used to render each opcode, and
dictates what info is actually printed into each line.
-concise
Use the author’s favorite set of formatting conventions. This is the default,
of course.
-terse
Use formatting conventions that emulate the output of B::Terse. The basic mode
is almost indistinguishable from the real B::Terse, and the exec mode looks
very similar, but is in a more logical order and lacks curly brackets. B::Terse
doesn’t have a tree mode, so the tree mode is only vaguely reminiscent of
B::Terse.
-linenoise
Use formatting conventions in which the name of each OP, rather than being
written out in full, is represented by a one- or two-character abbreviation.
This is mainly a joke.
-debug
Use formatting conventions reminiscent of B::Debug; these aren’t very concise
at all.
-env
Use formatting conventions read from the environment variables "B_CONCISE_FOR-
MAT", "B_CONCISE_GOTO_FORMAT", and "B_CONCISE_TREE_FORMAT".
Options for tree-specific formatting
-compact
Use a tree format in which the minimum amount of space is used for the lines
connecting nodes (one character in most cases). This squeezes out a few pre-
cious columns of screen real estate.
-loose
Use a tree format that uses longer edges to separate OP nodes. This format
tends to look better than the compact one, especially in ASCII, and is the
default.
-vt Use tree connecting characters drawn from the VT100 line-drawing set. This
looks better if your terminal supports it.
-ascii
Draw the tree with standard ASCII characters like "+" and "│". These don’t look
as clean as the VT100 characters, but they’ll work with almost any terminal (or
the horizontal scrolling mode of less(1)) and are suitable for text documenta-
tion or email. This is the default.
These are pairwise exclusive, i.e. compact or loose, vt or ascii.
Options controlling sequence numbering
-basen
Print OP sequence numbers in base n. If n is greater than 10, the digit for 11
will be ’a’, and so on. If n is greater than 36, the digit for 37 will be ’A’,
and so on until 62. Values greater than 62 are not currently supported. The
default is 36.
-bigendian
Print sequence numbers with the most significant digit first. This is the usual
convention for Arabic numerals, and the default.
-littleendian
Print seqence numbers with the least significant digit first. This is obvi-
ously mutually exclusive with bigendian.
Other options
These are pairwise exclusive.
-main
Include the main program in the output, even if subroutines were also speci-
fied. This rendering is normally suppressed when a subroutine name or refer-
ence is given.
-nomain
This restores the default behavior after you’ve changed it with ’-main’ (it’s
not normally needed). If no subroutine name/ref is given, main is rendered,
regardless of this flag.
-nobanner
Renderings usually include a banner line identifying the function name or
stringified subref. This suppresses the printing of the banner.
TBC: Remove the stringified coderef; while it provides a ’cookie’ for each
function rendered, the cookies used should be 1,2,3.. not a random hex-address.
It also complicates string comparison of two different trees.
-banner
restores default banner behavior.
-banneris => subref
TBC: a hookpoint (and an option to set it) for a user-supplied function to pro-
duce a banner appropriate for users needs. It’s not ideal, because the render-
ing-state variables, which are a natural candidate for use in concise.t, are
unavailable to the user.
Option Stickiness
If you invoke Concise more than once in a program, you should know that the options
are ’sticky’. This means that the options you provide in the first call will be
remembered for the 2nd call, unless you re-specify or change them.
ABBREVIATIONS
The concise style uses symbols to convey maximum info with minimal clutter (like
hex addresses). With just a little practice, you can start to see the flowers, not
just the branches, in the trees.
OP class abbreviations
These symbols appear before the op-name, and indicate the B:: namespace that repre-
sents the ops in your Perl code.
0 OP (aka BASEOP) An OP with no children
1 UNOP An OP with one child
2 BINOP An OP with two children
│ LOGOP A control branch OP
@ LISTOP An OP that could have lots of children
/ PMOP An OP with a regular expression
$ SVOP An OP with an SV
" PVOP An OP with a string
{ LOOP An OP that holds pointers for a loop
; COP An OP that marks the start of a statement
# PADOP An OP with a GV on the pad
OP flags abbreviations
These symbols represent various flags which alter behavior of the opcode, sometimes
in opcode-specific ways.
v OPf_WANT_VOID Want nothing (void context)
s OPf_WANT_SCALAR Want single value (scalar context)
l OPf_WANT_LIST Want list of any length (list context)
K OPf_KIDS There is a firstborn child.
P OPf_PARENS This operator was parenthesized.
(Or block needs explicit scope entry.)
R OPf_REF Certified reference.
(Return container, not containee).
M OPf_MOD Will modify (lvalue).
S OPf_STACKED Some arg is arriving on the stack.
* OPf_SPECIAL Do something weird for this op (see op.h)
FORMATTING SPECIFICATIONS
For each line-style (’concise’, ’terse’, ’linenoise’, etc.) there are 3 format-
specs which control how OPs are rendered.
The first is the ’default’ format, which is used in both basic and exec modes to
print all opcodes. The 2nd, goto-format, is used in exec mode when branches are
encountered. They’re not real opcodes, and are inserted to look like a closing
curly brace. The tree-format is tree specific.
When a line is rendered, the correct format-spec is copied and scanned for the fol-
lowing items; data is substituted in, and other manipulations like basic indenting
are done, for each opcode rendered.
There are 3 kinds of items that may be populated; special patterns, #vars, and lit-
eral text, which is copied verbatim. (Yes, it’s a set of s///g steps.)
Special Patterns
These items are the primitives used to perform indenting, and to select text from
amongst alternatives.
(x(exec_text;basic_text)x)
Generates exec_text in exec mode, or basic_text in basic mode.
(*(text)*)
Generates one copy of text for each indentation level.
(*(text1;text2)*)
Generates one fewer copies of text1 than the indentation level, followed by one
copy of text2 if the indentation level is more than 0.
(?(text1#varText2)?)
If the value of var is true (not empty or zero), generates the value of var
surrounded by text1 and Text2, otherwise nothing.
~ Any number of tildes and surrounding whitespace will be collapsed to a single
space.
# Variables
These #vars represent opcode properties that you may want as part of your render-
ing. The ’#’ is intended as a private sigil; a #var’s value is interpolated into
the style-line, much like "read $this".
These vars take 3 forms:
#var
A property named ’var’ is assumed to exist for the opcodes, and is interpolated
into the rendering.
#varN
Generates the value of var, left justified to fill N spaces. Note that this
means while you can have properties ’foo’ and ’foo2’, you cannot render ’foo2’,
but you could with ’foo2a’. You would be wise not to rely on this behavior
going forward ;-)
#Var
This ucfirst form of #var generates a tag-value form of itself for display; it
converts ’#Var’ into a ’Var => #var’ style, which is then handled as described
above. (Imp-note: #Vars cannot be used for conditional-fills, because the =>
#var transform is done after the check for #Var’s value).
The following variables are ’defined’ by B::Concise; when they are used in a style,
their respective values are plugged into the rendering of each opcode.
Only some of these are used by the standard styles, the others are provided for you
to delve into optree mechanics, should you wish to add a new style (see "add_style"
below) that uses them. You can also add new ones using add_callback.
#addr
The address of the OP, in hexadecimal.
#arg
The OP-specific information of the OP (such as the SV for an SVOP, the non-
local exit pointers for a LOOP, etc.) enclosed in parentheses.
#class
The B-determined class of the OP, in all caps.
#classsym
A single symbol abbreviating the class of the OP.
#coplabel
The label of the statement or block the OP is the start of, if any.
#exname
The name of the OP, or ’ex-foo’ if the OP is a null that used to be a foo.
#extarg
The target of the OP, or nothing for a nulled OP.
#firstaddr
The address of the OP’s first child, in hexidecimal.
#flags
The OP’s flags, abbreviated as a series of symbols.
#flagval
The numeric value of the OP’s flags.
#hyphseq
The sequence number of the OP, or a hyphen if it doesn’t have one.
#label
’NEXT’, ’LAST’, or ’REDO’ if the OP is a target of one of those in exec mode,
or empty otherwise.
#lastaddr
The address of the OP’s last child, in hexidecimal.
#name
The OP’s name.
#NAME
The OP’s name, in all caps.
#next
The sequence number of the OP’s next OP.
#nextaddr
The address of the OP’s next OP, in hexidecimal.
#noise
A one- or two-character abbreviation for the OP’s name.
#private
The OP’s private flags, rendered with abbreviated names if possible.
#privval
The numeric value of the OP’s private flags.
#seq
The sequence number of the OP. Note that this is a sequence number generated by
B::Concise.
#seqnum
5.8.x and earlier only. 5.9 and later do not provide this.
The real sequence number of the OP, as a regular number and not adjusted to be
relative to the start of the real program. (This will generally be a fairly
large number because all of B::Concise is compiled before your program is).
#opt
Whether or not the op has been optimised by the peephole optimiser.
Only available in 5.9 and later.
#static
Whether or not the op is statically defined. This flag is used by the B::C
compiler backend and indicates that the op should not be freed.
Only available in 5.9 and later.
#sibaddr
The address of the OP’s next youngest sibling, in hexidecimal.
#svaddr
The address of the OP’s SV, if it has an SV, in hexidecimal.
#svclass
The class of the OP’s SV, if it has one, in all caps (e.g., ’IV’).
#svval
The value of the OP’s SV, if it has one, in a short human-readable format.
#targ
The numeric value of the OP’s targ.
#targarg
The name of the variable the OP’s targ refers to, if any, otherwise the letter
t followed by the OP’s targ in decimal.
#targarglife
Same as #targarg, but followed by the COP sequence numbers that delimit the
variable’s lifetime (or ’end’ for a variable in an open scope) for a variable.
#typenum
The numeric value of the OP’s type, in decimal.
Using B::Concise outside of the O framework
The common (and original) usage of B::Concise was for command-line renderings of
simple code, as given in EXAMPLE. But you can also use B::Concise from your code,
and call compile() directly, and repeatedly. By doing so, you can avoid the com-
pile-time only operation of O.pm, and even use the debugger to step through B::Con-
cise::compile() itself.
Once you’re doing this, you may alter Concise output by adding new rendering
styles, and by optionally adding callback routines which populate new variables, if
such were referenced from those (just added) styles.
Example: Altering Concise Renderings
use B::Concise qw(set_style add_callback);
add_style($yourStyleName => $defaultfmt, $gotofmt, $treefmt);
add_callback
( sub {
my ($h, $op, $format, $level, $stylename) = @_;
$h->{variable} = some_func($op);
});
$walker = B::Concise::compile(@options,@subnames,@subrefs);
$walker->();
set_style()
set_style accepts 3 arguments, and updates the three format-specs comprising a
line-style (basic-exec, goto, tree). It has one minor drawback though; it doesn’t
register the style under a new name. This can become an issue if you render more
than once and switch styles. Thus you may prefer to use add_style() and/or
set_style_standard() instead.
set_style_standard($name)
This restores one of the standard line-styles: "terse", "concise", "linenoise",
"debug", "env", into effect. It also accepts style names previously defined with
add_style().
add_style()
This subroutine accepts a new style name and three style arguments as above, and
creates, registers, and selects the newly named style. It is an error to re-add a
style; call set_style_standard() to switch between several styles.
add_callback()
If your newly minted styles refer to any #variables, you’ll need to define a call-
back subroutine that will populate (or modify) those variables. They are then
available for use in the style you’ve chosen.
The callbacks are called for each opcode visited by Concise, in the same order as
they are added. Each subroutine is passed five parameters.
1. A hashref, containing the variable names and values which are
populated into the report-line for the op
2. the op, as a B<B::OP> object
3. a reference to the format string
4. the formatting (indent) level
5. the selected stylename
To define your own variables, simply add them to the hash, or change existing val-
ues if you need to. The level and format are passed in as references to scalars,
but it is unlikely that they will need to be changed or even used.
Running B::Concise::compile()
compile accepts options as described above in "OPTIONS", and arguments, which are
either coderefs, or subroutine names.
It constructs and returns a $treewalker coderef, which when invoked, traverses, or
walks, and renders the optrees of the given arguments to STDOUT. You can reuse
this, and can change the rendering style used each time; thereafter the coderef
renders in the new style.
walk_output lets you change the print destination from STDOUT to another open file-
handle, or (unless you’ve built with -Uuseperlio) into a string passed as a ref.
my $walker = B::Concise::compile(’-terse’,’aFuncName’, \&aSubRef); # 1
walk_output(\my $buf);
$walker->(); # 1 renders -terse
set_style_standard(’concise’); # 2
$walker->(); # 2 renders -concise
$walker->(@new); # 3 renders whatever
print "3 different renderings: terse, concise, and @new: $buf\n";
When $walker is called, it traverses the subroutines supplied when it was created,
and renders them using the current style. You can change the style afterwards in
several different ways:
1. call C<compile>, altering style or mode/order
2. call C<set_style_standard>
3. call $walker, passing @new options
Passing new options to the $walker is the easiest way to change amongst any pre-
defined styles (the ones you add are automatically recognized as options), and is
the only way to alter rendering order without calling compile again. Note however
that rendering state is still shared amongst multiple $walker objects, so they must
still be used in a coordinated manner.
B::Concise::reset_sequence()
This function (not exported) lets you reset the sequence numbers (note that they’re
numbered arbitrarily, their goal being to be human readable). Its purpose is
mostly to support testing, i.e. to compare the concise output from two identical
anonymous subroutines (but different instances). Without the reset, B::Concise,
seeing that they’re separate optrees, generates different sequence numbers in the
output.
Errors
All detected errors, (invalid arguments, internal errors, etc.) are resolved with a
die($message). Use an eval if you wish to catch these errors and continue process-
ing.
In particular, compile will die if you’ve asked for a non-existent function-name, a
non-existent coderef, or a non-CODE reference.
AUTHOR
Stephen McCamant, <smcc AT CSUA.EDU>.
perl v5.8.6 2001-09-21 B::Concise(3pm)
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