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B::Concise(3)
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 workings 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 displayed 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 an example of 2 outputs (aka 'renderings'), using the -exec and
-basic (i.e. default) formatting conventions on the same code snippet.
% perl -MO=Concise,-exec -e '$a = $b + 42'
1 <0> enter
2 <;> nextstate(main 1 -e:1) v
3 <#> gvsv[*b] s
4 <$> const[IV 42] s
* 5 <2> add[t3] sK/2
6 <#> gvsv[*a] s
7 <2> sassign vKS/2
8 <@> leave[1 ref] vKP/REFC
Each line corresponds to an opcode. The opcode marked with '*' is used in a
few examples below.
The 1st column is the op's sequence number, starting at 1, and is displayed
in base 36 by default. This rendering is in -exec (i.e. execution) order.
The symbol between angle brackets indicates the op's type, for example; <2>
is a BINOP, <@> a LISTOP, and <#> is a PADOP, which is used in threaded
perls. (see "OP class abbreviations").
The opname, as in 'add[t1]', which may be followed by op-specific
information in parentheses or brackets (ex '[t1]').
The op-flags (ex 'sK/2') follow, and are described in ("OP flags
abbreviations").
% 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
The default rendering is top-down, so they're not in execution order. This
form reflects the way the stack is used to parse and evaluate expressions;
the add operates on the two terms below it in the tree.
Nullops appear as "ex-opname", where opname is an op that has been
optimized away by perl. They're displayed with a sequence-number of '-',
because they are not executed (they don't appear in previous example),
they're printed here because they reflect the parse.
The arrow points to the sequence number of the next op; they're not
displayed in -exec mode, for obvious reasons.
Note that because this rendering was done on a non-threaded perl, the
PADOPs in the previous examples are now SVOPs, and some (but not all) of
the square brackets have been replaced by round ones. This is a subtle
feature to provide some visual distinction between renderings on threaded
and un-threaded perls.
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 program (outside any subroutines, and not including use'd
or require'd files) is rendered. Passing "BEGIN", "CHECK", "INIT", or
"END" will cause all of the corresponding 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, starting at the root). The indentation of each OP shows its
level in the tree, and the '->' at the end of the line indicates the
next opcode in execution order. 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
constructs 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_FORMAT", "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 precious 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 documentation 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
obviously mutually exclusive with bigendian.
Other options
These are pairwise exclusive.
-main
Include the main program in the output, even if subroutines were also
specified. This rendering is normally suppressed when a subroutine
name or reference 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 produce a banner appropriate for users needs. It's not ideal,
because the rendering-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 represents 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
OP flags are either public or private. The public flags alter the behavior
of each opcode in consistent ways, and are represented by 0 or more single
characters.
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)
Want is unknown
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)
Private flags, if any are set for an opcode, are displayed after a '/'
8 <@> leave[1 ref] vKP/REFC ->(end)
7 <2> sassign vKS/2 ->8
They're opcode specific, and occur less often than the public ones, so
they're represented by short mnemonics instead of single-chars; see op.h
for gory details, or try this quick 2-liner:
$> perl -MB::Concise -de 1
DB<1> |x \%B::Concise::priv
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 following 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 literal 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
rendering. 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 hexadecimal.
#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 hexadecimal.
#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 hexadecimal.
#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 hexadecimal.
#svaddr
The address of the OP's SV, if it has an SV, in hexadecimal.
#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 compile-time only operation of O.pm, and even
use the debugger to step through B::Concise::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 new #variables, you'll need to
define a callback 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 values 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 filehandle, or into a string passed as a ref (unless you've built perl
with -Uuseperlio).
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
Errors in rendering (non-existent function-name, non-existent coderef) are
written to the STDOUT, or wherever you've set it via walk_output().
Errors using the various *style* calls, and bad args to walk_output(),
result in die(). Use an eval if you wish to catch these errors and
continue processing.
AUTHOR
Stephen McCamant, <smcc@CSUA.Berkeley.EDU>.
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