ext::B::B::ConcisPerl)Programmers Referenceext::B::B::Concise(3p)
B::Concise - Walk Perl syntax tree, printing concise info
about ops
perl -MO=Concise[,OPTIONS] foo.pl
use B::Concise qw(set_style add_callback);
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.
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").
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% 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 execu-
tion 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.
Arguments that don't start with a hyphen are taken to be the
names of subroutines to print the OPs of; if no such func-
tions 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 interre-
late; 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
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document.
-basic
Print OPs in the order they appear in the OP tree (a
preorder traversal, starting at the root). The indenta-
tion 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
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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 charac-
ters, 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
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bigendian.
Other options
These are pairwise exclusive.
-main
Include the main program in the output, even if subrou-
tines 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 sub-
routine 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.
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.
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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 pub-
lic 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
For each line-style ('concise', 'terse', 'linenoise', etc.)
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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 encoun-
tered. 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), gen-
erates 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:
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#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 proper-
ties '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 oth-
ers 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
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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.
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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 pro-
gram 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 hexa-
decimal.
#svaddr
The address of the OP's SV, if it has an SV, in hexade-
cimal.
#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.
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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 call-
back 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
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populate (or modify) those variables. They are then avail-
able 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 sup-
plied 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
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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 multi-
ple $walker objects, so they must still be used in a coordi-
nated 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 sup-
port 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.
Stephen McCamant, <smcc@CSUA.Berkeley.EDU>.
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