| B::Concise(3pm) | Perl Programmers Reference Guide | B::Concise(3pm) |
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 two outputs (or '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
In this -exec rendering, each opcode is executed in the order shown. The add opcode, marked with '*', is discussed in more detail.
The 1st column is the op's sequence number, starting at 1, and is displayed in base 36 by default. Here they're purely linear; the sequences are very helpful when looking at code with loops and branches.
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]', may be followed by op-specific information in parentheses or brackets (ex '[t1]').
The op-flags (ex 'sK/2') 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.
Arguments that don't start with a hyphen are taken to be the names of subroutines or formats to render; 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", "UNITCHECK", "CHECK", "INIT", or "END" will cause all of the corresponding special blocks to be printed. Arguments must follow options.
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).
These options control the 'vertical display' of opcodes. The display 'order' is also called 'mode' elsewhere in this document.
These options select the line-style (or just style) used to render each opcode, and dictates what info is actually printed into each line.
These are pairwise exclusive, i.e. compact or loose, vt or ascii.
1 <0> enter
# 1: my $i;
2 <;> nextstate(main 1 junk.pl:1) v:{
3 <0> padsv[$i:1,10] vM/LVINTRO
# 3: for $i (0..9) {
4 <;> nextstate(main 3 junk.pl:3) v:{
5 <0> pushmark s
6 <$> const[IV 0] s
7 <$> const[IV 9] s
8 <{> enteriter(next->j last->m redo->9)[$i:1,10] lKS
k <0> iter s
l <|> and(other->9) vK/1
# 4: print "line ";
9 <;> nextstate(main 2 junk.pl:4) v
a <0> pushmark s
b <$> const[PV "line "] s
c <@> print vK
# 5: print "$i\n";
...
The following options are pairwise exclusive.
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.
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.
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
+ UNOP_AUX A UNOP with auxillary fields
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
. METHOP An OP with method call info
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 B::Op_private and regen/op_private for more details.
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.)
These items are the primitives used to perform indenting, and to select text from amongst alternatives.
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:
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".
$ strict refs
& strict subs
* strict vars
x$ explicit use/no strict refs
x& explicit use/no strict subs
x* explicit use/no strict vars
i integers
l locale
b bytes
{ block scope
% localise %^H
< open in
> open out
I overload int
F overload float
B overload binary
S overload string
R overload re
T taint
E eval
X filetest access
U utf-8
us use feature 'unicode_strings'
fea=NNN feature bundle number
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.
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 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.
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().
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.
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.
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.
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 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>.
| 2022-02-19 | perl v5.34.1 |