NAME
perlcompile - Introduction to the Perl Compiler-Translator
DESCRIPTION
Perl has always had a compiler: your source is compiled into an
internal form (a parse tree) which is then optimized before being run.
Since version 5.005, Perl has shipped with a module capable of
inspecting the optimized parse tree ("B"), and this has been used to
write many useful utilities, including a module that lets you turn your
Perl into C source code that can be compiled into a native executable.
The "B" module provides access to the parse tree, and other modules
("back ends") do things with the tree. Some write it out as semi-
human-readable text. Another traverses the parse tree to build a
cross-reference of which subroutines, formats, and variables are used
where. Another checks your code for dubious constructs. Yet another
back end dumps the parse tree back out as Perl source, acting as a
source code beautifier or deobfuscator.
Because its original purpose was to be a way to produce C code
corresponding to a Perl program, and in turn a native executable, the
"B" module and its associated back ends are known as "the compiler",
even though they don’t really compile anything. Different parts of the
compiler are more accurately a "translator", or an "inspector", but
people want Perl to have a "compiler option" not an "inspector gadget".
What can you do?
This document covers the use of the Perl compiler: which modules it
comprises, how to use the most important of the back end modules, what
problems there are, and how to work around them.
Layout
The compiler back ends are in the "B::" hierarchy, and the front-end
(the module that you, the user of the compiler, will sometimes interact
with) is the O module.
Here are the important back ends to know about, with their status
expressed as a number from 0 (outline for later implementation) to 10
(if there’s a bug in it, we’re very surprised):
B::Lint
Complains if it finds dubious constructs in your source code.
Status: 6 (it works adequately, but only has a very limited number
of areas that it checks).
B::Deparse
Recreates the Perl source, making an attempt to format it
coherently. Status: 8 (it works nicely, but a few obscure things
are missing).
B::Xref
Reports on the declaration and use of subroutines and variables.
Status: 8 (it works nicely, but still has a few lingering bugs).
Using The Back Ends
The following sections describe how to use the various compiler back
ends. They’re presented roughly in order of maturity, so that the most
stable and proven back ends are described first, and the most
experimental and incomplete back ends are described last.
The O module automatically enabled the -c flag to Perl, which prevents
Perl from executing your code once it has been compiled. This is why
all the back ends print:
myperlprogram syntax OK
before producing any other output.
The Cross Referencing Back End
The cross referencing back end (B::Xref) produces a report on your
program, breaking down declarations and uses of subroutines and
variables (and formats) by file and subroutine. For instance, here’s
part of the report from the pod2man program that comes with Perl:
Subroutine clear_noremap
Package (lexical)
$ready_to_print i1069, 1079
Package main
$& 1086
$. 1086
$0 1086
$1 1087
$2 1085, 1085
$3 1085, 1085
$ARGV 1086
%HTML_Escapes 1085, 1085
This shows the variables used in the subroutine "clear_noremap". The
variable $ready_to_print is a my() (lexical) variable, introduced
(first declared with my()) on line 1069, and used on line 1079. The
variable $& from the main package is used on 1086, and so on.
A line number may be prefixed by a single letter:
i Lexical variable introduced (declared with my()) for the first
time.
& Subroutine or method call.
s Subroutine defined.
r Format defined.
The most useful option the cross referencer has is to save the report
to a separate file. For instance, to save the report on myperlprogram
to the file report:
$ perl -MO=Xref,-oreport myperlprogram
The Decompiling Back End
The Deparse back end turns your Perl source back into Perl source. It
can reformat along the way, making it useful as a deobfuscator. The
most basic way to use it is:
$ perl -MO=Deparse myperlprogram
You’ll notice immediately that Perl has no idea of how to paragraph
your code. You’ll have to separate chunks of code from each other with
newlines by hand. However, watch what it will do with one-liners:
$ perl -MO=Deparse -e '$op=shift||die "usage: $0
code [...]";chomp(@ARGV=<>)unless@ARGV; for(@ARGV){$was=$_;eval$op;
die$@ if$@; rename$was,$_ unless$was eq $_}'
-e syntax OK
$op = shift @ARGV || die("usage: $0 code [...]");
chomp(@ARGV = <ARGV>) unless @ARGV;
foreach $_ (@ARGV) {
$was = $_;
eval $op;
die $@ if $@;
rename $was, $_ unless $was eq $_;
}
The decompiler has several options for the code it generates. For
instance, you can set the size of each indent from 4 (as above) to 2
with:
$ perl -MO=Deparse,-si2 myperlprogram
The -p option adds parentheses where normally they are omitted:
$ perl -MO=Deparse -e 'print "Hello, world\n"'
-e syntax OK
print "Hello, world\n";
$ perl -MO=Deparse,-p -e 'print "Hello, world\n"'
-e syntax OK
print("Hello, world\n");
See B::Deparse for more information on the formatting options.
The Lint Back End
The lint back end (B::Lint) inspects programs for poor style. One
programmer’s bad style is another programmer’s useful tool, so options
let you select what is complained about.
To run the style checker across your source code:
$ perl -MO=Lint myperlprogram
To disable context checks and undefined subroutines:
$ perl -MO=Lint,-context,-undefined-subs myperlprogram
See B::Lint for information on the options.
Module List for the Compiler Suite
B This module is the introspective ("reflective" in Java terms)
module, which allows a Perl program to inspect its innards. The
back end modules all use this module to gain access to the compiled
parse tree. You, the user of a back end module, will not need to
interact with B.
O This module is the front-end to the compiler’s back ends. Normally
called something like this:
$ perl -MO=Deparse myperlprogram
This is like saying "use O 'Deparse'" in your Perl program.
B::Concise
This module prints a concise (but complete) version of the Perl
parse tree. Its output is more customizable than the one of
B::Terse or B::Debug (and it can emulate them). This module useful
for people who are writing their own back end, or who are learning
about the Perl internals. It’s not useful to the average
programmer.
B::Debug
This module dumps the Perl parse tree in verbose detail to STDOUT.
It’s useful for people who are writing their own back end, or who
are learning about the Perl internals. It’s not useful to the
average programmer.
B::Deparse
This module produces Perl source code from the compiled parse tree.
It is useful in debugging and deconstructing other people’s code,
also as a pretty-printer for your own source. See "The Decompiling
Back End" for details about usage.
B::Lint
This module inspects the compiled form of your source code for
things which, while some people frown on them, aren’t necessarily
bad enough to justify a warning. For instance, use of an array in
scalar context without explicitly saying "scalar(@array)" is
something that Lint can identify. See "The Lint Back End" for
details about usage.
B::Showlex
This module prints out the my() variables used in a function or a
file. To get a list of the my() variables used in the subroutine
mysub() defined in the file myperlprogram:
$ perl -MO=Showlex,mysub myperlprogram
To get a list of the my() variables used in the file myperlprogram:
$ perl -MO=Showlex myperlprogram
[BROKEN]
B::Terse
This module prints the contents of the parse tree, but without as
much information as B::Debug. For comparison, "print "Hello,
world."" produced 96 lines of output from B::Debug, but only 6
from B::Terse.
This module is useful for people who are writing their own back
end, or who are learning about the Perl internals. It’s not useful
to the average programmer.
B::Xref
This module prints a report on where the variables, subroutines,
and formats are defined and used within a program and the modules
it loads. See "The Cross Referencing Back End" for details about
usage.
KNOWN PROBLEMS
BEGIN{} blocks are executed while compiling your code. Any external
state that is initialized in BEGIN{}, such as opening files, initiating
database connections etc., do not behave properly. To work around
this, Perl has an INIT{} block that corresponds to code being executed
before your program begins running but after your program has finished
being compiled. Execution order: BEGIN{}, (possible save of state
through compiler back-end), INIT{}, program runs, END{}.
AUTHOR
This document was originally written by Nathan Torkington, and is now
maintained by the perl5-porters mailing list perl5-porters@perl.org.