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PERLCOMPILE(1)	       Perl Programmers Reference Guide		PERLCOMPILE(1)

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.

perl v5.10.1			  2009-04-11			PERLCOMPILE(1)
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