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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 v5.12.5                                     2012-11-03                                   PERLCOMPILE(1)