


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



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           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.




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       &   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



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       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-



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           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{}.




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AUTHOR
       This document was originally written by Nathan Torkington,
       and is now maintained by the perl5-porters mailing list
       perl5-porters@perl.org.





















































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