=head1 NAME

perlcompile - Introduction to the Perl Compiler-Translator 

=head1 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 (C<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 C<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
C<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.

=head2 Layout

The compiler back ends are in the C<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):

=over 4

=item 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).

=item 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).

=item B::Xref

Reports on the declaration and use of subroutines and variables.
Status: 8 (it works nicely, but still has a few lingering bugs).

=back

=head1 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 B<-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.

=head2 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 I<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 C<clear_noremap>.  The
variable C<$ready_to_print> is a my() (lexical) variable,
B<i>ntroduced (first declared with my()) on line 1069, and used on
line 1079.  The variable C<$&> from the main package is used on 1086,
and so on.

A line number may be prefixed by a single letter:

=over 4

=item i

Lexical variable introduced (declared with my()) for the first time.

=item &

Subroutine or method call.

=item s

Subroutine defined.

=item r

Format defined.

=back

The most useful option the cross referencer has is to save the report
to a separate file.  For instance, to save the report on
I<myperlprogram> to the file I<report>:

  $ perl -MO=Xref,-oreport myperlprogram

=head2 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 B<-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 L<B::Deparse> for more information on the formatting options.

=head2 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 L<B::Lint> for information on the options.

=head1 Module List for the Compiler Suite

=over 4

=item 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.

=item 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 C<use O 'Deparse'> in your Perl program.

=item 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 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.

=item 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.

=item 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
L</"The Decompiling Back End"> for details about usage.

=item 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 C<scalar(@array)> is something that Lint
can identify.  See L</"The Lint Back End"> for details about usage.

=item 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]

=item B::Terse

This module prints the contents of the parse tree, but without as much
information as B::Debug.  For comparison, C<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.

=item 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 L</"The Cross-Referencing Back End"> for details about
usage.

=back

=head1 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{}.

=head1 AUTHOR

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

=cut
