=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
("backends") do things with the tree.  Some write it out as
bytecode, C source code, or a 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 backend 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 backends 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 backend modules,
what problems there are, and how to work around them.

=head2 Layout

The compiler backends 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.  Some backends (e.g., C<B::C>) have
programs (e.g., I<perlcc>) to hide the modules' complexity.

Here are the important backends 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::Bytecode

Stores the parse tree in a machine-independent format, suitable
for later reloading through the ByteLoader module.  Status: 5 (some
things work, some things don't, some things are untested).

=item B::C

Creates a C source file containing code to rebuild the parse tree
and resume the interpreter.  Status: 6 (many things work adequately,
including programs using Tk).

=item B::CC

Creates a C source file corresponding to the run time code path in
the parse tree.  This is the closest to a Perl-to-C translator there
is, but the code it generates is almost incomprehensible because it
translates the parse tree into a giant switch structure that
manipulates Perl structures.  Eventual goal is to reduce (given
sufficient type information in the Perl program) some of the
Perl data structure manipulations into manipulations of C-level
ints, floats, etc.  Status: 5 (some things work, including
uncomplicated Tk examples).

=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 Backends

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 backends are described first, and the most
experimental and incomplete backends 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 backends print:

  myperlprogram syntax OK

before producing any other output.

=head2 The Cross Referencing Backend

The cross referencing backend (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 Backend

The Deparse backend turns your Perl source back into Perl source.  It
can reformat along the way, making it useful as a de-obfuscator.  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 Backend

The lint backend (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.

=head2 The Simple C Backend

This module saves the internal compiled state of your Perl program
to a C source file, which can be turned into a native executable
for that particular platform using a C compiler.  The resulting
program links against the Perl interpreter library, so it
will not save you disk space (unless you build Perl with a shared
library) or program size.  It may, however, save you startup time.

The C<perlcc> tool generates such executables by default.

  perlcc myperlprogram.pl

For more information, see L<perlcc> and L<B::C>.

=head2 The Bytecode Backend

This backend is only useful if you also have a way to load and
execute the bytecode that it produces.  The ByteLoader module provides
this functionality.

To turn a Perl program into executable byte code, you can use C<perlcc>
with the C<-B> switch:

  perlcc -B myperlprogram.pl

The byte code is machine independent, so once you have a compiled
module or program, it is as portable as Perl source (assuming that
the user of the module or program has a modern-enough Perl interpreter
to decode the byte code).

See B<B::Bytecode> for information on options to control the
optimization and nature of the code generated by the Bytecode module.

=head2 The Optimized C Backend

The optimized C backend will turn your Perl program's run time
code-path into an equivalent (but optimized) C program that manipulates
the Perl data structures directly.  The program will still link against
the Perl interpreter library, to allow for eval(), C<s///e>,
C<require>, etc.

The C<perlcc> tool generates such executables when using the -O
switch.  To compile a Perl program (ending in C<.pl>
or C<.p>):

  perlcc -O myperlprogram.pl

To produce a shared library from a Perl module (ending in C<.pm>):

  perlcc -O Myperlmodule.pm

For more information, see L<perlcc> and L<B::CC>.

=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
backend modules all use this module to gain access to the compiled
parse tree.  You, the user of a backend module, will not need to
interact with B.

=item O

This module is the front-end to the compiler's backends.  Normally
called something like this:

  $ perl -MO=Deparse myperlprogram

This is like saying C<use O 'Deparse'> in your Perl program.

=item B::Asmdata

This module is used by the B::Assembler module, which is in turn used
by the B::Bytecode module, which stores a parse-tree as
bytecode for later loading.  It's not a backend itself, but rather a
component of a backend.

=item B::Assembler

This module turns a parse-tree into data suitable for storing
and later decoding back into a parse-tree.  It's not a backend
itself, but rather a component of a backend.  It's used by the
I<assemble> program that produces C<.plc> bytecode.

=item B::Bblock

This module is used by the B::CC backend.  It walks "basic blocks".
A basic block is a series of operations which is known to execute from
start to finish, with no possibility of branching or halting or 
jumps into inner ops.

=item B::Bytecode

This module is a backend that generates bytecode from a program's parse tree.
This bytecode is written to a C<.plc> file, from where it can later be
reconstructed back into a parse tree.  The goal is to do the expensive program
compilation once, save the interpreter's state into a file, and then restore the
state from the file when the program is to be executed.  See L</"The Bytecode
Backend"> for details about usage.

=item B::C

This module writes out C code corresponding to the parse tree and
other interpreter internal structures.  You compile the corresponding
C file, and get an executable file that will restore the internal
structures and the Perl interpreter will begin running the
program.  See L</"The Simple C Backend"> for details about usage.

=item B::CC

This module writes out C code corresponding to your program's
operations.  Unlike the C<B::C> module, which merely stores the
interpreter and its state in a C program, the C<B::CC> module makes a
C program that does not involve the interpreter.  As a consequence,
programs translated into C by C<B::CC> can execute faster than normal
interpreted programs.  See L</"The Optimized C Backend"> for
details about usage.

=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 useful for people who
are writing their own backend, 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 backend, 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 Backend"> for details about usage.

=item B::Disassembler

This module decodes C<.plc> bytecode back into a readable parse-tree, 
the reverse of the L</"B::Assembler">.
It's not a backend itself, but rather a component of a backend. 
It's used by the I<disassemble> program that produces bytecode.

=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 Backend"> 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 L</"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 backend,
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 Backend"> 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{}.

CC backend: goto, sort with non-default comparison. last for non-loop blocks.

improve XSUB handling (both static and dynamic)

sv_magic can do SvREFCNT_inc(obj) which messes up precalculated refcounts

allocation of XPV[INAHC]V structures needs fixing: Perl tries to free
  them, whereas the compiler expects them to be linked to a xpv[inahc]v_root

list the same as X[IPR]V structures.

ref counts

perl_parse replacement

fix cstring for long strings

compile-time initialisation of AvARRAYs

signed/unsigned problems with NV (and IV?) initialisation and elsewhere?

CvOUTSIDE for ordinary subs

DATA filehandle for standalone Bytecode program (easy)

DATA filehandle for multiple bytecode-compiled modules (harder)

DATA filehandle for C-compiled program (yet harder)

B::IO::SUBPROCESS

=head1 AUTHOR

This document was originally written by Nathan Torkington, and was
maintained by the perl5-porters mailing list I<perl5-porters@perl.org> 
up to Perl version 5.8.

This version with all the compiler options is now part of the L<B::C> 
compiler module, maintained by Reini Urban I<rurban@cpan.org>.

=head1 SEE ALSO

L<perlguts>

L<http://books.simon-cozens.org/index.php/Perl_5_Internals> 
with a simplier version at L<http://www.faqs.org/docs/perl5int/>.

=cut
