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control over the execution of the compile tree it is possible to provide your
own runops function.

It's probably best to copy one of the existing runops functions and
change it to suit your needs. Then, in the C<BOOT> section of your XS
file, add the line:

  PL_runops = my_runops;

This function should be as efficient as possible to keep your programs
running as fast as possible. See L<Jit> for an even faster just-in-time 
compilation runloop.

=head3 Walkers or runops

The standard op tree B<walker> or B<runops> is as simple as this fast
C<Perl_runops_standard()> in (F<run.c>). It starts with C<main_start> and walks
the C<op_next> chain until the end. No need to check other fields, strictly
linear through the tree.

  int
  Perl_runops_standard(pTHX)
  {
	dVAR;
	while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
		PERL_ASYNC_CHECK(); /* until 5.13.2 */
	}
	TAINT_NOT;
	return 0;
  }

To inspect the op tree within a perl program, you can also hook C<PL_runops> (see
above at L</"Pluggable runops">) to your own perl walker (see e.g. L<B::Utils>
for various useful walkers), but you cannot modify the tree from within the B
accessors, only via XS. Or via L<B::Generate> as explained in Simon Cozen's 
"Hacking the Optree for Fun..." L<http://www.perl.com/pub/a/2002/05/07/optree.html>.

I<Todo: Show the other runloops, and esp. the B:Utils ones.>
I<Todo: Describe the dumper, the debugging and more extended walkers.>

=head1 SEE ALSO

=head2 Internal and external modifications

See the short description of the internal optimizer in the "Brief Summary".

I<Todo: Describe the exported variables and functions which can be
hooked, besides simply adding code to the blocks.>

Via L</"Pluggable runops"> you can provide your own walker function, as it
is done in most B modules. Best see L<B::Utils>.

You may also create custom ops at runtime (well, strictly speaking at
compile-time) via L<B::Generate>.

=head2 Modules

The most important op tree module is L<B::Concise> by Stephen McCamant.

L<B::Utils> provides abstract-enough op tree grep's and walkers with
callbacks from the perl level.

L<Devel::Hook> allows adding perl hooks into the BEGIN, CHECK,
UNITCHECK, INIT blocks.

L<Devel::TypeCheck> tries to verify possible static typing for
expressions and variables, a pretty hard problem for compilers,
esp. with such dynamic and untyped variables as Perl 5.

Reini Urban maintains the interactive op tree debugger L<B::Debugger>, 
the Compiler suite (B::C, B::CC, B::Bytecode), L<B::Generate> and 
is working on L<Jit>.

=head2 Various Articles

The best source of information is the source. It is very well documented.

There are some pod files from talks and workshops in F<ramblings/>.
From YAPC EU 2010 there is a good screencast at L<http://vimeo.com/14058377>.

Simon Cozens has posted the course material to NetThink's
L<http://books.simon-cozens.org/index.php/Perl_5_Internals#The_Lexer_and_the_Parser>
training course. This is the currently best available description on
that subject.

"Hacking the Optree for Fun..." at
L<http://www.perl.com/pub/a/2002/05/07/optree.html> is the next step by
Simon Cozens.

Scott Walters added more details at L<http://perldesignpatterns.com/?PerlAssembly>

Joshua ben Jore wrote a 50 minute presentation on "Perl 5
VM guts" at L<http://diotalevi.isa-geek.net/~josh/Presentations/Perl%205%20VM/>
focusing on the op tree for SPUG, the Seattle Perl User's Group.

Eric Wilhelm wrote a brief tour through the perl compiler backends for
the impatient refactorerer. The perl_guts_tour as mp3
L<http://scratchcomputing.com/developers/perl_guts_tour.html> or as
pdf L<http://scratchcomputing.com/developers/perl_guts_tour.pdf>

This text was created in this wiki article:
L<http://www.perlfoundation.org/perl5/index.cgi?optree_guts>
The with B::C released version should be more actual.

=head1 Conclusion

So this is about 30% of the basic op tree information so far. Not speaking about
the guts. Simon Cozens and Scott Walters have more 30%, in the source are more
10% to copy&paste, and in the compilers and run-time information is the rest. I
hope with the help of some hackers we'll get it done, so that some people will
begin poking around in the B backends. And write the wonderful new C<dump>/C<undump>
functionality (which actually worked in the early years on Solaris) to
save-image and load-image at runtime as in LISP, analyse and optimize the
output, output PIR (parrot code), emit LLVM or another JIT optimized code or
even write assemblers. I have a simple one at home. :)

Written 2008 on the perl5 wiki with socialtext and pod in parallel 
by Reini Urban, CPAN ID C<rurban>.