# Copyright (C) 2001-2004, The Perl Foundation.
# $Id: debug.pod 14308 2006-08-22 17:14:31Z ambs $

=head1 NAME

docs/debug.pod - Debugging Parrot

=head1 ABSTRACT

This document describes how to debug various parts of Parrot.

=head1 VERSION

$Revision $

=head1 THE PARROT BINARY

=head2 Using a debugger

Per default the C<parrot> binary is being built with debugging symbols. This
means that you can run C<parrot> under an debugger like C<gdb>.

Debugging support can be explicitly enabled with:

    shell> perl Configure.pl --debugging
    shell> make

For testing it might be a good idea to make test runs without debug support. So
debugging can also be turned off with:

  shell> perl Configure.pl --debugging=0
  shell> make

=head2 Using a memory checker

You could, and should, also run the tests with a memory checker such as
C<valgrind>.  You can enable C<valgrind>, by running:

  shell> make test VALGRIND="valgrind --logfile=/tmp/grind"

Another possibility is to use Electric Fence, or ...

=head2 MEMORY MANAGEMENT

Some of the more frequent and exasperating C<parrot> bugs are related to memory
management in general, and garbage collection in particular.

Infant mortality

See F<docs/dev/infant.dev> for details of one frequent problem: infant
mortality. Infant mortality is when you create a Parrot object, but the garbage
collector runs before you put it into a Parrot register or in something else
that is itself within a Parrot register.

To help in resolving these issues, the parrot binary accepts a C<--gc-debug>
flag. This flag makes garbage collection occur as frequently as possible, which
maximizes the probability that any newborn objects will run afoul of the
garbage collector.

Within the C<--gc-debug> mode, there is another tool to help narrow down the
problem. You can edit F<src/dod.c> and C<#define> the C<GC_VERBOSE> flag to 1.
After recompiling C<parrot>, the garbage collector will perform additional
checks. After the garbage collector has traced all objects to find which ones
are still alive, it will scan through all of the dead objects to see if any of
them believe they are alive (which will happen for infants, since they come
into existence marked live.) If it finds any, it will print them out. You can
then re-run the program with a breakpoint set on the routine that allocated the
object (e.g. C<get_free_object> in F<src/smallobject.c>).  You'll probably want
to make the breakpoint conditional on the object having the version number that
was reported, because the same memory location will probably hold many
different objects over the lifetime of the program.

=head1 PIR AND PASM CODE

Let's say you have written (or generated) a huge .pasm or .pir file.  It's not
working. You'd like some help in figuring out why.

=head2 pdb

One possible tool is C<pdb>, the Parrot Debugger. See F<docs/debugger.pod> for
details on it.

=head2 stabs

If you are running on a jit-capable machine, you can also try using C<gdb> by
having the JIT compiler generate C<stabs> metadata and then stepping through
the code with C<gdb> as if it were any other language.

To use this, you'll want to use C<parrot> to generate your bytecode (.pbc
file). It is not strictly necessary, but you'll get more information into the
bytecode this way.

Let's say your file is named C<test.pasm>. (Note: these instructions will also
work if you use C<test.pir> everywhere C<test.pasm> occurs.)

Step 1: Generate the .pbc file with extra debugging information.

  shell> parrot -d -o test.pbc test.pasm

Step 2: Start up C<parrot> under C<gdb>

  % gdb parrot

or

  % emacs &
  (in emacs) M-x gdb
  (in emacs) type "parrot" so it says "gdb parrot"

Step 3: Set a breakpoint on runops_jit

  gdb> b runops_jit

Step 4: Run your program under C<gdb> with JIT and debugging on

  gdb> run -j -D4 test.pbc

Step 5: C<gdb> will stop at the beginning of runops_jit. Step through the lines
until just before the JITed code is executed (the line will be something like
C<(jit_code)(interpreter,pc)>.

  gdb> n
  gdb> n
  .
  .
  .

Step 6: load in the debugging information from the symbol file that the jit
just generated.

  gdb> add-symbol-file test.o 0

Step 7: Step into the JITed code

  gdb> s

At this point, you can step through the instructions, or print out the
various Parrot registers. FIXME: C<gdb> will know about I0-I31,
N0-N31, S0-S31, and P0-P31.


WARNING: Stepping too far

One thing to watch out for is that C<gdb> gets confused when attempting to step
over certain instructions. The only ones that I have noticed having problems is
keyed operations. With my version of C<gdb>, if I do 'n' to step over the
instruction, C<gdb> will start running and only stop when the entire parrot
program has finished. To work around this, do 'si' twice just before executing
any keyed op. For some reason, C<gdb> can then figure out when it's supposed to
stop next. If you know of a better technique, please let the mailing list know
(C<parrot-porters@perl.org>).

=head1 PIR CODE GENERATION

The C<parrot> binary has a bunch of debugging flags for spewing out information
about various aspects of its processing. See L<running.pod> for a
list of flags. Or have a look at the information provided by:

  shell> parrot --help

or

  shell> parrot --help-debug
