[Bug gcov-profile/47618] Collecting multiple profiles and using all for PGO

["steven at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47618

--- Comment #12 from Steven Bosscher <steven at gcc dot gnu.org> 2012-07-25 08:24:49 UTC ---
(In reply to comment #9)
> I think a tool to merge would be a good partial solution.

We will go with the tool solution. I'll take care of the tool before GCC 4.8,
if that's OK with apinski.

I think we shouldn't have a new tool, though. I'd prefer to teach the gcov
program to do it instead. What would you prefer?


> As far as I can see what would still be missing for user-friendly usage, is a
> mechanism to guarantee that all pre-merged files are saved with different
> names, so that different processes don't overwrite each others output files.

Deeply berried in the GCC manuals is this section:
http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/Cross_002dprofiling.html

With the right combination of GCOV_PREFIX_STRIP and GCOV_PREFIX, it should be
possible to send the gcda files to unique directories per MPI rank. But I think
that a more practical solution is necessary. (I also don't know how these
environment variables interact with -profile-dir. I doubt anyone looked into
this before now...)

I like the %q (and %p) variables from Valgrind, and I don't think it's very
hard to add support for them in libgcov.
(http://valgrind.org/docs/manual/manual-core.html)