Re: [patch] Prevent -fprofile-arcs and -fbranch-probabilities rtl from diverging

[Jan Hubicka <hubicka at ucw dot cz>]

> Zdenek,
> >in code that calls fork or exec functions, the rtl generated for
> >-fprofile-arcs slightly differ from the one generated for
> >-fbranch-probabilities, since in the former case we insert calls to
> >gcov_flush before fork/exec.  This may cause the code diverge (as indeed
> >happened to me during value profiling enhancement testing) enough so
> >that the counters issued with -fprofile-arcs do not those that
> >-fbranch-probabilities expects.
> >
> >This patch fixes it (or better said, moves it closer to the correct
> >state) by replacing the fork/exec call by a call to a library function
> >that calls gcov_flush and the original function, in the program compiled
> >with -fprofile-arcs.  This makes the code more similar, thus preventing
> >the divergence.
> Hmm, it's not immediately obvious to me as to why inserting the
> gcov_flush call causes the bb graph to change. Unless there's a
> no-return edge being inserted.  couldn't we supress that edge
> for this particular call?
> 
> >Of course this will become unnecesary once the profiling is done on
> >trees, since there it won't be a problem to insert the gcov_flush
> >calls after the profile feedback stage.
> This patch is large and it is a stopgap measure within
> the current development series. We run the significant risk of it
> remaining in the compiler after ssa obsoletes it.

It would not be terribly bad to have it in compiler even after
tree-profiling branch is merged (tree-ssa itself does not obsolette RTL
profilng yet).  I plan to keep the RTL feedback code working.  It is
usefull for verification of profile updating code and several copmilers
allow feedback to be read at multiple stages so the late stages of
compilation are not affected by incorrectly guessed profile updates.

Honza
> 
> nathan
> 
> -- 
> Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
> nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk
>