[Bug middle-end/35544] Error with -fprofile-use

[xinliangli at gmail dot com]

------- Comment #8 from xinliangli at gmail dot com  2008-03-12 07:17 -------
(In reply to comment #5)
> For reference see:
> http://gcc.gnu.org/ml/gcc-patches/2007-01/msg01205.html
> http://gcc.gnu.org/ml/gcc-patches/2005-03/msg00200.html
> http://gcc.gnu.org/ml/gcc/2005-12/msg00215.html
> 
> And most likely a couple others too.  I cannot find a good reference explaining
> why early inline helps a lot for C++ code like tramp3d.
> 
> >Again, how much performance gain can be achieved with early optimization for
> instrumented binary (which is slow anyway)?
> 
> IIRC there was a huge win (like a 10x speedup) for doing this.
> 

Yes, that is actually likely for C++ apps. For the compiler I worked with
before, the default optimization level is -O1 which performs user directed
inlining -- so the implicit requirement for instrumentation is actually -O1 and
above.
> 
> >  At least it is tricky with cross module inlining happening for instrumented
> > functions -- the inline instances should really updating counters in the
> > original routines.
> 
> Depends on if the inline is done with early inline or late and really really if
> it is done with late, the counters will be emitted with the LTO file before
> reading them back in so the counters will be correct already and inlining
> becomes obvious.
> 

I was referring to cross module inlining that happens late. Looks like for gcc,
this won't be an issue in theory.

David


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35544