Re: Loop unrolling-related SPEC regressions?

[Jan Hubicka <jh at suse dot cz>]

> > Honza writes:
> > 
> > > THe base/peak flags are not supposed to bring best performance,
> > > but be good for testing majority of gcc features.
> > 
> > gcc's competition, though, tends to use them that way (choosing options
> > that meet the criteria but give best performance).
> > 
> > Not that I want to get into a war on that front, but ...
> Hmm, perhaps we can try to make kind of "official" SPEC results when 3.1 release
> is out.  Andreas did some experimentation with the various options (it is
> linked from the page), and as I remember the loop unrolling -funroll-loops
> had neutral effect overal, while -funroll-all-loops caused slight performance
> drop.
Oops, sorry.
Looking at the numbers, -funrol-loops/-funroll-all-loops are equivalent in
Andreas testing and both slightly (6 seconds) better.
For 3.1 I would guess them to be more win because of code size savings
around the compiler.

Note that for Athlon the optimizer manual recommends heavy inlining and unrolling
as the cache sizes are pretty big.  This is the case for common benchmarks, even
for spec2000 that are often smaller than real world applications.

Honza