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Re: RFC: Improving GCC8 default option settings


> On Wed, Sep 13, 2017 at 3:46 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> > On Wed, Sep 13, 2017 at 03:41:19PM +0200, Richard Biener wrote:
> >> On its own -O3 doesn't add much (some loop opts and slightly more
> >> aggressive inlining/unrolling), so whatever it does we
> >> should consider doing at -O2 eventually.
> >
> > Well, -O3 adds vectorization, which we don't enable at -O2 by default.
> 
> As said, -fprofile-use enables it so -O2 should eventually do the same
> for "really hot code".

I don't see static profile prediction to be very useful here to find "really
hot code" - neither in current implementation or future. The problem of
-O2 is that we kind of know that only 10% of code somewhere matters for
performance but we have no way to reliably identify it.

I would make sense to have less agressive vectoriazaoitn at -O2 and more at
-Ofast/-O3.

Adding -Os and -Oz would make sense to me - even with hot/cold info it is not
desriable to optimize as agressively for size as we do becuase mistakes happen
and one do not want to make code paths 1000 times slower to save one byte
of binary.

We could handle this gratefully internally by having logic for "known to be cold"
and "guessed to be cold". New profile code can make difference in this.

Honza
> 
> Richard.
> 
> >         Jakub


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