On Thu, Oct 11, 2012 at 02:26:33PM -0400, Rafael Ávila de Espíndola wrote:
> On 10/11/2012 02:33 AM, Mike Hommey wrote:
> >On Wed, Oct 10, 2012 at 05:57:53PM -0400, Justin Lebar wrote:
> >>By "turning off Linux PGO testing", you really mean "stop making and
> >>distributing Linux PGO builds," right?
> >>
> >>The main reason I'd want Linux PGO is for mobile.  On desktop Linux,
> >>most users (I expect) don't run our builds, so it's not a big deal if
> >>they're some percent slower.
> >
> >Many people have made claims about that at several different occasions.
> >Can we once and for all come up with actual data on that?
> >
> >That being said, PGO on Linux is between 5 and 20% improvement on our
> >various talos tests. That's with the version of gcc we currently use,
> >which is 4.5. I'd expect 4.7 to do a better job even, especially if we
> >added lto to the equation (and since we are now building on x86-64
> >machines, we wouldn't have to worry about memory usage ; link time could
> >be a problem, though).
> >
> >Also note that disabling PGO currently also means disabling the
> >optimizations we do on omni.ja (central directory optimizations and
> >reordering). This is somehow covered by bug 773171.
> 
> I wouldn't be surprised if most of the pgo benefit is because of bad
> inline decisions by gcc. If we can narrow the gap by adding
> MOZ_ALWAYS_INLINE, then maybe we can drop pgo.

A non-unsignificant part of the performance improvements PGO gives come
from code reordering to improve branch prediction. Presumably, we can
use NS_LIKELY/NS_UNLIKELY to improve some branches manually.

Mike
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