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 _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform