On Tue, Jan 22, 2013 at 05:09:10PM -0500, Ehsan Akhgari wrote: > On 2013-01-22 4:40 PM, Robert O'Callahan wrote: > >On Wed, Jan 23, 2013 at 4:31 AM, Ehsan Akhgari <ehsan.akhg...@gmail.com > ><mailto:ehsan.akhg...@gmail.com>> wrote: > > > > But note that unless a given code path is examined throughout the > > profiling phase of a PGO build, PGO will probably have negligible > > effect on it, if any. The PGO compiler looks for hot code paths and > > tries to optimize those, so for example if the awesomebar doesn't > > get examined during the profiling (which it isn't), it is extremely > > unlikely that turning off PGO on the code responsible for it would > > have any noticeable change on performance. > > > > > >I don't think this is a safe assumption. Our PGO builds not only do PGO > >but also "Link Time Code Generation" which enables cross-module > >optimizations. I have seen code being heavily optimized under PGO that I > >would not have expected to be significant in our PGO profile. > > > >It wouldn't be that hard to do an experiment to test the impact of > >PGO/LTCG on code that's not in the profile. > > Yeah, that would probably be an interesting experiment.
FWIW, IIRC my experiments last time we had this problem, LTCG alone accounts for less than a third of the performance boost we get from PGO on Talos. Mike _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
