On Oct 2, 2012, at 12:48 AM, Ryosuke Niwa <rn...@webkit.org> wrote: > On Tue, Oct 2, 2012 at 12:17 AM, Mike Lawther <mikelawt...@google.com> wrote: > > On 2 October 2012 16:29, Maciej Stachowiak <m...@apple.com> wrote: > > Note, despite the stackoverflow thread cited, I would be highly surprised if > static branch predicition had no effect ever, even on modern architectures. > While recent intel CPUs have very good dynamic branch prediction, the basic > block reordering should still have a significant impact in some cases due to > cache effects. > > Aside: interestingly, the Core2 family of CPUs doesn't do 'static branch > prediction' in the traditional sense. When they encounter a branch not > already in the BTB, a BTB index is assigned to it. The prediction now > proceeds as though it were dynamic - meaning that this first time, it > predicts based on the previous data in that index. That is, it's effectively > a 50% chance of taken/not taken the first time the branch is encountered > (source: http://www.agner.org/optimize/microarchitecture.pdf, Section 3.15). > > Strange. I still observed ~10% performance gain from adding LIKELY/UNLIKELY > on Westmere.
Two reasons: code placement (i.e. icache behavior) and register allocation. -F > > - Ryosuke > > _______________________________________________ > webkit-dev mailing list > webkit-dev@lists.webkit.org > http://lists.webkit.org/mailman/listinfo/webkit-dev
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