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. - Ryosuke
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