Re: __builtin_expect in clang?
Re: __builtin_expect in clang?
- Subject: Re: __builtin_expect in clang?
- From: Wade Tregaskis <email@hidden>
- Date: Fri, 11 Nov 2011 20:01:15 -0800
I'm pretty sure i386 doesn't have branch prediction.
i386 processors most certainly do. Intel's became exceptionally good in the Netburst era, as a function of necessity thanks to their fantastically long pipelines. As far as I know that's mostly been retained, though I may recall that they scaled it back somewhat in later architectures.
Last I looked at it, a few years ago, they were able to truthfully claim successful branch prediction rates of ~98% on "typical" code - well, SPEC and similar, anyway. Shark had a profiling mode, for a while, for measuring the branch prediction accuracy, but I'm not sure it even shipped publicly since it was such a niche. IIRC it wasn't too hard to get a 50% success rate - i.e. statistically random, considered pathological - but it was really hard to get much below that. At least, I found it was; I'm sure it can be done, given sufficient time and investment. :) |
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