2016-09-06 3:56 GMT+02:00 Ilia Mirkin <imir...@alum.mit.edu>: > On Mon, Sep 5, 2016 at 9:54 PM, Michel Dänzer <mic...@daenzer.net> wrote: >> On 06/09/16 07:46 AM, Marek Olšák wrote: >>> From: Marek Olšák <marek.ol...@amd.com> >> >> Did you measure any significant performance boost with this change? >> Otherwise, using (un)likely can be bad because it can defeat the CPU's >> branch prediction, which tends to be pretty good these days. > > Is there a way to affect the branch predictor on x86 with instruction > encodings? I didn't think so. I was under the impression that all > likely/unlikely did was to affect placement of the code, i.e. where > the "if" code was placed.
If I may add to the discussion: there was a way to add branch prediction hints to the instruction encoding (using x86 prefixes that were to be ignored according to ISA), which was used by NetBurst architecture (Pentium 4). It is no longer recognized by any modern architecture and AFAIK compilers will not generate code that uses them. The compiler should be able to do two things using the (un)likely hints (there might be more tricks I am not aware of): 1. Make the likely branch not jump. When the CPU executes a jump without any branch prediction data cached for it, it assumes that it doesn't jump. 2. Move the unlikely parts of code outside of a function or to the end of a function. That increases instruction cache and fetch usage for likely code. In the end, it would be best to measure the performance of the (un)likely hints. Regards, Gustaw Smolarczyk > > -ilia > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
