[x264-devel] [PATCH 2/3] RFC: checkasm: Warn if a better SIMD function is slower than the simpler one

[Janne Grunau]

On 2015-08-14 00:26:55 +0300, Martin Storsjö wrote:
> On Thu, 13 Aug 2015, Henrik Gramner wrote:
> 
> >On Thu, Aug 13, 2015 at 11:00 PM, Martin Storsjö <martin at martin.st> wrote:
> >>---
> >>This naively assumes that a later tested SIMD function is supposed
> >>to be better than the earlier ones - this probably doesn't
> >>hold for all x86 SIMD flags.
> >
> >This would most likely result in a huge amount of false positives.
> >There are plenty of AVX functions for example that are neither slower
> >nor faster than non-AVX functions on many CPUs which would often
> >trigger the warning since the cycle counter can drift a bit from run
> >to run for multiple reasons.
> 
> Yeah, I guess so. With some amount of margin it might be more useful
> though (e.g. N * nop?). Even though it's prone to false positives,
> it can also be a useful hint to investigate things - at least for
> arm I found a few surprises where the C versions were faster than

Have you looked at the functions with slower asm than C? I guess 4 pixel 
wide predictions functions are a likely target. Even on ARM this is 
probably CPU dependent.

Janne