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

[Martin Storsjö]

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 the asm ones. I guess 
it shouldn't be enabled by default at least.

// Martin