Re: [PATCH v3 1/6] util/bufferiszero: remove SSE4.1 variant

[Richard Henderson <richard.henderson@linaro.org>]

On 2/7/24 06:48, Alexander Monakov wrote:
> The SSE4.1 variant is virtually identical to the SSE2 variant, except
> for using 'PTEST+JNZ' in place of 'PCMPEQB+PMOVMSKB+CMP+JNE' for testing
> if an SSE register is all zeroes. The PTEST instruction decodes to two
> uops, so it can be handled only by the complex decoder, and since
> CMP+JNE are macro-fused, both sequences decode to three uops. The uops
> comprising the PTEST instruction dispatch to p0 and p5 on Intel CPUs, so
> PCMPEQB+PMOVMSKB is comparatively more flexible from dispatch
> standpoint.
> 
> Hence, the use of PTEST brings no benefit from throughput standpoint.
> Its latency is not important, since it feeds only a conditional jump,
> which terminates the dependency chain.
> 
> I never observed PTEST variants to be faster on real hardware.
> 
> Signed-off-by: Alexander Monakov<amonakov@ispras.ru>
> Signed-off-by: Mikhail Romanov<mmromanov@ispras.ru>
> ---
>   util/bufferiszero.c | 29 -----------------------------
>   1 file changed, 29 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~