[FFmpeg-devel] [PATCH] libavutil: add x86 optimized av_popcount

[Ronald S. Bultje]

Hi,

On Tue, Feb 24, 2015 at 8:05 PM, James Almer <jamrial at gmail.com> wrote:
>
> +#if HAVE_FAST_POPCNT
> +#if AV_GCC_VERSION_AT_LEAST(4,5)
> +#ifndef av_popcount
> +    #define av_popcount   __builtin_popcount
> +#endif /* av_popcount */
> +#if HAVE_FAST_64BIT
> +#ifndef av_popcount64
> +    #define av_popcount64 __builtin_popcountll
> +#endif /* av_popcount64 */
> +#endif /* HAVE_FAST_64BIT */
> +#endif /* AV_GCC_VERSION_AT_LEAST(4,5) */
> +#endif /* HAVE_FAST_POPCNT */
>

Is this just to get the sse4 popcnt instruction if we compile with
-mcpu=sse4? The slightly odd thing is that we're using a built-in, yet
configure still does an arch/cpu check. I'd expect the built-in/compiler to
do that for us based on -mcpu, and we could always unconditionally use this
(as long as gcc >= 4.5); alternatively, you could use inline asm and then
have the configure check (HAVE_FAST_POPCNT). But doing both seems a little
odd. I have no objection to it, patch is still fine, just odd.

Ronald