[FFmpeg-devel] [PATCH] Optimisations for av_log2 and integer clip functions

[Jason Garrett-Glaser]

> +#define av_log2 av_log2
> +static inline av_const int av_log2(unsigned int v)
> +{
> + ? ?return v? 31 - __builtin_clz(v) : 0;
> +}
> +
> +#ifndef av_log2_16bit
> +#define av_log2_16bit av_log2
> +#endif

Won't ^31 be faster?  "31 - X" requires an extra mov on x86.

Also, __builtin_clz/ctz maps to bsr/bsf, which are extraordinarily
slow on Athlons.

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