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

Jason Garrett-Glaser darkshikari
Wed Jan 13 23:55:26 CET 2010

On Wed, Jan 13, 2010 at 5:49 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, Jan 13, 2010 at 10:10:30PM +0000, M?ns Rullg?rd wrote:
>> Jason Garrett-Glaser <darkshikari at gmail.com> writes:
> [...]
>> > Also, __builtin_clz/ctz maps to bsr/bsf, which are extraordinarily
>> > slow on Athlons.
>> Fabulous. ?So what shall we do? ?List the CPUs with good clz support
>> in configure like we do with cmov et al?
> yes, if someone wants to use it

Is it better to default to BSR/BSF and be slower on some CPUs, or is
it better to default to table lookup, and be slower on a _different_
set of CPUs (when people aren't compiling specifically for their own

Dark Shikari

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