[FFmpeg-cvslog] r23921 - trunk/libavcodec/vp56.h

[Reimar Döffinger]

On Thu, Jul 01, 2010 at 01:18:47AM +0200, darkshikari wrote:
> Author: darkshikari
> Date: Thu Jul  1 01:18:47 2010
> New Revision: 23921
> 
> Log:
> CMOV-ify vp56 arithcoder
> This incantation causes gcc 4.3 to generate cmov on x86, a vastly better option
> than a completely unpredictable branch.
> Hopefully this carries over to newer versions and other CPUs with conditionals.
> ~5 cycles saved per call on a Core i7.

Nice, I actually tried to make a branchless variant using a mask etc, unfortunately
it needs so many additional instructions that it ended up being the same speed.

> +    c->high = bit ? c->high - low : low;
> +    c->code_word = bit ? c->code_word - low_shift : c->code_word;

Does the compiler "get" that
bit == !((c->high - low) >> 31)
not sure that actually is any faster to calculate, but what the
compiler did looked rather bad to me...