[FFmpeg-devel] [PATCH 09/13] avcodec/svq1dec: clear MMX state after MB decode loop

Ronald S. Bultje rsbultje at gmail.com
Mon Oct 24 23:31:46 EEST 2016


On Mon, Oct 24, 2016 at 4:26 PM, Henrik Gramner <henrik at gramner.com> wrote:

> On Mon, Oct 24, 2016 at 9:59 PM, Ronald S. Bultje <rsbultje at gmail.com>
> wrote:
> > Good idea to reference Hendrik Gramner here, who keeps insisting we get
> rid
> > of all MMX code in ffmpeg (at least as an option) for future Intel CPUs
> in
> > which MMX will be deprecated.
> Replacing MMX with SSE2 is indeed the most "proper" fix in my opinion,
> but it's a fair amount of work and not done in an evening.
> The fact that a lot of assembly lacks unit tests is certainly not
> helping in that regard.
> Some MMX instructions are slower than the equivalent SSE2 code on
> Skylake. Intel hasn't officially commented on (as far as I know at
> least) if we should expect this trend to continue, but they certainly
> seem to treat MMX as legacy.
> I doubt they would completely remove support for it though, backwards
> compatibility is a big selling-point for x86.

Well, it gives us another way of fixing this issue (on x86-64 only): have
sse2 implementations for all code that has a mmx (register) path right now.

Given the complexity of this issue, it's worth considering that fix along
with the other possibilities...


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