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

[Hendrik Leppkes]

On Mon, Oct 24, 2016 at 10:31 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> Hi,
>
> 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.
>

I don't think the argument for pre-sse2 CPUs is that strong on 32-bit
systems, either.
I wouldn't necessarily limit this to x86-64 at that, considering its
probably another half year at least, probably longer, until such a
conversion could be completed.

- Hendrik