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

[Andreas Cadhalpun]

On 24.10.2016 16:14, Ronald S. Bultje wrote:
> On Mon, Oct 24, 2016 at 8:47 AM, wm4 <nfxjfg at googlemail.com> wrote:
>> On Mon, 24 Oct 2016 07:54:47 -0400
>> "Ronald S. Bultje" <rsbultje at gmail.com> wrote:
>>> On Mon, Oct 24, 2016 at 3:36 AM, wm4 <nfxjfg at googlemail.com> wrote:
>>>> I was under the impression that it is UB to have the FPU in MMX state
>>>> at any time while in C, not just while e.g. calling the stdlib. Maybe I
>>>> got that wrong (how would MMX intrinsics even work?) - can anyone shed
>>>> light on the exact requirements? (Possibly again, sorry.)
>>>
>>> I'm under the impression that it's part of the calling convention. That
>> is,
>>> any code anywhere (including mmx intrinsics, indeed) can - when called -
>>> expect the state to be cleared by the caller, just like you'd expect
>>> eax/edx to be caller-save (whereas esi/edi are callee-save).
>>>
>>> However, if you never call external code (including intrinsics), you can
>>> ignore this, just as you can ignore / create your own calling
>>> convention (remember fastcall etc.?). However, when calling any external
>>> code, this could (in theory) crash; it's just that right now it only
>>> crashes with musl when calling malloc/free. So basically, ffmpeg has its
>>> own calling convention, and manually calling emms_c() fixes "ffmpeg"
>>> calling convention to be compatible with "standard" calling
>> convention...?
>>
>> It can't really be a calling convention unless the compiler is aware of
>> it?

It is defined as part of the System V Application Binary Interface [1]:
"The CPU shall be in x87 mode upon entry to a function. Therefore, every
function that uses the MMX registers is required to issue an emms or femms
instruction after using MMX registers, before returning or calling another
function."

>> We're doing things behind the compiler's back here. The safest
>> assumption would be that leaving the FPU state invalid while in C is
>> not allowed, period.

That's what the standard says.

>> The next safest assumption is that it's fine as long as we explicitly
>> don't use floating point or call external functions that aren't
>> MMX-aware. This would mean calling any libc functions or user callbacks
>> (including indirectly through e.g. av_log) is not fine.

This is probably OK in practice and likely has a significant performance
benefit.

Best regards,
Andreas


1: https://software.intel.com/sites/default/files/article/402129/mpx-linux64-abi.pdf