[FFmpeg-devel] [PATCH] enable auto vectorization for gcc 7 and higher

[Martin Storsjö]

On Wed, 27 Jul 2022, Hendrik Leppkes wrote:

> On Wed, Jul 27, 2022 at 7:39 PM James Almer <jamrial at gmail.com> wrote:
>>
>> On 7/27/2022 2:34 PM, Swinney, Jonathan wrote:
>>> I recognize that this patch is going to be somewhat controversial. I'm submitting it mostly to see what the opinions are and evaluate options. I am working on improving performance for aarch64. On that architecture, there are fewer hand written assembly implementations of hot functions than there are for x86_64 and allowing gcc to auto-vectorize yields noticeable improvements.
>>>
>>> Gcc vectorization has improved recently and it hasn't been evaluated on the mailing list for a few years. This is the latest discussion I found in my searches: http://ffmpeg.org/pipermail/ffmpeg-devel/2016-May/193977.html
>>
>> Every time this was done, it was inevitably reverted after complains and
>> crash reports started piling up because gcc can't really handle all the
>> inline code our codebase has, among other things.
>>
>
> No need to wait for issues, I just tested, and the same issues still
> persist that have existed for years with GCC now. They don't seem to
> care to make it compatible with inline asm, which might be fair
> enough, but it means it just can't work here.
>
> In file included from libavcodec/cabac_functions.h:49,
>                 from libavcodec/h264_cabac.c:36:
> libavcodec/h264_cabac.c: In function 'ff_h264_decode_mb_cabac':
> libavcodec/x86/cabac.h:199:5: error: 'asm' operand has impossible constraints

This particular bit of inline assembly has historically been very 
problematic in many configurations (although primarily on i386 I think) - 
see e.g. 8990c5869e27fcd43b53045f87ba251f42e7d293. Would something like 
that be enough for that build configuration to succeed, or are there many 
other cases that break?

// Martin