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

[Hendrik Leppkes]

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

GCC 11.3, configure --cpu=haswell, mingw32

So this is a NACK. It just flat out breaks builds.

- Hendrik