[FFmpeg-trac] #11116(avutil:new): av_frame_get_buffer automatic alignment is incorrect for AVX512 CPUs

[FFmpeg]

#11116: av_frame_get_buffer automatic alignment is incorrect for AVX512 CPUs
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             Reporter:  Aras         |                     Type:  defect
  Pranckevicius                      |
               Status:  new          |                 Priority:  normal
            Component:  avutil       |                  Version:  git-
                                     |  master
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Summary of the bug:
 av_frame_get_buffer with "automatic" (align=0) alignment ends up picking
 32 alignment, even if on an AVX512 CPU it should have picked 64.

 Actual get_video_buffer function implementation, in the original commit
 that added automatic alignment (2017 Feb)
 https://git.ffmpeg.org/gitweb/ffmpeg.git/commitdiff/4de220d2e3751c4?hp=f44ec22e095
 used av_cpu_max_align. However, a later merge of that commit (2017 Sep)
 https://git.ffmpeg.org/gitweb/ffmpeg.git/commitdiff/7aa6b8a68fce93d84?hp=24ee1b8c6343f
 replaced usage of av_cpu_max_align with hardcoded value of 32, with a
 comment saying that it should really be av_cpu_max_align.

 The repro case is a bit involved, currently I don't have a clean one, only
 an indirect one through investigating a Blender bug report
 (https://projects.blender.org/blender/blender/issues/125446). Where it
 ends up being a problem, is when a video frame is decoded through ffmpeg,
 and then YUV -> RGBA conversion is done through libswscale. On an AVX512
 CPU, where video width is not multiple of 16 pixels (i.e. RGBA image
 stride is not multiple of 64 bytes), the swscale conversion ends up
 emitting "wrapping artifacts" on the left side of the image.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11116>
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