On Wed, Jul 27, 2022 at 11:02 PM Martin Storsjö <mar...@martin.st> wrote: > > On Wed, 27 Jul 2022, Hendrik Leppkes wrote: > > > On Wed, Jul 27, 2022 at 7:39 PM James Almer <jamr...@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? >
I can test tomorrow, but if we start influencing optimizer decisions just to run another optimizer flag, such a change would need to be backed with (positive!) performance numbers, and _very_ thorough testing (as we all know, trying to prove that something is not an issue is practically impossible, as the combinations are infinite) - Hendrik _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
