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

[James Almer]

On 7/27/2022 10:02 PM, Soft Works wrote:
> 
>> -----Original Message-----
>> From: ffmpeg-devel<ffmpeg-devel-bounces at ffmpeg.org>  On Behalf Of
>> Hendrik Leppkes
>> Sent: Wednesday, July 27, 2022 10:42 PM
>> To: FFmpeg development discussions and patches <ffmpeg-
>> devel at ffmpeg.org>
>> Subject: Re: [FFmpeg-devel] [PATCH] enable auto vectorization for gcc
>> 7 and higher
>>
>> 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
> I wonder why it doesn't fail when I try the same on MINGW32:
> 
> gcc -I. -Isrc/ -D_FORTIFY_SOURCE=0 -D__USE_MINGW_ANSI_STDIO=1 -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -U__STRICT_ANSI__ -D__USE_MINGW_ANSI_STDIO=1 -D__printf__=__gnu_printf__ -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -DOPJ_STATIC -DZLIB_CONST -DHAVE_AV_CONFIG_H -DBUILDING_avcodec -mthreads -DLIBTWOLAME_STATIC -std=c11 -IV:/ffbuild/mas/local32/include -IV:/ffbuild/mas/msys64/mingw32/include -I/mingw32/include -IF:/ffbuild/mas/local32/include -DLIBARCHIVE_STATIC -Wdeclaration-after-statement -Wall -Wdisabled-optimization -Wpointer-arith -Wredundant-decls -Wwrite-strings -Wtype-limits -Wundef -Wmissing-prototypes -Wstrict-prototypes -Wempty-body -Wno-parentheses -Wno-switch -Wno-format-zero-length -Wno-pointer-sign -Wno-unused-const-variable -Wno-bool-operation -Wno-char-subscripts -O3 -Werror=format-security -Werror=implicit-function-declaration -Werror=missing-prototypes -Werror=return-type -Werror=vla -Wformat -fdiagnostics-color=auto -Wno-maybe-uninitialized -
>   ftree-vectorize -MMD -MF libavcodec/h264_cabac.d -MT libavcodec/h264_cabac.o -c -o libavcodec/h264_cabac.o src/libavcodec/h264_cabac.c

You didn't set CPU to haswell (Which will add -march=haswell to the 
command line).