Re: [FFmpeg-devel] gcc: Remove auto-vectorization limitation.

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Martin Storsjö:
> On Wed, 21 May 2025, Andreas Rheinhardt wrote:
> 
>> Jiawei:
>>> This patch modifies the FFmpeg build system to remove the explicit
>>> disabling
>>> of GCC's auto-vectorization feature.
>>>
>>> Modern GCC versions (>= 10.0) have demonstrated stable auto-
>>> vectorization
>>> capabilities through extensive optimizations in loop analysis and SIMD
>>> code generation. The explicit -fno-tree-vectorize flag originally added
>>> in commit 973859f (2009) to workaround early GCC vectorization
>>> instability
>>> is no longer necessary.
>>>
>>> Key improvements justifying this change:
>>> 1. Enhanced heuristics for loop vectorization cost models
>>> 2. Mature handling of alignment and memory access patterns
>>> 3. Robust fallback mechanisms for unsupported architectures
>>>
>>> This change allows FFmpeg to benefit from automated SIMD optimizations
>>> when built with -O3 optimization level, particularly improving
>>> performance on x86_64 (AVX), ARM64 (SVE) and RISC-V(RVV) architectures.
>>>
>>> [1] https://git.ffmpeg.org/gitweb/ffmpeg.git/
>>> commit/973859f5230e77beea7bb59dc081870689d6d191
>>>
>>> ---
>>>  configure | 1 -
>>>  1 file changed, 1 deletion(-)
>>>
>>> diff --git a/configure b/configure
>>> index 3730b0524c..b9e95ce4ec 100755
>>> --- a/configure
>>> +++ b/configure
>>> @@ -7656,7 +7656,6 @@ if enabled icc; then
>>>              disable aligned_stack
>>>      fi
>>>  elif enabled gcc; then
>>> -    check_optflags -fno-tree-vectorize
>>>      check_cflags -Werror=format-security
>>>      check_cflags -Werror=implicit-function-declaration
>>>      check_cflags -Werror=missing-prototypes
>>
>> FYI: The last discussion about auto-vectorization is here:
>> https://ffmpeg.org/pipermail/ffmpeg-devel/2022-July/299405.html
>> It contains a report about a failing build with vectorization enabled:
>> https://ffmpeg.org/pipermail/ffmpeg-devel/2022-July/299421.html
>> I don't know whether this is still reproducible with the latest GCC.
> 
> The issue which was reported last time, when compiling for i686 mingw32
> with --cpu=haswell, seems to have gone away in
> 182663a58a7a099e02e76da3b0f96d63e5c26a6d, where we made the whole
> problematic x86 inline cabac assembly noinline on i386. (That whole
> inline assembly block has been problematic in a large number of cases
> anyway.)
> 

So there are currently no known miscompilations due to vectorization
with GCC?

- Andreas

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