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

[Jiawei <jiawei@iscas.ac.cn>]

在 2025/5/21 15:46, Michael Niedermayer 写道:
> On Wed, May 21, 2025 at 02:17:50PM +0800, Jiawei wrote:
>> 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
> Your text speaks about this change being ok in a gcc version dependant
> way
>
> Your patch has no gcc version dependancy
>
> If you claim that all issues where solved, please show the issues happening
> in version v and no longer happening in w>v . Then it make sense to
> change the flags for version w
>
> Thx
> [...]


Sorry I forgot about that, thanks for reminding me. Here still exist 
many old version gcc user,

And I am not sure how will this impact them.

Maybe a later version gcc checking is good, like gcc 13-15, what you 
think about it?

>   
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