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

[Michael Niedermayer <michael@niedermayer.cc>]

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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
[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When the tyrant has disposed of foreign enemies by conquest or treaty, and
there is nothing more to fear from them, then he is always stirring up
some war or other, in order that the people may require a leader. -- Plato

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