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

[Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>]

> On May 21, 2025, at 14:17, Jiawei <jiawei@iscas.ac.cn> 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.

This isn’t the whole story.

The flag was added by 973859f in 2009.
Then it was reverted by cb8646af in 2016.
Shortly after that, the revert was reverted again by fd6dbc5 in 2016.

> 
> 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.

Those flags can only be enabled in tightly controlled environments (e.g., built and run on the same
machine), while FFmpeg has hand written assembly, runtime cpu probe and dynamic binding/dispatch.

Those auto-vectorization and ARCH flags can be enabled manually, but be careful.

> 
> [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
> -- 
> 2.43.0
> 
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