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

[Andreas Rheinhardt]

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.

- Andreas