[FFmpeg-devel] [FFmpeg-devel, v2] gcc: Relaxing auto-vectorization limitation.

[Zhao Zhili]

> On May 29, 2025, at 15:03, Jiawei <jiawei at 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 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 for recent gcc versions.
> 
> 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
> 
> Version log:
>  Only allow GCC versions >= 13 to use auto-vectorization.
> Disscussion see:
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250521061750.54882-1-jiawei@iscas.ac.cn/
> 
> ---
> configure | 1 -
> 1 file changed, 1 deletion(-)
> 
> Signed-off-by: Jiawei <jiawei at iscas.ac.cn>
> ---
> configure | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 3730b0524c..91e3e107c2 100755
> --- a/configure
> +++ b/configure
> @@ -7656,7 +7656,11 @@ if enabled icc; then
>             disable aligned_stack
>     fi
> elif enabled gcc; then
> -    check_optflags -fno-tree-vectorize
> +    gcc_version=$($cc -dumpversion)
> +    major_version=${gcc_version%%.*}
> +    if [ $major_version -lt 13 ]; then
> +        check_optflags -fno-tree-vectorize
> +    fi
>     check_cflags -Werror=format-security
>     check_cflags -Werror=implicit-function-declaration
>     check_cflags -Werror=missing-prototypes
> -- 
> 2.43.0
> 
> This patch modifies the FFmpeg build system to remove the explicit disabling
> of GCC's auto-vectorization feature.
> 
> Modern GCC versions have demonstrated stable auto-vectorization capabilities
> through extensive optimizations in loop analysis and SIMD code generation.
> The explicit -fno-tree-vectorize flag originally added in commit 973859f
> (2009) to workaround early GCC vectorization instability is no longer
> necessary for recent gcc versions.
> 
> Key improvements justifying this change:
> 1. Enhanced heuristics for loop vectorization cost models
> 2. Mature handling of alignment and memory access patterns
> 3. Robust fallback mechanisms for unsupported architectures
> 
> This change allows FFmpeg to benefit from automated SIMD optimizations
> when built with -O3 optimization level, particularly improving
> performance on x86_64 (AVX), ARM64 (SVE) and RISC-V(RVV) architectures.
> 
> [1] https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/973859f5230e77beea7bb59dc081870689d6d191
> 
> Version log:
>  Only allow GCC versions >= 13 to use auto-vectorization.
> Disscussion see:
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250521061750.54882-1-jiawei@iscas.ac.cn/
> 
> ---
> configure | 1 -
> 1 file changed, 1 deletion(-)
> 
> Signed-off-by: Jiawei <jiawei at iscas.ac.cn>
> ---
> configure | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 3730b0524c..91e3e107c2 100755
> --- a/configure
> +++ b/configure
> @@ -7656,7 +7656,11 @@ if enabled icc; then
>             disable aligned_stack
>     fi
> elif enabled gcc; then
> -    check_optflags -fno-tree-vectorize
> +    gcc_version=$($cc -dumpversion)
> +    major_version=${gcc_version%%.*}
> +    if [ $major_version -lt 13 ]; then
> +        check_optflags -fno-tree-vectorize
> +    fi
>     check_cflags -Werror=format-security
>     check_cflags -Werror=implicit-function-declaration
>     check_cflags -Werror=missing-prototypes
> -- 
> 2.43.0
> 
> This patch modifies the FFmpeg build system to remove the explicit disabling
> of GCC's auto-vectorization feature.
> 
> Modern GCC versions have demonstrated stable auto-vectorization capabilities
> through extensive optimizations in loop analysis and SIMD code generation.
> The explicit -fno-tree-vectorize flag originally added in commit 973859f
> (2009) to workaround early GCC vectorization instability is no longer
> necessary for recent gcc versions.
> 
> Key improvements justifying this change:
> 1. Enhanced heuristics for loop vectorization cost models
> 2. Mature handling of alignment and memory access patterns
> 3. Robust fallback mechanisms for unsupported architectures
> 
> This change allows FFmpeg to benefit from automated SIMD optimizations
> when built with -O3 optimization level, particularly improving
> performance on x86_64 (AVX), ARM64 (SVE) and RISC-V(RVV) architectures.
> 
> [1] https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/973859f5230e77beea7bb59dc081870689d6d191
> 
> Version log:
>  Only allow GCC versions >= 13 to use auto-vectorization.
> Disscussion see:
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250521061750.54882-1-jiawei@iscas.ac.cn/
> 
> ---
> configure | 1 -
> 1 file changed, 1 deletion(-)
> 
> Signed-off-by: Jiawei <jiawei at iscas.ac.cn>
> ---
> configure | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 3730b0524c..91e3e107c2 100755
> --- a/configure
> +++ b/configure
> @@ -7656,7 +7656,11 @@ if enabled icc; then
>             disable aligned_stack
>     fi
> elif enabled gcc; then
> -    check_optflags -fno-tree-vectorize
> +    gcc_version=$($cc -dumpversion)
> +    major_version=${gcc_version%%.*}
> +    if [ $major_version -lt 13 ]; then
> +        check_optflags -fno-tree-vectorize
> +    fi
>     check_cflags -Werror=format-security
>     check_cflags -Werror=implicit-function-declaration
>     check_cflags -Werror=missing-prototypes
> -- 
> 2.43.0
> 

It looks like the patch format is corrupted.

I’m OK with the code change. However, the commit message is misleading. As already pointed out
by multiple developers, this option doesn’t help with AVX, SVE and RVV because we can’t assume
they are available at runtime, unless build and run on a particular hardware.