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

[Michael Niedermayer]

Hi

On Thu, May 29, 2025 at 04:37:16PM +0800, Zhao Zhili wrote:
> 
> 
> > 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.

can gcc or clang not build code like our runtime cpudetect ?
i mean build functions for each major type and detect cpu once
and switch accordingly ?

I cannot be the first person thinking of that

thx

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

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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