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

[Jiawei]

在 2025/5/29 16:37, Zhao Zhili 写道:
>
>> 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.
Sorry, I don't know how this happened.
>
> 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.

Do you think change it into 'gcc: Allow `-fno-tree-vectorize` when gcc 
version greater than 13.' will be better?


BR,

Jiawei