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

[Frank Plowman <post@frankplowman.com>]

On 29/05/2025 11:20, Jiawei wrote:
> 
> 在 2025/5/29 16:37, Zhao Zhili 写道:
>>
>>> On May 29, 2025, at 15:03, 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 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@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@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@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?
> 

I don't think it's the header line (gcc: Relaxing auto-vectorization
limitation.) that's being called problematic, though maybe the full stop
could be removed.  Moreso it's the body text beneath it: "particularly
improving performance on x86_64 (AVX)..."  Note that when using git
send-email, lines below the subject line will also become part of the
commit message.  Specifically, all text before the first line that
begins --- will form the commit message, so if you wish to add extra
text to your email but not to your commit message, you can insert it
after the first ---.

-- 
Frank
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