Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

[Martin Storsjö]

On Fri, 13 Jun 2025, Jiawei wrote:

> This patch modifies the FFmpeg build system to allow GCC to use the
> `-ftree-vectorize` flag when the compiler version is 13 or newer.
> Enabling this flag can improve performance through better loop analysis
> and auto-vectorization (SIMD) opportunities in modern GCC versions.
>
> The explicit `-fno-tree-vectorize` flag was originally added in commit
> 973859f5230e (2009). A previous attempt to enable `-ftree-vectorize`
> was made in commit cb8646af24bd (2016), but it was reverted in
> fd6dbc53855f. The reason for the revert was that the inline x86 CABAC
> assembly code caused compiler errors ??? the compiler would run out of
> available registers during vectorization, making it unable to compile
> some functions.
>
> In commit 182663a58a7a (2023), the problematic CABAC function was made
> non-inline. This significantly reduces the risk of register exhaustion
> caused by inlining large assembly blocks, making the vectorizer safer
> to enable for other functions.
>
> Given improvements in GCC's vectorizer and the mitigation of the
> original issue, we now re-enable `-ftree-vectorize` for GCC version 13
> and above.
>
> Version log:
>  Only allow GCC versions >= 13 to use -ftree-vectorize.
>  Modify the commit description.
>  Add historical context and rationale for the change.
>
> Disscussion see:
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250521061750.54882-1-jiawei@iscas.ac.cn/
>
>
> Signed-off-by: Jiawei <jiawei@iscas.ac.cn>
> ---
> configure | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)

Thanks; I'm ok with this version. Whoever applies it can touch up the 
commit message to remove the ???? word (maybe just replace it with an 
"as"), it sounds reasonably right like that.

// Martin

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Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

[Michael Niedermayer]

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Hi Martin

On Fri, Jun 13, 2025 at 11:42:11AM +0300, Martin Storsjö wrote:
> On Fri, 13 Jun 2025, Jiawei wrote:
> 
> > This patch modifies the FFmpeg build system to allow GCC to use the
> > `-ftree-vectorize` flag when the compiler version is 13 or newer.
> > Enabling this flag can improve performance through better loop analysis
> > and auto-vectorization (SIMD) opportunities in modern GCC versions.
> > 
> > The explicit `-fno-tree-vectorize` flag was originally added in commit
> > 973859f5230e (2009). A previous attempt to enable `-ftree-vectorize`
> > was made in commit cb8646af24bd (2016), but it was reverted in
> > fd6dbc53855f. The reason for the revert was that the inline x86 CABAC
> > assembly code caused compiler errors ??? the compiler would run out of
> > available registers during vectorization, making it unable to compile
> > some functions.
> > 
> > In commit 182663a58a7a (2023), the problematic CABAC function was made
> > non-inline. This significantly reduces the risk of register exhaustion
> > caused by inlining large assembly blocks, making the vectorizer safer
> > to enable for other functions.
> > 
> > Given improvements in GCC's vectorizer and the mitigation of the
> > original issue, we now re-enable `-ftree-vectorize` for GCC version 13
> > and above.
> > 
> > Version log:
> >  Only allow GCC versions >= 13 to use -ftree-vectorize.
> >  Modify the commit description.
> >  Add historical context and rationale for the change.
> > 
> > Disscussion see:
> > https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250521061750.54882-1-jiawei@iscas.ac.cn/
> > 
> > 
> > Signed-off-by: Jiawei <jiawei@iscas.ac.cn>
> > ---
> > configure | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> 
> Thanks; I'm ok with this version. Whoever applies it can touch up the commit
> message to remove the ???? word (maybe just replace it with an "as"), it
> sounds reasonably right like that.

the patch seems not to be on the mailing list
I have v4 in my inbox but not v5, so if the patch is ok, maybe you are
the only one who can apply it

Maybe Jiawei is not subscribed to the ML so his mails would go to the
moderation que (i didnt check)

thx

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

Dictatorship naturally arises out of democracy, and the most aggravated
form of tyranny and slavery out of the most extreme liberty. -- Plato

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Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

[Martin Storsjö]

On Sun, 15 Jun 2025, Michael Niedermayer wrote:

> Hi Martin
>
> On Fri, Jun 13, 2025 at 11:42:11AM +0300, Martin Storsjö wrote:
>> On Fri, 13 Jun 2025, Jiawei wrote:
>>
>>> This patch modifies the FFmpeg build system to allow GCC to use the
>>> `-ftree-vectorize` flag when the compiler version is 13 or newer.
>>> Enabling this flag can improve performance through better loop analysis
>>> and auto-vectorization (SIMD) opportunities in modern GCC versions.
>>>
>>> The explicit `-fno-tree-vectorize` flag was originally added in commit
>>> 973859f5230e (2009). A previous attempt to enable `-ftree-vectorize`
>>> was made in commit cb8646af24bd (2016), but it was reverted in
>>> fd6dbc53855f. The reason for the revert was that the inline x86 CABAC
>>> assembly code caused compiler errors ??? the compiler would run out of
>>> available registers during vectorization, making it unable to compile
>>> some functions.
>>>
>>> In commit 182663a58a7a (2023), the problematic CABAC function was made
>>> non-inline. This significantly reduces the risk of register exhaustion
>>> caused by inlining large assembly blocks, making the vectorizer safer
>>> to enable for other functions.
>>>
>>> Given improvements in GCC's vectorizer and the mitigation of the
>>> original issue, we now re-enable `-ftree-vectorize` for GCC version 13
>>> and above.
>>>
>>> Version log:
>>>  Only allow GCC versions >= 13 to use -ftree-vectorize.
>>>  Modify the commit description.
>>>  Add historical context and rationale for the change.
>>>
>>> Disscussion see:
>>> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250521061750.54882-1-jiawei@iscas.ac.cn/
>>>
>>>
>>> Signed-off-by: Jiawei <jiawei@iscas.ac.cn>
>>> ---
>>> configure | 6 +++++-
>>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> Thanks; I'm ok with this version. Whoever applies it can touch up the commit
>> message to remove the ???? word (maybe just replace it with an "as"), it
>> sounds reasonably right like that.
>
> the patch seems not to be on the mailing list
> I have v4 in my inbox but not v5, so if the patch is ok, maybe you are
> the only one who can apply it
>
> Maybe Jiawei is not subscribed to the ML so his mails would go to the
> moderation que (i didnt check)

That's possible; I don't see v3 or any of the newer versions on the 
mailing list archive.

Content-wise, the patch is the same since v2 I think, it's just been work 
on the commit message.

But I guess I can repost a version that I would be ok with getting 
applied.

// Martin
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Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

[Martin Storsjö]

On Thu, 10 Jul 2025, Jiawei wrote:

> Hi Martin,
>
> Is there any progress for this patch, should I resend it to the mailing list 
> again?

When I posted the updated version of the patch, the patchwork test runners 
noticed that this patch causes test failures on Loongarch. So modern 
versions of GCC still do have vectorization bugs, at least on some 
architectures.

I'm unsure if we should allow it everywhere, and just single out loongarch 
as a case where it is known broken, or if we should only let GCC enable it 
on the major architectures that we've tested.

In either case, we should isolate the compiler bugs on loongarch and 
report them, as well, so that they have a chance of getting the 
vectorization to work properly.

// Martin

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Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

[Martin Storsjö]

On Thu, 10 Jul 2025, Jiawei wrote:

> In addition, can you provide a bug record or a reproduction method? I think 
> it should also be fixed in the GCC community, and I will report the problems 
> you encounter in test to GCC Bugzilla.

https://patchwork.ffmpeg.org/check/125495/

If you build ffmpeg with this patch, "make fate" fails on the monkeysaudio 
tests, on loongarch. That's about as much as I have so far.

It should indeed be fixed in GCC. You can of course just report this 
above, but with that only that info, it is somewhat unlikely that anybody 
will tend to the bug and fix it. To make it properly actionable, you need 
to isolate the code which gets miscompiled, so GCC developers can compile 
and observe that the resulting code is wrong (which may require some 
loongarch knowledge to see what is wrong) without needing to build and run 
the full ffmpeg fate.

// Martin

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Re: [FFmpeg-devel] [PATCH v5] gcc: Don't disable '-ftree-vectorize' if gcc version higher than 13.

[compn]

On Sun, 15 Jun 2025 00:49:42 +0200, Michael Niedermayer wrote:

> the patch seems not to be on the mailing list
> I have v4 in my inbox but not v5, so if the patch is ok, maybe you are
> the only one who can apply it
> 
> Maybe Jiawei is not subscribed to the ML so his mails would go to the
> moderation que (i didnt check)

no mails in mod queue last few days, including checked right now.

-compn
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