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

[Kacper Michajlow <kasper93@gmail.com>]

On Fri, 11 Jul 2025 at 23:41, Michael Niedermayer
<michael@niedermayer.cc> wrote:
>
> Hi Martin
>
> On Thu, Jul 10, 2025 at 02:35:10PM +0300, Martin Storsjö wrote:
> > 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.
>
> I think for architectures where there is an active maintainer,
> that maintainer should decide.
>
> For architectures where there is noone looking after them, its
> safer to leave it disabled. Otherwsie users could be having
> complex to debug issues to deal with and noone in ffmpeg able to
> replicate

I agree. In a perfect world we would enable it everywhere, but if
there is a worry that GCC is not a mature enough compiler to enable
it, it's better to gradually roll it out on an opt-in basis. We can
start with major architectures and for other architectures allow
maintainers to decide.

Alternatively enable it everywhere for start and selectively disable
it, once issues are identified. This may cause breakage, but is easy
to workaround (by adding a compiler flag), and will likely identify
issues with GCC faster.

Bottom line is that adding `-fno-tree-vectorize` in current year is
quite a heavy hammer to avoid possible issues and especially on major
architectures like x86 it should be safe to enable vectorization. It
also currently gives clang and advantage over gcc if we compare pure c
code in some cases.

- Kacper
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