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Re: Altivec in baseline for ppc64?

Hi Mathieu,

Le mardi 13 juillet 2021 à 18:56 +0200, Mathieu Malaterre a écrit :
> On Tue, Jul 13, 2021 at 2:04 PM Sébastien Villemot <sebastien@debian.org> wrote:
> > 
> > The wiki page that synthesizes architecture specificities indicates
> > that Altivec is included in the baseline for the ppc64 port:
> > https://wiki.debian.org/ArchitectureSpecificsMemo#ppc64
> > 
> > However my understanding is that this port supports any powerpc64 CPU,
> > including some that don’t have Altivec (e.g. POWER4 or POWER5). This is
> > also what the main wiki page for PPC64 says:
> > https://wiki.debian.org/PPC64
> > 
> > Can someone please clarify the situation?
> > 
> > (I’m asking because I’m the maintainer of the openblas package, and
> > knowing whether Altivec is available or not, and more generally what is
> > in the baseline, is essential for proper packaging).
> I do not believe that you can do much as a packager. You cannot assume
> anything on the target arch. You need to do the same thing as ffmpeg
> is doing for avx2/sse4 on amd64, you need to do runtime detection. So
> unless upstream is doing something very clever you cannot compile blas
> using any of the fancy altivec instructions :(
> The man page for ld.so mentions something about optimized libraries
> (search for "/usr/lib/sse2/"), but this is currently not in use in
> Debian (AFAIK).

Actually OpenBLAS has its own runtime detection mechanism, which is
used to select the best linear algebra kernel for the current CPU
(those kernels are mainly written in assembly, and take advantage of
available ISA extensions). This mechanism is used on several archs,
including ppc64el (so at runtime, OpenBLAS chooses between a POWER8 and
a POWER9 kernel; there is even a POWER10 kernel already available).

However, I cannot enable this mechanism on ppc64 and powerpc, because
the runtime detection only works for POWER6 and above, and my
understanding is that for these two ports the baseline is lower. Hence
on these two archs, only one kernel is included in the package binaries
(currently POWER4 for ppc64 and PPCG4 for powerpc). For optimal
performance, users should recompile OpenBLAS locally (as indicated in
the package description and in README.Debian).

I am however not sure that my current choices for the ppc64 and powerpc
baselines are optimal, hence this thread.

⢀⣴⠾⠻⢶⣦⠀  Sébastien Villemot
⣾⠁⢠⠒⠀⣿⡁  Debian Developer
⢿⡄⠘⠷⠚⠋⠀  https://sebastien.villemot.name
⠈⠳⣄⠀⠀⠀⠀  https://www.debian.org

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