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Re: Altivec in baseline for ppc64?

Le mardi 13 juillet 2021 à 21:04 +0200, John Paul Adrian Glaubitz a
écrit :
> On 7/13/21 1:55 PM, Sébastien Villemot 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?
> The ppc64 originally used the ppc64 baseline including AltiVec e.g PowerPC970,
> (64-Bit PowerMac). However, the previous port maintainer decided he wanted to
> support embedded systems such as the PowerPC E5500 which does not support
> AltiVec.

Thanks for this clarification. I have updated the architectures wiki
page accordingly.

> > (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).
> Please go ahead and enabled AltiVec as I don't think it makes much sense to use BLAS
> on machines without any SIMD support. If any user complains about compatibility issues,
> please feel free to bring up the issue here again.

I think I disagree with this idea. OpenBLAS can be pulled in by chains
of dependencies, even for users who do not even know what BLAS is.
Violating the baseline can lead to hard-to-understand crashes.
Since I think that reliability is more important than performance, I 
prefer to strictly respect the baseline in the binary package.

However note that locally recompiling OpenBLAS is a supported and
documented procedure, for those who want to take full advantage of
their hardware.

Regarding the kernel that is currently built in the official binary, I
could do with some help to determine which one is the best. You can see
the list of kernels at this address:
Each KERNEL.* file lists a bunch of source files, many of which are
assembly files.
Currently, I use POWER4 for ppc64 and PPCG4 for powerpc, but I’m unsure
that those are the right choice. I want a kernel that respects the
baseline, but still taking advantage of all that is in the baseline.

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

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