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Re: sse{2,3,4.2}, altivec, neon, ...

On Sat, Aug 05, 2017 at 07:53:02PM +0200, Adam Borowski wrote:
> Dropping baseline support is giving up, but let's at least surrender nicely.
> Thus, here's a proposed solution: in unstable, there's now a bunch of
> packages that do such checking in preinst, and thus refuse (overridably) to
> even install such software.
> Currently this includes:
> * sse2: i386 (inc. hurd-i386, kfreebsd-i386)
> * sse3: i386, amd64, x32 (〃, kfreebsd-amd64)
> * sse4.2: i386, amd64, x32 (〃)
> * neon: armhf
> * altivec: powerpc
> (amd64 always has sse2, arm64 neon, ppc64el altivec)
> Package names are $(ISA)-support, thus sse2-support if you need SSE2.
> To use: make your package:
> Depends: sse2-support [any-i386]
> Lemme know if anything is amiss; in particular, adding a new extension is a
> two-line change.

The problem is not the technical implementation, the problem is that
no longer considering these issues as bugs will quickly result in a 
fractured archive.

Packages that genuinely cannot work on the architecture baseline are
very rare, these are a tiny part of the packages that are not binary-any.

Quite common are possible performance gains by violating the baseline.
Do we want to continue to push upstreams to support runtime detection,
or do we accept that large parts of the archive might stop working on
the architecture baseline?

To give an example of the problem:

We likely have multimedia libraries that are dependencies of many 
packages, that do not have CPU autodetection, and where enabling 
unconditional usage of AVX on amd64/i386 would bring significant 
performance improvements.

Would a dependency on avx-support [any-i386 any-amd64]
be OK for such a library?

Would a dependency on avx-512-ifma-support [any-i386 any-amd64]
be OK for such a library?

> Meow!



       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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