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Re: About VMX/Altivec support in Debian ppc64 port

On Sat, Feb 16, 2013 at 09:31:59PM +0900, Hiroyuki Yamamoto wrote:
> Hi, All.
> I am a ppc64 porter, Hiroyuki Yamamoto.
> Because ppc64 port is NOT a official port yet,
> please DON'T report to official BTS the bugs which have NOT a relation to the packages AT ALL,
> and which have a relation ONLY to porting to ppc64 port,
> Instead, please report to this debian-powerpc ML, and for making sure,
> please add ppc64@debian-ports.org (ppc64 buildd maintainers of Debian Ports) to CC.
> On this time, unless the package maintainer is fixing build option specially, the packages of ppc64 port are built
> with build options "-pipe -maltivec -mabi=altivec -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt".
> This has an experimental intention for evaluating merits and demerits of building with these build options,
> and being able to run various powerpc64 machines.
> Because official powerpc port also support powerpc64 machines,
> I think that machines supported by ppc64 port may be able to restricted to some extent.
> Further build options which should be adopted may be adopted,
> but I also think that it is not good to carry out FATAL restriction to ppc64.
> So, I run ppc64 porting and offer the ppc64 packages, and I wait reports of bugs, benchmarks, opinions, and so on.
> This report is the first one, so I welcome this report very much.
> Hmmm,
> I didn't consider for qemu.
> It may be inconvenience that it cannot use as an image of qemu.
> About not supporting for POWER4/5 machines, it is intentional on this time powerpc port already existing.
> >>According Debian ppc64 wiki page[2], xenon, the cpu used by
> >>xbox360, supports altivec, but altivec don't work in my box,
> >>I think it is another bug or misconfigure.
> >>
> >>I'm not a subscriber of this list, please CC me.
> >>
> >>[1] http://buildd.debian-ports.org/status/fetch.php?pkg=util-linux&arch=ppc64&ver=2.20.1-5.3&stamp=1355261928
> >>[2] http://wiki.debian.org/PPC64
> Is it wrong although I think that Xenon CPUs support the VMX instructions?

Yes it is wrong.  The Xenon supports VMX128 which is an enhanced version
of VMX, which unfortunately also leaves out a few integer operations
that they didn't think they needed to save die space.

So the Xenon does in fact not support normal altivec.

> However, I think that the phenomenon which ppc64 port doesn't work on Xbox360 with really Xenon CPU may happen
> because of memory shortage or other reasons.

It is certainly way lower on ram than any other ppc64 machine is likely
to be.

> It is good or not to adopt these build options, I think that it depends on evaluation of merits and demerits.
> It is glad that this subject comes up at the now in front of the release of wheezy,
> because the great part of packages of sid will interchange immediately after wheezy release.

Well the ports page for ppc64 claims to have Xenon as a goal currently,
which certainly means altivec is NOT an option for ppc64 by default.
It is I believe the only CPU listed as a target goal that does not have
standard Altivec support.

Now some code might work with altivec enabled as long as it doesn't use
any of the missing instructions.  That sounds risky unless there is a gcc
option to use only the subset of altivec that is common to VMX and VMX128.

Len Sorensen

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