Re: [PATCH] Mention Intel Atom in Kconfig.cpu

From: Phil Endecott
Date: Mon Oct 13 2008 - 15:22:34 EST


Arjan van de Ven wrotes:
On Mon, 13 Oct 2008 17:02:30 +0300
Adrian Bunk <bunk <at> kernel.org> wrote:

> On Mon, Oct 13, 2008 at 09:30:14AM -0400, Arjan van de Ven wrote:
> > On Mon, 13 Oct 2008 15:30:51 +0200
> > Andi Kleen <andi <at> firstfloor.org> wrote:
> > > > > > > Core2 instruction set with tune=generic is still the best to
> > > > set.
> > > > > > Not sure that is true. These option are mostly for the compiler.
> > > > exactly, and our benchmarks show that tune=generic is best right now
> > for Atom. > > (586 scheduling sounds nice, but the pipelines are rather different.
> > And the benchmarks don't lie.. > > That sounds a bit dangerous since tune=generic is documented to
> change the semantics between gcc versions to better fit more recent
> CPUs (there's even a small difference between gcc 4.2 and gcc 4.3):
>

reality is that tune=generic avoids the things that are "really bad"
for a wide generation of cpus; the world of x86 is such that there
really are many common things that are good for the vast majority of
the cpus out there (or at least neutral).

Future versions of GCC might have a specific ATOM model. Until they do,
tune=generic is the right thing based on tests over a few gcc versions.
Yes it's a bit fluid, but no gcc isn't going to suddenly go do stupid
things for currently mass-sold cpus.

Well, if the Intel experts can't even agree, what hope do I have of getting it right :-( I chose Core2 because I read somewhere that Atom was "feature compatible" with it, but of course that doesn't say anything about the optimal optimisations. I trust that someone will update Kconfig.cpu with their idea of the right choice for Atom eventually.

(Maybe there should be a way to auto-suggest the right setting for a native build based on /proc/cpuinfo? I think the gcc build process can do something like that.)

Cheers, Phil.




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