Re: Noob question. Why is the for-pentium4 kernel built with -march=i686 ?

[Ivan Yosifov <ivan@yosifov.net>]

On Wed, 2005-07-20 at 09:03 +0100, Kerin Millar wrote:
> On Tue, 19 Jul 2005 21:35:51 +0300, Ivan Yosifov wrote:
> 
> <snip>
> 
> > -march implies -mtune and also implies thing like -msse2 for the
> > instruction set where applicable. 
> > I think -march=pentium4 is equivalent to -mmmx -msse -msse2
> > -mtune=pentium4 ( if I have not fogotten anything ).  
> > Pentium4 supports things like sse2 and mmx which AFAIK plain i686 does
> > not. I first thought that maybe the kernel was destabilized by such
> > optimizations, but k8 has all of them and more ( sse3 ). 
> > So, if it is ok to build the k8 kernel with -march=k8 why is it not ok
> > to built the p4 kernel with -march=pentium4 ? 
> > I may be wrong, but any way I think of it it looks like a performance
> > hit to build a p4 kernel with -march=i686.
> > 
> 
> Well, it may seem logical to think that but proving it is another matter
> entirely. The flags that you refer to (-mmmx, -msse and -msse2) enable
> support for vector extensions through various built-in functions. But this
> does not necessarily mean that the code is somehow transformed to make use
> of these functions, nor that the compiler decides to makes effective use
> of these extensions via the optimisation process. As far as I'm aware,
> only >=gcc-4.0 has support for auto-vectorisation although I am not
> certain as to how effective it is. Jakub Jellinek has some wise words to
> say on the topic in general:
> 
> https://www.redhat.com/archives/fedora-devel-list/2005-January/msg00742.html

Interesting. 

> 
> Where genuine performance tests are conducted the results are not always
> in accordance with what one might expect. Only hours ago I was reading a
> LFS thread where someone had noted poorer performance using the "prescott"
> target as opposed to "i386" for example. That Red Hat also choose to use
> -march=i386 is interesting (as noted in the link above, with the exception
> of the kernel and glibc) ...
> 
> Also, I believe that the -march=pentium4 option /was/ actually used up
> until kernel 2.6.10 where it was dropped because of a risk that some
> versions of gcc would cause the kernel to use SSE registers for data
> movement (which is a no-no).
> 

You seem right. I fetched a 2.6.9 tarball and it is really built with
-march=pentium4. Do you know which are versions of gcc in question ?

> Cheers,
> 
> --Kerin Millar
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/