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

[Ivan Yosifov <ivan@yosifov.net>]

On Wed, 2005-07-20 at 13:38 +0300, Denis Vlasenko wrote:
> On Wednesday 20 July 2005 12:25, Ivan Yosifov wrote:
> > > > > 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 ?
> > > > 
> > > 
> > > No, I'm afraid not. I only know that the advice came from Richard
> > > Henderson who (I think) is one of the core glibc hackers. You can see
> > > the point at which it was introduced by Linus in the ChangeLog (2nd
> > > message from last):
> > > 
> > > http://www.kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.10
> > 
> > Seems to be this one:
> > 
> > <torvalds@ppc970.osdl.org>
> > 	Don't use "-march=pentium3" for gcc tuning.
> > 	
> > 	rth tells me that some versions of gcc may end up using the
> > 	SSE registers for data movement when you do that.
> > 	
> > 	Use "-march=i686 -mtune=xxxx" instead.
> > 	
> > 	(We do the same thing for march=pentium2/4 too, just for
> > 	consistency).
> > 
> > 
> > The way it is worded it seems that it is a problem with *some* versions
> > of gcc only on p3, not p4.
> 
> Why do you care? I bet that differences between i686 code and pentium4 code
> are well below noise level.

Ah, well.

I was curious why p4 got special treatment other CPUs ( like amd ) do
not.

Cheers,
Ivan Yosifov.


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