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 [EMAIL PROTECTED] > 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 [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
