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 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). 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/
