On Tue, 2005-07-19 at 19:52 +0200, Jan Engelhardt wrote: > >Hello, > > > >If I set the CPU type to be amd64 in kernel config, the kernel is built > >with -march=k8. If I set it to be k6, the kernel is built with > >-march=k6. If I set the CPU type to be Pentium4, the kernel is built > >with -march=i686 -mtune=pentium4. Why is not the for-P4 kernel built > >with -march=pentium4 ? > >I tried building the kernel with -march=pentium4 for the sake of > >experiment and got no ill effects. > > -march= specifies the instruction set, -mcpu= / -mtune= the tuning factor. > Maybe it is that the instruction set is the same on i686 and > pentium4. cmov for example is not present in k6, and k8 is something > completely different at all. > > > Jan Engelhardt
-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. Ivan Yosifov. - 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/
