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On 22 November 2002 08:54, Dave Jones wrote: > On Fri, Nov 22, 2002 at 01:04:39PM -0200, Denis Vlasenko wrote: > > I consider 16-byte code alignment as way too big. > > P4 zealots can demand even more I guess :( > > I will happily change my mind when/if I'll see > > favorable speed/kernel size benchmarks. Until then, > > I think there's a misunderstanding here. > The march=pentium4 option is only used when you select > "build me a pentium 4 kernel" You do realise that right? > Generic kernels don't change 1 bit. > > > I think 4-byte alignment is closest to sanity. > > You know where to find the Intel P4 optimisation manuals.. > > > Not exactly P4 related but: if you tell gcc your > > processor has cmov, gcc will try to use it. > > So what ? Show me a P4 without cmov. > > > Results: > > * gcc code is worse with cmov than without > > * some CPUs (Cyrix?) have slow cmovs (microcoded?) > > * you lose whenever you try to use your code > > on cmov-less CPU. > > <------------ The point. > --------------> You. I find it's far too easy for me to talk on lkml than to do something useful... I'll try to *not* reply next time :) Let's start from the start. This is an original message which started this thread: On 21 November 2002 18:18, Margit Schubert-While wrote: > Maybe a dumb question - > Is it possible to use the "-march=pentium4 -mfpmath=sse -msse2" > options for a P4 ? > I notice anything over a P2 just gets "-march=i686". It sounds like: "I've got a P4, I want my kernel use each and every P4ism possible. I want. I want. I want. How to do it?" (a bit exagerrated :) An important question is missing here: is a particular P4ism useful? It is ok to use P4isms *if* one is sure they lead to better kernel. What -mfpmath=sse or -msse2 will give us? Probably nothing since kernel do not use fp (well, almost). And if it would use 'em, use of sse2 can *slow down* context switch. I'm not sure, but _it needs testing_ before we "optimize" kernel with such options. Margit Schubert-While <margitsw@t-online.de> has an excellent opportunity to compile a handful of kernels, run, say, contest with them and report his findings. ;) -- vda - 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/ | ||||||||||||
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