Messages in this thread | | | From | Denis Vlasenko <> | Subject | Re: Why does C3 CPU downgrade in kernel 2.4.20? | Date | Wed, 11 Dec 2002 17:09:34 -0200 |
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On 11 December 2002 08:58, Dave Jones wrote: > On Wed, Dec 11, 2002 at 11:19:23AM -0200, Denis Vlasenko wrote: > > > Prolly I would have to do more benchmarking to find out about > > > aligment advantages. > > > > I heard cmovs are microcoded in Centaurs. > > s...l...o...w... > > Hardly surprising given that the chip isn't targetted at the > performance market.
*We Support 686 Instruction Set* plastered everywhere? ;) Who cares that a single cmov take some tens of cycles... (btw, can someone measure that? I have no C3...)
On 7 July 2002 12:32, Willy TARREAU wrote: > because GCC's output is really ugly. In fact, it is > also ugly when it generates cmov. I disassembled my > libc and found that it subobtimizes the code at the > point that it's far worse with cmov than without ! > (more instructions, more memory accesses, more > registers used).
Do not try to optimize "pedal to the metal" without actually looking at the results. With "-march=i686" on C3 one will get:
* Non-optimal GCC code generation * Really Slow (tm) cmovs * Buggy code (cmov with mem operands) if one don't think above two are not enough ;)
On 10 December 2002 05:22, Daniel Egger wrote: > Am Die, 2002-12-10 um 06.52 schrieb Dave Jones: > > I believe someone (Jeff Garzik?) benchmarked gcc code generation, > > and the C3 executed code scheduled for a 486 faster than it did for > > -m586 > > I'm not sure about the alignment flags. I've been meaning to look > > into that myself... > > Interesting. I have no clue about which C3 you're talking about here > but a VIA Ezra has all 686 instructions including cmov and thus > optimising for PPro works best for me.
Such things need testing. A kernel compile would suffice I guess. -- 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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