Messages in this thread | | | From | Denis Vlasenko <> | Subject | Re: Why does C3 CPU downgrade in kernel 2.4.20? | Date | Sat, 14 Dec 2002 16:46:41 +0000 |
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On 12 December 2002 18:09, Pavel Machek wrote: > Hi! > > > > > 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. > > > > > > 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... > > It still might be faster then a branch... or not if centaurs are > really that simple. > Pavel
I did not measure it myself, but rumors were they took tens of cycles.
Well, a IFcc prefix meaning 'execute next instruction if' would be way more cool that CMOVcc. Because I want CADDcc, CTESTcc, CBSWAPcc too ;)
But since all 1 byte opcodes are taken and
Jcc skip # <- 2 byte opcode opcode op1,op2 skip:
I think some CPU magic can detect such short jumps and handle'em just like they were such a prefix, saving potential branch (mis-)prediction. -- 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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