On Wednesday 03 January 2007 17:03, Linus Torvalds wrote: > On Wed, 3 Jan 2007, Grzegorz Kulewski wrote: > > Could you explain why CMOV is pointless now? Are there any benchmarks > > proving > > that? > > CMOV (and, more generically, any "predicated instruction") tends to > generally a bad idea on an aggressively out-of-order CPU. It doesn't > always have to be horrible, but in practice it is seldom very nice, and > (as usual) on the P4 it can be really quite bad. > > On a P4, I think a cmov basically takes 10 cycles. > > But even ignoring the usual P4 "I suck at things that aren't totally > normal", cmov is actually not a great idea. You can always replace it by > > j<negated condition> forward > mov ..., %reg > forward: ... ... > In contrast, if you use a predicated instruction, ALL of it is on the > critical path. Calculating the conditional is on the critical path. > Calculating the value that gets used is obviously ALSO on the critical > path, but so is the calculation for the value that DOESN'T get used too. > So the cmov - rather than speeding things up - actually slows things down, > because it makes more code be dependent on each other.
Why CPU people do not internally convert cmov into jmp,mov pair? -- vda - 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/
