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/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
