On Mon, Nov 29, 2004 at 01:30:11AM -0800, Nicholas Miell wrote: > On Mon, 2004-11-29 at 10:16 +0100, Arjan van de Ven wrote: > > On Mon, Nov 29, 2004 at 01:02:46AM -0800, Nicholas Miell wrote: > > > CMOVcc will use less space in the instruction cache than the Jcc/MOV > > > pair, though. > > > > only sometimes.... since cmov doens't work on all register/memory > > combinations extra code might be needed to glue that together... > > > > > > .... and we're suddenly talking about 0.01% performance ;) > > Well, yeah. :) > > There's also branch prediction and decode bandwidth issues that I didn't > bother to mention. Although P4 have the ds/cs segment prefixes for static branch prediction, the non-preproduction chips actually don't use it, so it only makes code bigger. http://gcc.gnu.org/ml/gcc-patches/2004-07/msg02200.html > But, if you're going to optimize for i686 or better for other reasons, > there's no reason not to use CMOVcc instead of Jcc/MOV, where possible. Well, there is a reason aside from some CPUs not having those insns at all: on some recent Intel CPUs CMOVcc is actually slower than Jcc/MOV. Jakub
