Author: Tom Kerrigan
Date: 12:02:46 07/22/03
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On July 22, 2003 at 14:18:23, Robert Hyatt wrote: >>Of course, this is contrary to the point of a conditional move instruction. My >>only comment to that is that Intel must have decided to add the conditional move >>after they were done designing the relevant parts of the core. The decision to >>add the instruction makes sense for forward-compatibility, i.e., "use this >>instruction and you will see a performance improvement with it on later >>processors." > >That could be. However, the idea was not new. The alpha did this 10+ years >ago. So the advantage to a real CMOV implementation should be real. Did I ever say it was new? Did I say that Intel's implementation is ideal? No, I didn't. And conditional move is just a poor man's predication, which has been implemented in processors LONG before Alpha. (And, IIRC, conditional move was a recently added Alpha instruction. I don't think it was in the 21064.) I see nothing wrong with what Intel did. If my hunch is right and they only thought of adding the instruction after the P6 datapaths were planned/designed, then the net effect is that they added an instruction that doesn't increase performance, or increases it only marginally. So it's a little something extra they threw in there to future-proof the processor which isn't hurting you because you DON'T EVEN HAVE TO USE IT. So you're being an ass for criticizing them for it. They could have just as easily left it out and then you would have had no complaints. -Tom
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