Author: Robert Hyatt
Date: 17:56:44 06/20/02
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On June 20, 2002 at 14:07:50, Tom Kerrigan wrote: >On June 20, 2002 at 13:03:10, Robert Hyatt wrote: > >>It could certainly be done. However, I don't see what it would prove. >>Other than that 64 bit operations are more efficient when done in one >>"chunk" than in two. That seems intuitive anyway. It would also present >>a few problems, with the FirstOne() and LastOne() PopCnt() functions that >>use assembly on the PC but not on the 64 bit machines (yet). > >How would this be a problem? Why are you talking about PCs? The experiment is to >force a 64-bit chip to use 32-bit ints for bitboards. The PC is not a 64-bit >platform (yet) so we're OBVIOUSLY not talking about it. > >As for not seeing what the experiment would prove, I assume you're joking. > >-Tom Not joking. When you have multiple degrees of freedom, things change and it is not easy to attribute results to a specific change. Does the compiler or cpu do better with a larger number of 32 bit instructions? Or better with a smaller number of 64 bit operations? Do the 32 bit operations cause unnecessary pipeline stalls due to things like the carry bit and whatever, or do they not? Does the compiler produce as elegant a code for 32 and 64 or does it do better on one or the other? When the 64 bit version runs 2x faster than the 32 bit version is it because of the 64 bit advantage or because of a bad 32 bit executable from the compiler? When the 64 bit runs only 5% faster than the 32 bit version, same question? How do you attribute a specific result to a specific variable when there are _several_ variables, all beyond your control and in the hands of the compiler guys?
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