Author: Keith Evans
Date: 18:48:10 06/20/02
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On June 20, 2002 at 20:56:44, Robert Hyatt wrote: >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? It sounds a little like you're being disingenuous. If you did the experiment and got a result like "the 64 bit runs only 5% faster than the 32 bit version" then would you ignore it because you're not sure why? And still tout the performance advantages of bitboards for 64-bit machines? Are you interested in validating the idea that bitboards are a win on 64-bit machines? We're just trying to propose an experiment which although imperfect would be more reliable than mere intuition. Any ideas? -Keith
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