Author: Robert Hyatt
Date: 12:03:56 06/21/02
Go up one level in this thread
On June 20, 2002 at 21:48:10, Keith Evans wrote: >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 So I don't trust the experiment, but if it produces results favorable to me I would tout 64 bit programs as the cat's meow? But if it produces results unfavorable to me I would say "the test is no good"?? Sorry, that isn't _me_. The test is flawed from the _beginning_. And no matter what result it shows, it won't mean a thing. Therefore, what would be the point unless you have a lot of time to burn and nothing to prove???
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