Author: Keith Evans
Date: 14:07:54 06/21/02
Go up one level in this thread
On June 21, 2002 at 15:03:56, Robert Hyatt wrote: >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??? The test may be flawed, but is really it any more flawed than your method of comparing performance on a P3 to performance on a McKinley and attributing the gains in performance to the wider datapath? I have seen statements of yours ranging from: "Bitboards really don't provide anything useful as far as move generation goes, 'today'.. because everything is done with 64 bit words. If you move to a 64 bit architecture, then they begin to pay off, but on 32 bit machines, they likely just 'break even.'" to: "with perfect programming, I think they should be 2x faster than offset representations, *unless* an offset generator can somehow take advantage of 64 bit words in a way that has not been done yet..." Is it "begin to pay off" (maybe the 10% the Eugene mentions in this thread), or is it "2x faster"? I would honestly like to know. Let's say that you were one of Knuth's grad students and he was preparing a tome on chess programming. Are you going to tell him that there's no valid way to compare the 32-bit and 64-bit performance of your bitboards? We offered up an experiment and you shot it down. Any better ideas? I know that Mr. Corbit is content to wait at least a year for results, but I'll that if a thread titled "bitboard performance analysis" appeared tomorrow that he would click on it. -Keith
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.