Author: Matt Taylor
Date: 07:29:50 12/04/02
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On December 03, 2002 at 07:00:37, Sune Fischer wrote: >On December 03, 2002 at 06:40:36, Gian-Carlo Pascutto wrote: > >>On December 02, 2002 at 19:43:15, Gerd Isenberg wrote: >> >>>Congrats, Walter!! >>> >>>10-bit pattern bsf PI2FD btr c LSB_64 >>>0x0000000011111133 15.3 18.0 19.1 22.8 17.8 >>>0x1010111010101110 19.7 18.5 19.6 23.4 17.8 >>>0x1111113300000000 20.6 18.0 19.1 22.8 17.8 >> >>Does this mean that this is the fastest currently known >>FindFirstAndRemove *and* that it doesn't need assembly? >> >>Impressive for sure! >> >>Any similar tricks for lightning-speed popcounting? >> >>What's the fastest sequence you've got for popcounting so >>far? (preferably one that doesn't depend too much on new >>instructions) >> >>-- >>GCP > >Yes very impressive. >But how do one make sure the table remains in the cache? > >I think maybe in a real world senario things perform a little different than in >a small loop, I suspect perft might be a little more realistic test for this >one. > >-S. If you access any part of the table, the CPU will store that part of the table into its cache. The exact size depends on the size of a data cache line. I think most CPUs use the LRU algorithm for cache discards, meaning the table won't leave the cache until you access a large amount of data without accessing the table again. The exact amount depends on the size of the caches, plural. In P4, the L1 cache contains data that is already in the L2 cache, meaning that the value is the size of the L2 cache, 512 KB for Northwood or 256 KB for Williamette. I am not sure whether the Athlon is built the same way, but I don't think it is. (Otherwise it wouldn't make any sense to have a humongous 64 KB L1 data cache.) Assuming I'm right, you have 320 KB on existing Athlon chips. Now, to directly answer the question of real-world performance, that depends only on what a chess engine does in between calls to the function. If the function is usually called in quick succession, performance will be near the synthetic test because the table and all of the bit scanning code remain in the L1 caches. If the function is called sparsely, it will suffer. Unless I'm completely wrong, the first case is more accurate for a chess engine, isn't it? -Matt
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