Author: Tim Foden
Date: 03:46:31 08/04/03
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On August 04, 2003 at 06:26:08, Zach Wegner wrote: >On August 04, 2003 at 06:11:20, Gerd Isenberg wrote: > >>Hi all, >> >>One common search/evaluation test is to compare the search of a position with >>it's mirrored position. >> >>There is one principle problem related to bitboard traversal in move-generation. >> >>Black pieces and targets are traversed in an other order than the "mirrored" >>pieces and targets, regardless if one uses bsf or bsr and the square-bitindex >>mapping. >> >>Lets take some white pawns with following bit indicies: b2(9),d2(11) and c3(18). >>With bsf you'll find them in ascending order: b2,d2,c3. Now with "mirrored" >>black pawns on b7(49),d7(51),c6(42) we will foreward scan the ascending c6,b7,d7 >>squares. >> >>Even if there is further move sorting, the different initial order of generated >>moves in mirrored positions implies different search trees - and therefore >>slightly different search results. >> >>How do you deal with this behaviour in your bitboard based program? >>I ignore it so far. >> >>One possible solution with x86-64 is to use the fast "bswap reg64" instruction >>(direct path,1 cycle) before scanning black pieces or targets for movegen. >>"bswap" mirrors the bits rankwise (8 becomes 1, 7 become 2...) and a remirror of >>the scanned bit index is a simple xor with 0x38. >> >>According to the sample above we mirror the black b7,d7,c6 pawns to b2',d2',c3' >>and we get the same order as with original white pawns: b7,d7,c6. >> >>One drawback is of course the need of a color2move paramter for bitScan - or >>separate black/white traversal. >> >>Any remarks or implementaion hints? >> >>Regards, >>Gerd > >What I have always done, which avoids this problem, is to use either MSB or LSB >for removing a piece depending on which side of the board it is most likely to >be on. e.g. for white pawns, use LSB; for black pawns, use MSB. I dont know if >this helps, but it couldn't hurt. Yes, I basically do the same, for the same reason... it is a very simple optimisation. For the white pieces I always use bsf, and for the black pieces I always use bsr. This also has the added advantage of solving the problem that Gerd is talking about. Cheers, Tim.
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