Author: Gerd Isenberg
Date: 04:00:22 08/04/03
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On August 04, 2003 at 06:46:31, Tim Foden wrote: >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. Sorry Tim, i meant exclusive rank-symmetry, but no file-symmetry. In a (rank) mirrored position queen wing and king wing are still on the same side of the board. Cheers Gerd
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