Author: Zach Wegner
Date: 03:26:08 08/04/03
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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.
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