Author: Gerd Isenberg
Date: 02:54:16 08/27/05
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On August 27, 2005 at 05:33:17, Alessandro Damiani wrote: >On August 27, 2005 at 03:12:23, Gerd Isenberg wrote: > >>On August 26, 2005 at 16:11:53, Dan Honeycutt wrote: >> >>>Run the lowest bits down the a or h file and you can do pawn captures without >>>having to mask those files. >>> >>>Best >>>Dan H. >> >>Hi Dan, >> >>a very nice idea! Keeping the a-file in least significant byte and the h-file in >>the most significant byte (or vice versa) to avoid a-h-wraps for pawn attacks >>and further ands. >> >>eg. with a1 == 0: >> >>a1 a2 a3 a4 a5 a6 a7 a8 >>b1 b2 b3 b4 b5 b6 b7 b8 >>c1 c2 c3 c4 c5 c6 c7 c8 >>d1 d2 d3 d4 d5 d6 d7 d8 >>e1 e2 e3 e4 e5 e6 e7 e8 >>f1 f2 f3 f4 f5 f6 f7 f8 >>g1 g2 g3 g4 g5 g6 g7 g8 >>h1 h2 h3 h4 h5 h6 h7 h8 >> >>"left" attacks of white pawns ::= white pawns >> 7 ; eg. e4:d5 >>"right" attacks of white pawns ::= white pawns << 9 ; eg. e4:f5 >>"left" attacks of black pawns ::= black pawns >> 9 ; eg. e4:d3 >>"right" attacks of black pawns ::= black pawns << 7 ; eg. e4:f3 >> >>Gerd > >A nice idea indeed, but then vertical shifting needs masking, e.g. shiftUp(e8) = >f1. So, no real gain in the end, I guess. > >Or do I miss something? > >Alessandro Since pawns usually don't stay on back ranks, there is no need to worry about rank1/8 pawn push wraps. So for pawns there is a small gain for their attacks. For x86-32 it might be negligible since the double register shifts are much more expensive than two "ands" with immediate operands. For x86-64 there is only one reg64 shift. The "and reg64, mem64" reads the mask from memory - so the relative gain might be more significant here. Gerd
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