Author: Vincent Diepeveen
Date: 16:09:23 11/19/02
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On November 19, 2002 at 15:48:57, Gerd Isenberg wrote: ><snip >> >>If you agree with me that the focus in the future is on evaluation, >>it will not come as a surprise that more complex patterns get more >>crucial. >> >>Let me present you a simple example. suppose you are scanning the board >>for example for your king safety, mobility or whatever, for complex >>patterns to evaluate: >> >>We are now at square sq and want to know whether a complex pattern is >>happening. For example: >> >>if( sq >= 8 && board[sq-8] == bishop && board[sq-7] == rook && ... >> >>What's your bitboard equivalent here? >> >>snelbord is in the L1 cache of course. Let's not worry about L1 or L2 >>caches here. Also let's not worry about that both in bitboards *and* >>non bitboards you can get rid of at least 1 compare here. >> >>AFAIK you need an array for each value sq-8 and sq-7 to lookup the >>mask needed for the bitboard to AND with. >> >>lemme give a shot out of the hand, writing compatible code >>(so not writing out black & white : >> >>So here the bitboard equivalent without caring that in both >>examples we can save out a CMP: >> >>if( sq >= 8 && (Bishops[side]&Mask[sq-8]) && (Rooks[side]&Mask[sq-7]) && ... >> >>Best regards, >>Vincent > >Yes Vincent, exactly. > >But the sequence is wrong and that's the point. > >Instead of a loop over all squares looking for one ore more properties, one get >a set of squares with the desired property, simply with a few bitwise >operations. Then you may traverse the found bits, if any. > >Bitboards work like SIMD. That is not possible with a big evaluation. That is like doing n operations at n elements instead of log n. Also another effect is there which is very intuitively true and very easy to understand for everyone except Bob. That's that with more difficult knowledge patterns, the bitboard pseudo code is too difficult to read and bugfix a month later. >Btw. instead of Mask[sq] you should use 1<<sq on 64 bit hardware. Often, if you SHL is too slow on P4 to use. 2 cycles. Array lookup might be faster than that. >traverse bitboards, it's even not necesseray to have a bitindex of a singular >bitboard at all. The singular bitboard (bb & -bb) is fine to get attacks or >attacks from with Kogge-Stone of a particulare square. That saves even the >bsf-instruction. >But the advantage of Kogge-Stone is to avoid a loop totally, the routine may be >supplied with a set of multiple squares. If you want to know whether a white >rook may reach a set of squares in n moves, without get being captured... >Regards, >Gerd
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