Author: Dan Newman
Date: 11:13:05 04/19/00
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On April 19, 2000 at 04:31:48, Tony Werten wrote: >On April 19, 2000 at 03:27:16, Dan Newman wrote: > >>On April 18, 2000 at 22:44:25, Flemming Rodler wrote: >> >>>Hi, >>> >>>I am trying to implement a bitboard based chess program on a Pentium or AMD >>>computer. I need to be able to find the following information fast: >>> >>>1) The position of the first and/or last bit in a sequence of 64 bits. >>>2) Count the number of bits that are 1 in a sequence of 64 bits. >>> >>>I know there is a method that works linear in the number of on-bits for >>>problem 2: >>> >>> for(count = 0; bitboard; count++, bitboard &= (bitboard -1)); >>> >>> >>>Is there anything faster, ie. such lookuptables or machine code intrutions? >>> >>>What about problem 1? >>> >>>Thanks in advance for any reply >>>Flemming >> >> >>The following is what I use in my program to find bit indices >>(it works only with MSVC). >> >>#pragma warning( push) >>#pragma warning( disable: 4035) >>inline int bsf( unsigned long bitpat) >>{ >> __asm mov eax, bitpat > >Just a small question. Can you leave this mov away ? Since bitpat will be passed >in the eax register anyway ? Or doesn't this work for inline functions ? >( I'm a delphi man, and delphi doesn't support inline ) > >Tony > Well, I haven't tried that. I think that MSVC's default argument passing scheme is by stack rather than register. I'll have to try it (with passing via registers turned on, I guess)... (It could be, since this is inline, that the optimizer gets rid of any extra move anyway and just arranges for a copy of the argument to end up in eax just before the BSF.) -Dan. >> __asm bsf eax, eax >>} >>inline int bsr( unsigned long bitpat) >>{ >> __asm mov eax, bitpat >> __asm bsr eax, eax >>} >>#pragma warning( pop) >> >> >>-Dan.
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