Author: Bo Persson
Date: 11:21:45 04/19/00
Go up one level in this thread
On April 19, 2000 at 14:13:05, Dan Newman wrote:
>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.
>
I have tried it :-)
MSVC does not optimize the __asm instructions, so there will be a redundant
store/load to a temporary variable (the parameter). However, the resulting code
is pretty fast anyway!
Also, it ignores all requests for register parameter passing for inline asm
functions.
Bo Persson
bop@malmo.mail.telia.com
>>> __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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