Author: Robert Hyatt
Date: 08:42:35 06/27/02
Go up one level in this thread
On June 26, 2002 at 04:38:25, Sune Fischer wrote:
>On June 26, 2002 at 02:02:28, Bruce Moreland wrote:
>
>>On June 25, 2002 at 05:26:34, Sune Fischer wrote:
>>
>>>On June 24, 2002 at 20:33:32, Russell Reagan wrote:
>>>
>>>>I am not an experienced bitboard user. I've been thinking about trying them out
>>>>just to see how they work in comparison with my current 0x88 implementation. Is
>>>>there any potential problem in mixing the two approaches? One problem that I can
>>>>see would be that if you have an index for a bitboard, it's not going to index
>>>>into the same square in your 0x88 array. I suppose the only real reason to keep
>>>>0x88 would be for efficient edge detection. Do bitboards offer a solution to
>>>>edge detection? Or do they even need edge detection when using bitboards (ex.
>>>>using the BSF asm instuction would seem to avoid edge detection altogheter)?
>>>>
>>>>Thanks,
>>>>Russell
>>>
>>>For edge detection you can do this:
>>>
>>>if (((uint64)1<<square)&0xFF818181818181FF)
>>> ...
>>>
>>>it will tell you when you are on the first and last file or rank.
>>>I use that in my SEE to raytrace behind the attacking piece, something like
>>>
>>>while (!(((uint64)1<<square)&0xFF818181818181FF)) {
>>> square+=direction;
>>> if (there-is-an-attacker-on-square)
>>> add-it-to-the-list-of-attackers
>>>}
>>>
>>
>>That shift has to be lots of fun for the processor.
>
>That is why I really use a Mask[square] table, but for the sake of clarity... :)
>Anyway I changed it to a macro, so now I can quickly test which is faster, and
>it seems the Mask[] lookup is faster, for the moment.
>
>-S.
>
If it is faster, I am surprised. IE the shift should run like the blazes
since there are no memory references (an immediate value of 1, shifted N
bits avoids memory) while a memory load can be expensive depending on
whether the mask is in memory, in L1/L2/L3 cache...
That was a common trick on the Cray where _all_ memory references (prior to
the T90) were real memory accesses (no cache).
>>bruce
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