Author: Robert Hyatt
Date: 14:19:41 06/27/02
Go up one level in this thread
On June 27, 2002 at 11:49:39, Sune Fischer wrote:
>On June 27, 2002 at 11:42:35, Robert Hyatt wrote:
>
>>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).
>
>To be honest I think I would need a specially designed test to get an answer
>here, the overall speed difference is so small that it's just lost in noise.
>
>-S.
>>
Even better then. As the shift doesn't disturb the contents of cache at
all. Which can be important later.
>>>>bruce
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