Author: Gerd Isenberg
Date: 08:43:31 05/28/04
Go up one level in this thread
On May 28, 2004 at 11:13:09, Dieter Buerssner wrote:
>On May 28, 2004 at 10:56:59, Gerd Isenberg wrote:
>
>>int popCount(BitBoard bb)
>>{
>> unsigned int l = low(bb);
>> unsigned int h = high(bb);
>> l -= (l >> 1) & 0x55555555; // count 2-bit groups
>> h -= (h >> 1) & 0x55555555; // count 2-bit groups
>> l = (l & 0x33333333) + ((l >> 2) & 0x33333333); // count 4-bit groups
>> h = (h & 0x33333333) + ((h >> 2) & 0x33333333); // count 4-bit groups
>
>l += h; // and do the next stage on l only
>
>> l = (l + (l >> 4)) & 0x0f0f0f0f; // count 8-bits groups
>> h = (h + (h >> 4)) & 0x0f0f0f0f; // count 8-bits groups
>> return ((h+l) * 0x01010101) >> 24; // sum 8-bit results
>>}
>
>Gerd, I think, you can combine the 2 halves already one stage earlier. Or did I
>miscaclulate this? I thought, the individual counters cannot overflow at that
>stage already. See also my post
>http://chessprogramming.org/cccsearch/ccc.php?art_id=349781
>
>For me, adding two counter combining stages was faster, than the multiplication.
>
>Regards,
>Dieter
I wrote it from scratch to verify the "packing" trick.
But it seems your add is a bit too early, since 8+8 may overflow to 0x10.
Cheers,
Gerd
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