Author: Dieter Buerssner
Date: 04:54:20 01/07/03
Go up one level in this thread
On January 07, 2003 at 03:22:02, Russell Reagan wrote:
>int PopCnt(register BITBOARD a) {
> register int c=0;
>
> while(a) {
> c++;
> a &= a - 1;
> }
> return(c);
>}
On architectures, that don't have native 64 bit words, the following will most
probably be significantly faster:
int PopCount(unsigned long long a)
{
unsigned long w;
int n = 0;
w = a&0xffffffffUL;
if (w)
do
{
n++;
}
while ((w &= w-1) != 0);
w = (a>>32)&0xffffffffUL;
if (w)
do
{
n++;
}
while ((w &= w-1) != 0);
return n;
}
Note, that any slighly decent compiler will optimize the masks away, but they
make the code portable. It produces almost the same code, as the Crafty assembly
version, and possibly is even faster, when inlined (the bitboard may already be
in some register pair). Some unportable microoptimization can be added by using
a union or some (not Standard C) pointer casting tricks like
w = *(((unsigned long *)&a)+1);
Although, the use of the adress operator may also confuse the optimizer.
For highly populated bitboard, other methods certainly will be faster. Perhaps,
other methods are allways faster ...
Regards,
Dieter
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