Author: Dieter Buerssner
Date: 11:46:10 06/12/04
Go up one level in this thread
On June 12, 2004 at 13:09:30, Gian-Carlo Pascutto wrote:
>The best thing seems to be one of the "Magic bitscans".
>
>const BITBOARD magic = 0x03f08c5392f756cdULL;
>
>const unsigned int magictable[64] = {
>0, 1, 12, 2, 13, 22, 17, 3,
>14, 33, 23, 36, 18, 58, 28, 4,
>62, 15, 34, 26, 24, 48, 50, 37,
>19, 55, 59, 52, 29, 44, 39, 5,
>63, 11, 21, 16, 32, 35, 57, 27,
>61, 25, 47, 49, 54, 51, 43, 38,
>10, 20, 31, 56, 60, 46, 53, 42,
>9, 30, 45, 41, 8, 40, 7, 6,
>};
>
>unsigned int FindFirst (const BITBOARD b) {
^^^^^
I think, this const buys nothing.
> const BITBOARD lsb = b & -b;
> return magictable[(lsb * magic) >> 58];
>}
Perhaps, doing the multiplication "manually" will produce better code.
const unsigned int magicl = 0x92f756cd, magich=0x03f08c53;
unsigned int FindFirst(BITBOARD b)
{
unsigned int bl, bh, r;
bh = b >> 32;
bl = b;
r = (bl*(BITBOARD)magicl)>>32;
r += bl*magich;
r += bh*magicl;
return magictable[r>>26];
}
Totally untested. But I compiled it with MSVC and GCC and looked at assembler
output. Seems better than above. Basically one mul, 2 imul, 2 add, one shr and a
few mov to get the index into the table.
Regards,
Dieter
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