Computer Chess Club Archives


Search

Terms

Messages

Subject: Re: SSE2 bit[64] * byte[64] dot product

Author: Anthony Cozzie

Date: 09:31:59 07/20/04

Go up one level in this thread


What do you think of the following C code:

int bb_dot_product(bitboard a, unsigned char *weights)
{
   bitboard t, t1, *_weights = weights;
   static bitboard table[256] = {correct translations, e.g. 0xFF -> 0xffffffff}

   //we count on the compiler to unroll this loop.
   for(i = 0; i < 8; i++, a ) {
      t = table[(a >> i*8) & 0xFF] & weights[i];
      t1 = t;
      t << 8;
      sum += (t & 0x00FF00FF00FF00FF) + (t1 & 0x00FF00FF00FF00FF);
   }

   sum = (sum & 0x0000FFFF0000FFFF) + ((sum >> 16) & 0x0000FFFF0000FFFF);
   sum = ((sum >> 32) + sum & 0x00000000FFFFFFFF);
   return sum;
}

It has several advantages:  Can use full 0-255 for each weight, the table does
not have to be rotated, and there is no penalty for moving between the integer
and MMX pipes.

OTOH, this solution is also much less cache friendly, requiring maybe 2x the
number of instructions and also needed 2KB of data cache.

anthony



This page took 0.02 seconds to execute

Last modified: Thu, 07 Jul 11 08:48:38 -0700

Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.