Author: Gerd Isenberg
Date: 10:54:12 07/21/04
Go up one level in this thread
On July 20, 2004 at 12:31:59, Anthony Cozzie wrote: >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 probably some minor improvements. Save the inner shift by building two intermediate results. Whether the byte access it worth instead of shift/and? On x86-32 it was. unsigned char *bptr = & (unsigned char) a; sum0 = 0, sum1 = 0; for(i = 0; i < 8; i++, ptr++ ) { t = table[*bptr] & weights[i]; sum0 += t & 0x00FF00FF00FF00FF sum1 += t & 0xFF00FF00FF00FF00; } sum = sum0 + (sum1>>8); sum = (sum & 0x0000FFFF0000FFFF) + ((sum >> 16) & 0x0000FFFF0000FFFF); sum = ((sum >> 32) + sum & 0x00000000FFFFFFFF); return sum;
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