Author: Anthony Cozzie
Date: 09:39:17 07/20/04
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
Could someone with a 64-bit MSVC compile this code and post the assembly? (and
if they were really nice, benchmark it? :)
anthony
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