Author: Anthony Cozzie
Date: 08:05:30 03/11/04
Go up one level in this thread
On March 11, 2004 at 10:31:22, milix wrote:
>Hello to everyone.
>I'm trying to optimize various parts of my bitboard engine and I found an
>optimization for my popcount() routine. I was using the routine:
>
>int popcount_1(const bitboard bb) {
> register int cnt=0;
> register bitboard a=bb;
> while (a) {
> cnt += bytecount[a & 0xff];
> a >>= 8;
> }
> return cnt;
>}
>
>where bytecount is a looup table for byte counts.
>
>I changed this routine a bit to
>
>int popcount_2(const bitboard bb) {
> register int cnt=0;
> unsigned long a1 = *((unsigned long*)&bb);
> unsigned long a2 = *(((unsigned long*)&bb)+1);
> while (a1) {
> cnt += bytecount[a1 & 0xff];
> a1 >>= 8;
> }
> while (a2) {
> cnt += bytecount[a2 & 0xff];
> a2 >>= 8;
> }
> return cnt;
>}
>
>and the routine popcount_2 was 2x times faster than popcount_1. It is even
>faster than the assembly routine provided by AMD for popcounts. I tested the
>routines using 1000 randomly generated bitboards.
>Do I miss something here or it is an improvement? Have anyone else tried a
>similar idea?
>
>My computer specs are:
>AMD Athlon XP 1600+
>512 MB RAM
In my experience the fastest popcount for Zappa is the simple
for(a = 0; b != 0; b &= b -1, a++) ;
where a is the counter and b is the bitboard.
This works because most bitboards that I call it on are sparsely populated.
anthony
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