Author: Matt Taylor
Date: 16:52:06 01/03/03
Go up one level in this thread
On January 03, 2003 at 12:03:45, Uri Blass wrote:
>code B is slightly faster than code A.
>I know that side can get only 0 or 1(something that the compiler does not know)
>and B is eqvivalent to A if you assume that side gets only 0 or 1.
>
>Is it possible to write a third code that will be even faster than B?
>
>I think that if the compiler can know that side is or 0 or 1 it can do B even
>faster.
>
>code A:
>
>if (side==LIGHT)
>{
> if (to>=56)
> {
> gen_promote(from,to,bits);
> return;
> }
>}
>else
>{
> if (to<=7)
> {
> gen_promote(from,to,bits);
> return;
> }
>}
>
>code B:
>if ((to+side*(63-2*to))>=56)
>{
> gen_promote(from,to,bits);
> return;
>}
>
>Uri
Try this:
if ((to + (-side & (63 - 2 * to))) >= 56)
{
gen_promote(from,to,bits);
return;
}
If side is 0 or 1, -side will be 0 or -1 and can be used as a mask here. It is
cheaper to do one's compliment and a bitwise and operation than a
multiplication.
The compiler -should- recognize that 2 * to = to << 1
FYI my gut instinct is that there is still potential for more optimization in
the code that I posted. I believe one could reduce either the number of ops
spent, data dependency, or something similar. I can't see it right now.
-Matt
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