Author: Robert Hyatt
Date: 20:04:56 06/23/04
Go up one level in this thread
On June 23, 2004 at 18:30:57, Russell Reagan wrote:
>I am curious about a few design choices in Crafty. Some of the questions are
>general questions that anyone who is knowledgable could answer, so I ask this
>publicly instead of via email to Dr. Hyatt.
>
>First issue. Crafty is written with a lot of color-specific code, like:
>
>if (wtm) {
> // ...
>}
>else {
> // ...
>}
>
>This is what I think. The downside to this approach is that you double the size
>of the code, which is not cache friendly. This should be a bigger problem in a
>rotated bitboard engine where there are already cache issues.
>
>Another potential downside is that you introduce an extra branch. However, I
>think that this branch would be easily predictable, since the side to move will
>alternate back and forth, and so the branch decision will alternate.
>
>So it seems that as long as you don't overuse this method and ruin the cache,
>this is a good method as far as speed is concerned. However, I think it is nicer
>to have a single function that works for both colors. It is less to maintain,
>and less error prone.
It would be nicer. However there may well be a speed penalty since bitboards
have specific elements for each type and color of piece. As a result it might
be harder to read.
>
>Second issue. Crafty uses a lot of switch statements, using special code for
>each case, increasing the code size. Same issues. Could be bad on cache, harder
>to maintain. Crafty uses a lot of switch statements to determine the type of a
>piece and update the appropriate bitboard. What about having an array of
>bitboards, indexed by the type of the piece? There are no branches involved, and
>the code is much smaller.
That would work ok. However the switch is not a performance killer at all, and
early on it made things easier to change. Turning the bitboards into arrays is
ok, but then it requires a register to index the arrays and the X86 doesn't have
many to go around...
>
>I'd like to know what people think about these design choices. Obviously they
>work well for Dr. Hyatt, but I wonder if alternatives would be better choices
>that would be faster or less error prone.
>
>Thanks,
>Russell
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