Author: Robert Hyatt
Date: 12:21:46 08/08/02
Go up one level in this thread
On August 08, 2002 at 04:49:43, Russell Reagan wrote:
>First question...
>
>In Crafty, there is a function FirstOne() which will return the least
>significant bit set in a bitboard.
>
>FORCEINLINE int FirstOne(BITBOARD a) {
> __asm {
> bsr edx, dword ptr a+4
> mov eax, 31
> jnz l1
> bsr edx, dword ptr a
> mov eax, 63
> jnz l1
> mov edx, -1
> l1: sub eax, edx
> }
>}
>
>Now I've always heard that you should pass by pointer or reference if you are
>dealing with a non-standard type, which I assume a 64-bit int would be on a
>32-bit machine. Is there any speed hit from passing by value in this function?
>Or is the reason it works due to inlining the function?
>
>Second question...
>
>I have seen different ASM used in different engines to accomplish this same task
>of determining the least significant set bit. This seems to be the shortest, but
>I don't see it in any engines.
>
>bsf eax, dword ptr a+4
>add eax, 32
>bsf eax, dword ptr a
That is extremely dangerous. It takes advantage of a quirk in the intel
BSF/BSR instructions that works today. But maybe not tomorrow. If a word
is all zero, then the target register is not modified. The problem is that
the Intel docs say "undefined" for the result of a bit scan on a word with
all zeros. Today "undefined" means "unchanged". Tomorrow it might not...
>
>The closest I see is Beowulf with:
>
>mov ecx,dword ptr [bits+4]
>mov ebx,dword ptr [bits]
>bsf eax,ecx
>add eax,32
>bsf eax,ebx
>
>Is the "problem" with this that you have to assume that the bitboard isn't equal
>to zero?
>
>If so, why is that a problem? Where do you need to use FirstOne() where you
>don't know if there are any bits set or not? It seems logical to me that the
>since you don't need to verify that the bitboard is not null most of the time,
>it is redundant and only wastes time. For example, if your FirstOne() function
>tests to make sure that the bitboard isn't null, you are doing extra work in the
>case of generating legal moves:
>
>while (whiteKnights) { // Make sure whiteKnights isn't null
> int index = FirstOne(whiteKnights); // Make sure _AGAIN_
> DoKnightStuff(index);
> Clear(whiteKnights,index);
>}
>
>Thanks,
>Russell
That is done simply because you can't emit a move if whiteKnights has no bits
set... Its simply the shortest way to write that piece of code.
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