Author: Tim Foden
Date: 12:06:27 01/13/02
Go up one level in this thread
On January 13, 2002 at 08:48:13, David Rasmussen wrote:
>Even though I try to keep my program extremely portable and ANSI/ISO C++
>compliant, except for the parts that can't be, I would like to see how much
>faster my program would be with an inline assembler version of my FirstBit()
>function for use in VC++ (jeez, when will 64-bit processors be common...).
>
>My board setup is a1=0,...,h8=63. For some reason, I can't get it to work.
>
>int FirstBit(BitBoard bitboard)
>{
> __asm {
> bsr edx, dword ptr bitboard
> mov eax, 0
> jnz l1
> bsr edx, dword ptr bitboard+4
> mov eax, 32
> jnz l1
> mov edx, -1
> l1: add eax, edx
> sub eax, 1
> }
>}
>
>This is written from me head, as I have deleted what I previously did, so I
>haven't even tried to compile this, but you get the idea. The problem was that
>for some bitboards, it returned the correct value, but for some other bitboards,
>bsr just returned some weird wrong number, which made the entire function return
>some weird wrong number. What's the problem?
1) Why are you subtracting 1 at the end? This would seem to return -2..62
instead of -1..63.
2) mov edx, -1 seems wrong. eax contains 32 here, so the final result will be
32-1-1 == 30. Maybe remove the final sub, and make this: mov edx, -33?
Chees, Tim.
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