Author: Robert Hyatt
Date: 13:16:39 09/17/99
Go up one level in this thread
On September 17, 1999 at 15:57:28, Inmann Werner wrote:
>Hello.
>
>Implementing bitboards I came to the old and known problem of finding the
>number of a bit in the bitboard on Intel machines (32 bit).
>Trying a little around this evening, I came to following routine. It is simple
>in C, but looking at the assemmbler outcome, I do not know, if it is good.
>How do you implement such a extreme critical routine?
>
>Werner
>
>
>***************
>//Testroutine, not tuned but readable
>//hope no silly bug in it :)
>// hope also, the idea is clear
>
>typedef unsigned __int64 BITBOARD;
>long lookup1[65];
>BITBOARD teil;
>
>init_bits()
>{
>int i;
>BITBOARD a,b,c;
>long erg;
>
>
> lookup1[0]=0;
> a=1;
> teil=67; /*## OK ##*/
> for(i=1;i<64;i++)
> {
> lookup1[i]=erg;
> a=a<<1;
> }
>}
>
>long findbit(BITBOARD x)
>{
>BITBOARD c;
>long bitnr,erg;
>
> c=x-(x&(x-1));
> bitnr=c%teil;
> erg=lookup1[bitnr];
> return(erg);
>}
>******************************************
That is not the way to do it. There are two good ways to do this.
One is to take each byte and use that as an index into a table that
gives the first 1 bit set in a byte with that particular subscript.
The other is to use the assembly code that is included with crafty so
you can access the BSF/BSR hardware instructions that are _very_ fast.
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