Author: Robert Hyatt
Date: 08:14:30 02/19/04
Go up one level in this thread
On February 19, 2004 at 08:38:43, Dieter Buerssner wrote:
>While looking at your code:
>
>int static __inline__ FirstOne(BITBOARD word) {
> int dummy, dummy2;
> asm ("movl $-1, %1" "\n\t"
> "bsr %3, %0" "\n\t"
> "cmovz %1, %0" "\n\t"
> "bsr %2, %1" "\n\t"
> "setnz %b3" "\n\t"
> "addl $32,%1" "\n\t"
> "testb %b3, %b3" "\n\t"
> "cmovz %0, %1" "\n\t"
> "movl $63, %0" "\n\t"
> "subl %1, %0" "\n\t"
> : "=&q" (dummy), "=&q" (dummy2)
> : "q" ((int) (word>>32)), "q" ((int) word)
> : "cc");
> return (dummy);
>}
Where did that come from? That is not what I am using, so I now see why we are
seeing different results. That was an experiment to write a FirstOne() with no
jumps, just to see what happened. Here is the current code (inlinex86.h):
int static __inline__ FirstOne(BITBOARD word) {
int dummy, dummy2;
asm (" movl $63, %0" "\n\t"
" bsr %2, %1" "\n\t"
" jnz 1f" "\n\t"
" bsr %3, %1" "\n\t"
" jnz 2f" "\n\t"
" movl $64, %0" "\n\t"
" jmp 3f" "\n\t"
"1: addl $32,%1" "\n\t"
"2: subl %1, %0" "\n\t"
"3:"
: "=&q" (dummy), "=&q" (dummy2)
: "q" ((int) (word>>32)), "q" ((int) word)
: "cc");
return (dummy);
}
This has more jumps, but the other did two bsf/bsr's each time and then used
cmovxx to choose between the results. This is what is in the current version of
crafty on my ftp machine. And this is what I used when I said the inline was
faster than the old external asm that required a call.
>
>This seems risky. You are changing %3 by setnz %b3 without notifying the
>compiler. It is probably better, to have another output var dummy3, to notify
>the compiler of this. Perhaps the & for dummy2 is not needed, because the input
>is already consumed, when you first need it..
It was dangerous, and I had fixed that a while back, but I never really used
that code in a production version that I know of. Where did you find this? IE
if it is in any of the released... wait a minute. This was probably in older
versions but it was not used as there was no include for it. The new version
has a specific include for this header file now, but it also has the non-cmovxx
firstone/lastone asm too.
>
>Regards,
>Dieter
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