On Sun, Nov 7, 2010 at 5:22 PM, Artyom Tarasenko <atar4q...@gmail.com> wrote:
> Can it be that bneg,a branches unconditionally, or annuls unconditionally?
>
> 0xf0071520: subcc %g3, %g2, %g3
> => 0xf0071524: bneg,a 0xf007152c
> 0xf0071528: clr %g3
> 0xf007152c: st %g3, [ %i0 + 0x58 ]
> (gdb) info registers g3 psr
> g3 0x18 24
> psr 0x4000ae7 [ #0 #1 #2 ET PS S #9 #11 #26 ]
> (gdb) nexti
> 0xf007152c in ?? ()
>
> 0xf0071528 is supposed to be executed. Or it a gdb stub bug?
It should not be executed. Since N flag is not set and this is an
ICC-conditional branch, the delay instruction is annulled. See V8
manual B.21, page 120.
The following program produces the same results natively and with QEMU:
$ cat bneg.c
#include <stdio.h>
long f(long val)
{
long ret;
asm("tst %1\n\t"
"clr %0\n\t"
"bneg,a 1f\n\t"
"or %0, 1, %0\n\t"
"or %0, 2, %0\n\t"
"or %0, 4, %0\n\t"
"1: \n\t"
: "=r" (ret) : "r" (val));
return ret;
}
int main(int argc, const char **argv)
{
long x;
x = -1;
printf("f(0x%lx) = 0x%lx\n", x, f(x));
x = 0;
printf("f(0x%lx) = 0x%lx\n", x, f(x));
return 0;
}
$ gcc -o bneg bneg.c
$ ./bneg
f(0xffffffff) = 0x1
f(0x0) = 0x6
$ qemu-sparc32plus ./bneg
f(0xffffffff) = 0x1
f(0x0) = 0x6
