On Thu, Aug 16, 2007 at 08:50:30PM -0700, Linus Torvalds wrote:
> Just try it yourself:
>
> volatile int i;
> int j;
>
> int testme(void)
> {
> return i <= 1;
> }
>
> int testme2(void)
> {
> return j <= 1;
> }
>
> and compile with all the optimizations you can.
>
> I get:
>
> testme:
> movl i(%rip), %eax
> subl $1, %eax
> setle %al
> movzbl %al, %eax
> ret
>
> vs
>
> testme2:
> xorl %eax, %eax
> cmpl $1, j(%rip)
> setle %al
> ret
>
> (now, whether that "xorl + setle" is better than "setle + movzbl", I don't
> really know - maybe it is. But that's not thepoint. The point is the
> difference between
>
> movl i(%rip), %eax
> subl $1, %eax
>
> and
>
> cmpl $1, j(%rip)
gcc bugzilla bug #33102, for whatever that ends up being worth. ;-)
Thanx, Paul
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