On 26-07-2011 18:37, Andrew Haley wrote:
On 07/26/2011 11:35 AM, Andrew Haley wrote:
Consider
for (i = 0; i< limit; i++)
f(i * 2);
which can be rewritten to
int tmp = limit * 2;
for (i = 0; i< tmp; i += 2)
f(i);
I tried your example. It doesn't make an induction variable because it
can do i*2 with the LEA instruction at no extra cost. With i*11 it makes
two counters:
for (i=0, k=0; i<limit; i++, k+=11) f(k);
I can't make it do what we both expect unless limit is a compile-time
constant:
for (k=0; k<limit*11; k+=11) f(k);
The reason I'm asking is that it would be interesting to know whether it
is better to use signed or unsigned integers in order to get the best
optimizations. I would publish that in my optimization manuals (
http://www.agner.org/optimize/#manuals )
Can you give any examples where optimization is better for signed
integers than for unsigned, or vice versa?
