On Wed, 2009-10-07 at 17:23 +0200, Richard Guenther wrote:
> On Wed, Oct 7, 2009 at 5:16 PM, Tobias Grosser
> <gros...@fim.uni-passau.de> wrote:
> > On Wed, 2009-10-07 at 16:42 +0200, Richard Guenther wrote:
> >> On Wed, Oct 7, 2009 at 4:37 PM, Tobias Grosser
> >> <gros...@fim.uni-passau.de> wrote:
> >> > I try to analyse this code:
> >> > ------------------------------------------------------
> >> > int foo (int N)
> >> > {
> >> > int ftab[257];
> >> > int i, j;
> >> >
> >> > for (i = 0; i < N - 7488645; i++)
> >> > j = ftab [i];
> >> >
> >> > return j;
> >> > }
> >> > ------------------------------------------------------
> >> >
> >> > The number of iterations I get is:
> >> >
> >> > (unsigned int) N_5(D) + 0x0ffffffff
> >> >
> >> > However I expect it to be
> >> >
> >> > (unsigned int) N_5(D) + (-1)
> >>
> >> No, that would be (unsigned int) (N_5(D) + -1) instead.
> >>
> >> It's fold that canonicalizes this to the above form - you
> >> simply have to deal with it (unsigned arithmetic, that is).
> >
> > OK. So I need to understand this better.
> >
> > E.g:
> >
> > int foo (int N)
> > {
> > int ftab[257];
> > int i, j;
> >
> > for (i = 0; i < N - 50; i++)
> > j = ftab [i];
> >
> > return j;
> > }
> >
> > Number of latch executions: (unsigned int) N_5(D) + 0x0ffffffcd
> >
> > What happens if N == 5? I would expect the number of latch iterations to
> > be 0 as i < 5 - 50 is always false. However using the upper expression I
> > get something like
> > 5 + 0x0ffffffcd == 0x0ffffffd2
> > what is a lot bigger than 0.
>
> It's undefined if N == 5 because the loop counter would overflow.
Why?
The loop
for (i=0; i < 5 - 50; i++)
is equivalent to
for (i=0; i < -45; i++)
Which just evaluates to false and will not be executed. How can the loop
counter overflow?
Tobi
