Scev analysing number of loop iterations returns (2^32-1) instead of -1

Richard Guenther richard.guenther@gmail.com
Wed Oct 7 16:30:00 GMT 2009


On Wed, Oct 7, 2009 at 5:35 PM, Tobias Grosser
<grosser@fim.uni-passau.de> wrote:
> On Wed, 2009-10-07 at 17:23 +0200, Richard Guenther wrote:
>> On Wed, Oct 7, 2009 at 5:16 PM, Tobias Grosser
>> <grosser@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
>> >> <grosser@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?

Ah, indeed.  Sorry for being confused.  Is tree-niter-desc->assumptions
or ->may_be_zero non-NULL?

RIchard.

> Tobi
>
>



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