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. Richard. > Thanks for your help > > Tobi > > > >