On Wed, 2009-10-07 at 18:30 +0200, Tobias Grosser wrote: > On Wed, 2009-10-07 at 17:44 +0200, Richard Guenther wrote: > > On Wed, Oct 7, 2009 at 5:35 PM, Tobias Grosser > > <gros...@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 > > >> <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? > > > > Ah, indeed. Sorry for being confused. Is tree-niter-desc->assumptions > > or ->may_be_zero non-NULL? > > Yes both. I attached the gdb content for both.
(gdb) call debug_generic_expr (ndesc.assumptions) 1 (gdb) call debug_generic_expr (ndesc.may_be_zero) 0 (gdb) call debug_tree (ndesc.assumptions) <integer_cst 0x29605658 type <boolean_type 0x296145b0 _Bool> constant 1> (gdb) call debug_tree (ndesc.may_be_zero) <integer_cst 0x29605620 type <boolean_type 0x296145b0 _Bool> constant 0> So it seems the "niter" expression should contain the correct solution for every N. The cases where "niter" is not valid are not fullfilled following the description in tree-flow.h Tobias