[Bug tree-optimization/32073] Loop unrolling does not exploit VRP for loop bound

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32073

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note there is a few things here that can be improved even more.
With a slightly different testcase:
void short_loop(int* __restrict dest, int* __restrict src, int count) {
  // same happens for assert(count <= 4) and if(count > 4) exit(-1)
  if(count > 4)
    count = 4;

  for(int i=0; i < count; i++)
    dest[i] = src[i] + 1;
}

In VRP2 we have:
  _5 = MIN_EXPR <count_10(D), 4>;
  if (count_10(D) > 0)
    goto <bb 3>; [80.00%]
  else
    goto <bb 9>; [20.00%]

  <bb 3> [local count: 171798694]:
  _23 = (unsigned int) _5;
  niters.4_24 = _5 > 0 ? _23 : 1;

Which we don't optimize niters.4_24 to just _23.

Without the +1 we optimize the above to memcpy (and still have the above issue
too) and the expansion there needs help (which I thought I filed as another bug
but I can't find it).
There might be other issues where the vectorizer and such don't see that the
count <= 4 and count >= 1.