[Bug tree-optimization/114403] [14 regression] LLVM miscompiled with -O3 -march=znver2 -fno-vect-cost-model since r14-6822-g01f4251b8775c8

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114403

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
Maybe easier to understand testcase:

long x[9];
long a[20];
struct { long x; long b[40]; } b;
int __attribute__((noipa))
foo (int n)
{
  int i = 0;
  int k = 0;
  do
    {
      if (x[k++])  // early exit, loop upper bound is 8 because of this
        break;
      a[i] = b.b[2*i]; // the misaligned 2*i access causes peeling for gaps
    }
  while (++i < n);
  return i;
}

int main()
{
  x[8] = 1;
  if (foo (20) != 8)
    __builtin_abort ();
  return 0;
}

with -O3 -msse4.1 -fno-vect-cost-model we return 20 instead of 8.  Adding
-fdisable-tree-cunroll avoids the issue.  The upper bound we set on the
vector loop causes us to force taking the IV exit which continues
with i == (niter - 1) / VF * VF, but 'niter' is 20 here.