[Bug tree-optimization/66652] New: try_transform_to_exit_first_loop_alt generates incorrect loop

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66652

            Bug ID: 66652
           Summary: try_transform_to_exit_first_loop_alt generates
                    incorrect loop
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

This testcase fails on trunk:
...
$ cat libgomp/testsuite/libgomp.c/parloops-exit-first-loop-alt-5.c
/* { dg-do run } */
/* { dg-options "-O2 -ftree-parallelize-loops=2" } */

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

unsigned int
__attribute__((noclone,noinline))
f (unsigned int n, unsigned int sum)
{
  unsigned int i;

  i = UINT_MAX;
  do
    {
      sum += i % 13;
      i++;
    }
  while (i < n - 1);

  return sum;
}

int
main (void)
{
  unsigned int actual = f (0, 12);
  unsigned int expected = 4294967290;

  if (actual != expected)
    abort ();

  return 0;
}
...

try_transform_to_exit_first_loop_alt changes the loop into this, and the loop
is never entered:
...
  i = UINT_MAX;
  while (i < n)
    {
      sum += i % 13;
      i++;
    }
...