[Bug sanitizer/63697] New: -fsanitize=undefined doesn't detect some subtraction overflows

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63697

            Bug ID: 63697
           Summary: -fsanitize=undefined doesn't detect some subtraction
                    overflows
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: jakub at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org

__attribute__((noinline, noclone)) int
foo1 (int x, int y)
{
  return x - y;
}

__attribute__((noinline, noclone)) int
foo2 (int x, int y)
{
  unsigned int xa = (unsigned int) x - (__INT_MAX__ - 3);
  xa &= 3;
  x = __INT_MAX__ - 3 + xa;
  unsigned int ya = y + 1U;
  ya &= 1;
  y = ya - 1;
  return x - y;
}

int
main ()
{
  int xm1, y;
  for (xm1 = __INT_MAX__ - 4; xm1 < __INT_MAX__; xm1++)
    for (y = -1; y <= 0; y++)
      if (foo1 (xm1 + 1, y) != (int) (xm1 + 1U - y)
          || foo2 (xm1 + 1, y) != (int) (xm1 + 1U - y))
        __builtin_abort ();
  return 0;
}

detects, due to bug in VRP, at -O2 -fsanitize=undefined just the overflow in
foo1, but not the one in foo2.