[Bug tree-optimization/80776] -Wformat-overflow false positive for %d on integer bounded by __builtin_unreachable

[eggert at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80776

--- Comment #3 from Paul Eggert <eggert at gnu dot org> ---
(In reply to Richard Biener from comment #1)
> Possibly the walk in remove_range_assertions visits the latter before the
> former block but in principle we do have code to handle this there.

I just ran into the same problem again, with the following code derived from
GNU Emacs, and it suggests that your diagnosis is correct. Perhaps this can be
used in a test case once the bug is fixed. If I remove the "if (! (0 <= i))
__builtin_unreachable ();" the bogus warning goes away, which suggests that the
earlier test is hiding the later one somehow. This is with GCC 7.2.1 20170915
(Red Hat 7.2.1-2) on x86-64.

extern __inline __attribute__ ((__always_inline__)) __attribute__
((__gnu_inline__)) __attribute__ ((__artificial__)) int
__attribute__ ((__nothrow__ , __leaf__)) sprintf (char *__restrict __s, const
char *__restrict __fmt, ...)
{
  return __builtin___sprintf_chk (__s, 2 - 1,
      __builtin_object_size (__s, 2 > 1), __fmt, __builtin_va_arg_pack ());
}
char number[sizeof "999999"];
int somerandom (void);
void
Foo (void)
{
  int i = somerandom ();
  if (! (0 <= i))
    __builtin_unreachable ();
  if (! (0 <= i && i <= 999999))
    __builtin_unreachable ();
  sprintf (number, "%d", i);
}

$ gcc -Wformat-overflow -O2 -S v.i
v.i: In function ‘Foo’:
v.i:17:21: warning: ‘%d’ directive writing between 1 and 10 bytes into a region
of size 7 [-Wformat-overflow=]
   sprintf (number, "%d", i);
                     ^~
v.i:17:20: note: directive argument in the range [0, 2147483647]
   sprintf (number, "%d", i);
                    ^~~~
v.i:4:10: note: ‘__builtin___sprintf_chk’ output between 2 and 11 bytes into a
destination of size 7
   return __builtin___sprintf_chk (__s, 2 - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       __builtin_object_size (__s, 2 > 1), __fmt, __builtin_va_arg_pack ());
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~