[Bug tree-optimization/90753] New: missing -Warray-bounds with an extern index in out-of-bounds range

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90753

            Bug ID: 90753
           Summary: missing -Warray-bounds with an extern index in
                    out-of-bounds range
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Only the out-of-bounds access in f() below is diagnosed.  The same invalid
access in g() is not.  The 

$ cat a.c && gcc -O2 -S -Wall -Wextra a.c
extern char a[4];

int f (int i)
{
  if (i < 9 || 99 < i)
    i = 9;

  return a[i];
}

int g (void)
{
  extern int i;

  if (i < 9 || 99 < i)
    i = 9;

  return a[i];
}
a.c: In function ‘f’:
a.c:8:11: warning: array subscript 9 is above array bounds of ‘char[4]’
[-Warray-bounds]
    8 |   return a[i];
      |          ~^~~
a.c:1:13: note: while referencing ‘a’
    1 | extern char a[4];
      |             ^

The VRP dump for f shows

    i_4: int [9, 99]
    ...
    <bb 4> [local count: 1073741824]:
    # i_4 = PHI <9(2), i_5(D)(3)>
    _3 = a[i_4];
    _7 = (int) _3;
    return _7;
  }

while the same dump for g:

    i.3_4: VARYING
    ...
    <bb 4> [local count: 1073741824]:
    i.3_4 = i;
    _5 = a[i.3_4];
    _9 = (int) _5;
    return _9;
  }

For some reason the range for the index is not available in g.