[Bug middle-end/96900] New: bogus -Warray-bounds on strlen with valid pointer obtained from just-past-the-end

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96900

            Bug ID: 96900
           Summary: bogus -Warray-bounds on strlen with valid pointer
                    obtained from just-past-the-end
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When a valid pointer into an array that has been derived from a past-the-end
pointer to a member array of an initialized constant struct is used in a call
to a string built-in like strlen GCC issues a bogus -Warray-bounds warning
indicating that the offset into the array is out of its bounds.

$ cat q.c && gcc -S -Wall q.c
struct S { char n, a[3]; };

const char a[3] = { 2, 1, 0 };
const struct S s = { 3, { 2, 1, 0 } };

int f (void)
{
  const char *p = &a[sizeof a];
  return __builtin_strlen (p - sizeof a);      // no warning (good)
}

int g (void)
{
  const char *p = &s.a[sizeof s.a];
  return __builtin_strlen (p - sizeof s.a);    // bogus -Warray-bounds
}

q.c: In function ‘g’:
q.c:15:10: warning: offset ‘1’ outside bounds of constant string
[-Warray-bounds]
   15 |   return __builtin_strlen (p - sizeof s.a);    // bogus -Warray-bounds
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
q.c:4:16: note: ‘s’ declared here
    4 | const struct S s = { 3, { 2, 1, 0 } };
      |                ^