[Bug c/102103] New: missing warning for arrays

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102103

            Bug ID: 102103
           Summary: missing warning for arrays
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC issues -Waddress only for the equality test in f() but not those in g() or
h(), even though all three evaluate to false (and even though GCC folds all
three to false even with no optimization).  It would be helpful to diagnose all
three.

$ cat z.c && gcc -S -Wall -fdump-tree-optimized=/dev/stdout z.c
void f (void)
{ 
  int i;
  if (&i == 0)   // -Waddress (good)
    __builtin_abort ();
}

void g (void)
{
  int a[1];
  if (a == 0)   // missing warning
    __builtin_abort ();
}

void h (void)
{
  int a[1][2];
  if (a[0] == 0)   // missing warning
    __builtin_abort ();
}

z.c: In function ‘f’:
z.c:4:10: warning: the comparison will always evaluate as ‘false’ for the
address of ‘i’ will never be NULL [-Waddress]
    4 |   if (&i == 0)   // -Waddress (good)
      |          ^~

;; Function f (f, funcdef_no=0, decl_uid=1943, cgraph_uid=1, symbol_order=0)

void f ()
{
  int i;

  <bb 2> :
  i ={v} {CLOBBER};
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1947, cgraph_uid=2, symbol_order=1)

void g ()
{
  int a[1];

  <bb 2> :
  a ={v} {CLOBBER};
  return;

}



;; Function h (h, funcdef_no=2, decl_uid=1951, cgraph_uid=3, symbol_order=2)

void h ()
{
  int a[1][2];

  <bb 2> :
  a ={v} {CLOBBER};
  return;

}


Clang diagnoses both the one in f() and in g() (but not the one in h()).  GCC
4.4.7 does as well but the latter has been lost since GCC 4.5  (see
https://gcc.godbolt.org/z/8dbjxv89E) so technically this could be viewed as a
regression.