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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102103] New: missing warning for arrays
Date: Fri, 27 Aug 2021 19:21:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102103

            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=3D/dev/stdout z.c
void f (void)
{=20
  int i;
  if (&i =3D=3D 0)   // -Waddress (good)
    __builtin_abort ();
}

void g (void)
{
  int a[1];
  if (a =3D=3D 0)   // missing warning
    __builtin_abort ();
}

void h (void)
{
  int a[1][2];
  if (a[0] =3D=3D 0)   // missing warning
    __builtin_abort ();
}

z.c: In function =E2=80=98f=E2=80=99:
z.c:4:10: warning: the comparison will always evaluate as =E2=80=98false=E2=
=80=99 for the
address of =E2=80=98i=E2=80=99 will never be NULL [-Waddress]
    4 |   if (&i =3D=3D 0)   // -Waddress (good)
      |          ^~

;; Function f (f, funcdef_no=3D0, decl_uid=3D1943, cgraph_uid=3D1, symbol_o=
rder=3D0)

void f ()
{
  int i;

  <bb 2> :
  i =3D{v} {CLOBBER};
  return;

}



;; Function g (g, funcdef_no=3D1, decl_uid=3D1947, cgraph_uid=3D2, symbol_o=
rder=3D1)

void g ()
{
  int a[1];

  <bb 2> :
  a =3D{v} {CLOBBER};
  return;

}



;; Function h (h, funcdef_no=3D2, decl_uid=3D1951, cgraph_uid=3D3, symbol_o=
rder=3D2)

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

  <bb 2> :
  a =3D{v} {CLOBBER};
  return;

}


Clang diagnoses both the one in f() and in g() (but not the one in h()).  G=
CC
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.=