[Bug c/103310] null comparison with a weak symbol eliminated

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103310

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |c

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The test case below shows that storing the address of the alias in a local
pointer avoids the problem.  The dumps show that the problem is in the front
end which folds the test of the address of the weak symbol to true.

$ cat pr103310.c && gcc -Wall -O2 -S -fdump-tree-original=/dev/stdout
-fdump-tree-optimized=/dev/stdout pr103310.c
extern void alias (void);

void call_alias (void)
{
  __builtin_printf ("in %s: alias = %p\n", __func__, alias);

  if (alias)
    alias ();
}

void call_ptr_alias (void)
{
  void (*p)(void) = alias;

  __builtin_printf ("in %s: alias = %p\n", __func__, p);

  if (p)
    p ();
}

extern void alias (void)  __attribute__((weak));
pr103310.c: In function ‘call_alias’:
pr103310.c:7:7: warning: the address of ‘alias’ will always evaluate as ‘true’
[-Waddress]
    7 |   if (alias)
      |       ^~~~~

;; Function call_alias (null)
;; enabled by -tree-original


{
  static const char __func__[11] = "call_alias";

    static const char __func__[11] = "call_alias";
  __builtin_printf ((const char *) "in %s: alias = %p\n", (const char *)
&__func__, alias);
  if (1)
    {
      alias ();
    }
}


;; Function call_ptr_alias (null)
;; enabled by -tree-original


{
  void (*<T349>) (void) p = alias;
  static const char __func__[15] = "call_ptr_alias";

    static const char __func__[15] = "call_ptr_alias";
    void (*<T349>) (void) p = alias;
  __builtin_printf ((const char *) "in %s: alias = %p\n", (const char *)
&__func__, p);
  if (p != 0B)
    {
      p ();
    }
}


;; Function call_alias (call_alias, funcdef_no=0, decl_uid=1945, cgraph_uid=1,
symbol_order=0)

void call_alias ()
{
  static const char __func__[11] = "call_alias";

  <bb 2> [local count: 1073741824]:
  __builtin_printf ("in %s: alias = %p\n", &__func__, alias);
  alias (); [tail call]
  return;

}



;; Function call_ptr_alias (call_ptr_alias, funcdef_no=1, decl_uid=1949,
cgraph_uid=2, symbol_order=1)

Removing basic block 5
void call_ptr_alias ()
{
  static const char __func__[15] = "call_ptr_alias";

  <bb 2> [local count: 1073741824]:
  __builtin_printf ("in %s: alias = %p\n", &__func__, alias);
  if (alias != 0B)
    goto <bb 3>; [53.47%]
  else
    goto <bb 4>; [46.53%]

  <bb 3> [local count: 574129753]:
  alias (); [tail call]

  <bb 4> [local count: 1073741824]:
  return;

}