[Bug middle-end/103310] New: null comparison with a weak symbol eliminated

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

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

            Bug ID: 103310
           Summary: null comparison with a weak symbol eliminated
           Product: gcc
           Version: 12.0
            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: ---

The following test case was prompted by the discussion in the review of a
-Waddress enhancement:
  https://gcc.gnu.org/pipermail/gcc-patches/2021-November/584749.html

Before folding the test in call_alias() to true, GCC issues a -Waddress.  At
runtime the program then crashes.  In the review Jason suggests GCC should
reject the subsequent declaration of the alias with an error because it has
been used (and because the test for null may have been eliminated).

cat a.c && gcc -DUSE_ALIAS -O2 -Wall -c a.c && gcc -O2 -Wall a.c a.o && ./a.out
#if USE_ALIAS

extern void alias (void);

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

  if (alias)
    alias ();
}

extern void alias (void)  __attribute__((weak));

#else

extern void alias (void)  __attribute__((weak));   // not defined

extern void call_alias (void);

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

  call_alias ();
}

#endif
a.c: In function ‘call_alias’:
a.c:9:7: warning: the address of ‘alias’ will always evaluate as ‘true’
[-Waddress]
    9 |   if (alias)
      |       ^~~~~
in main: alias = (nil)
in call_alias: alias = (nil)
Segmentation fault (core dumped)