[Bug tree-optimization/89145] GCC does not assume that two different external variables have different addresses

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89145

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-02-01
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
I can confirm this, though it's a known limitation documented in the
symtab_node::equal_address_to() function in symtab.c, and there probably is at
least one bug about it in Bugzilla.

  /* TODO: Alias oracle basically assume that addresses of global variables
     are different unless they are declared as alias of one to another while
     the code folding comparsions doesn't.
     We probably should be consistent and use this fact here, too, but for
     the moment return false only when we are called from the alias oracle.  */

This refers to assumptions about extern variables being distinct such as in the
test case below:

  int bar (void)
  {
    int tmp = a;
    b = 0;
    if (tmp != a)           // folded to false
      __builtin_abort ();   // eliminated
  }

As a data point, Clang folds the the equality in the test case in comment #0 to
false.

That said, GCC itself makes it easy to violate this assumptions by making it
possible to define aliases such as in:

  int a;
  extern __attribute__ ((alias ("a"))) int b;

  extern int foo (void);

  int main (void)
  {
    int i = foo ();
    __builtin_printf ("%i\n", i);
  }