[Bug c++/12277] eliminate and warn on dynamic casts with known NULL results

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
           Priority|P2                          |P3
   Last reconfirmed|2006-02-26 19:21:22         |2017-1-11
                 CC|                            |msebor at gcc dot gnu.org
            Summary|Warn on dynamic casts with  |eliminate and warn on
                   |known NULL results.         |dynamic casts with known
                   |                            |NULL results
      Known to fail|                            |3.3, 4.5.3, 4.8.3, 4.9.3,
                   |                            |5.3.0, 6.3.0, 7.0

--- Comment #10 from Martin Sebor <msebor at gcc dot gnu.org> ---
Besides warning, GCC could also optimize the dynamic_cast away when it's known
to fail.  The GCC 7.0 dump below for a slightly modified version of the test
case from comment #3 shows that it does neither.  At the same time, since this
is a rare case, I don't think it justifies P2.  Lowering to P3.

$ gcc$ cat t.C && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout
t.C
struct Base { virtual ~Base () {} }; 
class Derived : Base { friend int main (); }; 

int main () 
{ 
  Base *p = new Derived; 

  if (dynamic_cast<Derived *>(p) != 0)
      __builtin_abort ();
} 

...

;; Function int main() (main, funcdef_no=4, decl_uid=2303, cgraph_uid=10,
symbol_order=10) (executed once)

int main() ()
{
  void * _1;
  void * _4;

  <bb 2> [100.00%]:
  _4 = operator new (8);
  MEM[(struct Derived *)_4].D.2316._vptr.Base = &MEM[(void *)&_ZTV7Derived +
16B];
  _1 = __dynamic_cast (_4, &_ZTI4Base, &_ZTI7Derived, -2);
  if (_1 != 0B)
    goto <bb 3>; [0.04%]
  else
    goto <bb 4>; [99.96%]

  <bb 3> [0.04%]:
  __builtin_abort ();

  <bb 4> [99.96%]:
  return 0;

}