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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/46507] std::get and devirtualization on non-automatic variables
Date: Thu, 16 Jan 2014 06:10:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46507

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2014-01-16
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |rguenther at suse dot de
           Assignee|unassigned at gcc dot gnu.org      |hubicka at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #7 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
OK, GIMPLE representation is pre-inlining as follows:
void arg_tuple_test(const std::pair<A, A>&) (const struct pair & t)
{
  const struct type & _2;
  const struct type & _3;
  int (*__vtbl_ptr_type) () * _5;
  int (*__vtbl_ptr_type) () _6;

  <bb 2>:
  _2 = std::get<0ul, A, A> (t_1(D));
  _3 = _2;
  _5 = _3->_vptr.A;
  _6 = *_5;
  OBJ_TYPE_REF(_6;(const struct A)_3->0) (_3);
  return;

}
Here I do not think we can devirtualize, because std::get can return a derived
type in general.

After inlining we get:
void arg_tuple_test(const std::pair<A, A>&) (const struct pair & t)
{
  int (*__vtbl_ptr_type) () * _3;
  int (*__vtbl_ptr_type) () _4;
  const struct A & _6;

  <bb 2>:
  _6 = &t_1(D)->first;
  _3 = MEM[(const struct type *)t_1(D)]._vptr.A;
  _4 = *_3;
  OBJ_TYPE_REF(_4;(const struct A)_6->0) (_6); [tail call]
  return;

}
Here we look all the way to figuring out that the object comes as parameter of
arg_tuple_test and give up on this point.
I think GIMPLE typing rules does not really allow us to take the type from
COMPONENT_REF in statement defining _6, so it seems we completely lose track of
the type info here. (and it indeed looks like a common case)

Richard?