[Bug c++/26577] [4.0/4.1/4.2 regression] ICE in cp_expr_size with volatile and call to static

[mmitchel at gcc dot gnu dot org]

------- Comment #6 from mmitchel at gcc dot gnu dot org  2006-06-05 22:37 -------
An even smaller test case is:

==
struct A
{
  A(const A&);
  A& operator=(const A&);
  void baz() volatile;
};
void A::baz() volatile
{
  *this;
}
==

The problem is that the middle-end is creating a copy of an object of type A --
and C++ front-end has an assert designed to prevent exactly that, since copies
are supposed to be done by the copy constructor.

The code in the middle-end is in fact unsafe (and so we cannot just remove the
C++ assert) because for:

  struct B { ... };
  struct D : virtual public B { ... };

  void f(volatile D* p) {
    *p;
  }

the type of "*p" will be "D" -- but we cannot safely read sizeof (D) bytes from
*p, as sizeof (D) includes the size of the virtual base, and the virtual base
may be located elsewhere.  (If X derives from D virtually, then the B part of X
may be located in memory below the D part of X.)  If the C++ front end said
that "p" points only to a D-as-base (as agreed elsewhere would be a good idea),
then the middle end would still do the wrong thing, as it would touch only part
of *p.

Therefore, for a TREE_ADDRESSABLE type, I don't think the middle-end can or
should do anything for a volatile access.  It should just ignore the
volatility.

Diego, what say you?


-- 

mmitchel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dnovillo at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26577