Re: [Bug c++/44186] New: Wrong code generated with -O2 and above

[Andrew Pinski]

Sent from my iPhone

On May 17, 2010, at 8:37 PM, "eyakubovich at gmail dot com" <gcc-bugzi...@gcc.gnu.org 
> wrote:

This is a stripped down code from proposed Boost.Move library.  
Asserts don't
fire with -O0 and -O1 but do with -O2 and -O3

#include <assert.h>

template <class T>
class rv : public T
{
  rv();
  ~rv();
  rv(rv const&);
  void operator=(rv const&);
};

template <class T>
rv<T>& move(T& x)
{
  return *static_cast<rv<T>* >(&x);
}

//A movable class
class movable
{
  movable(movable &);
  movable& operator=(movable&);
public:
  operator rv<movable>&()
  {  return *reinterpret_cast< rv<movable>* >(this);  }
  operator const rv<movable>&() const
  {  return *reinterpret_cast<const rv<movable>* >(this);  }

private:
  int value_;

public:
  movable() : value_(1){}

  //Move constructor and assignment
  movable(rv<movable>& m)
  {  value_ = m.value_;   m.value_ = 0;  }

  movable & operator=(rv<movable>& m)
  {  value_ = m.value_;   m.value_ = 0;  return *this;  }

  bool moved() const //Observer
  {  return value_ == 0; }
};

movable function(movable m)
{
  return movable(move(m));
}

int main()
{
  {
     movable m;
     movable m2(move(m));

The above code cause aliasing violations because you acess a movable  
object via a rv<movable> (from now on just refered to as rv for short)  
one. So you have a type rv who's base type is movable. note both rv  
and movable are non pods which I think removes the case for upcasting  
and then accessing the first member.


     assert(m.moved());
     assert(!m2.moved());
  }

  {
     movable m;
     movable m3(function(movable(move(m))));
     assert(m.moved());
     assert(!m3.moved());
  }

  {
     movable m;
     movable m4(function(move(m)));
     assert(m.moved());
     assert(!m4.moved());
  }

  return 0;
}


--
          Summary: Wrong code generated with -O2 and above
          Product: gcc
          Version: 4.4.4
           Status: UNCONFIRMED
         Severity: normal
         Priority: P3
        Component: c++
       AssignedTo: unassigned at gcc dot gnu dot org
       ReportedBy: eyakubovich at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44186