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

[Andrew Pinski <pinskia@gmail.com>]

Sent from my iPhone

On May 17, 2010, at 8:37 PM, "eyakubovich at gmail dot com" <gcc-bugzilla@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
>