[Bug c++/45246] New: optimizer dereference

[attardi at di dot unipi dot it]

Code produced using -O2 handles dereferencing incorrectly.
Here is a program that shows the bug:

#include <iostream>
#include <vector>

class Derived : public std::vector<char *>
{
public:
   Derived() {}
};

void* foo(void* arg) {
     void* baseptr = 0;
     *(std::vector<char*> **)&baseptr = (Derived *)arg;
     return baseptr;
}

void* foo2(void* arg) {
     void* baseptr = 0;
     *(std::vector<char*> **)&baseptr = *(Derived **)&arg;
     return baseptr;
}

int main()
{
   Derived* d = new Derived();
   void* upcast = foo(d);
   void* upcast2 = foo2(d);
   std::cerr << d << ", " << upcast << ", " << upcast2 << std::endl;
}

If you compile without -O2 the two values upcast and upcast2 are the 
same, as they should be.
Using -O2 the results are different.

The bug occurrs with both:

g++ (GCC) 4.1.2 20070925 (Red Hat 4.1.2-27)
g++ (Ubuntu 4.4.1-4ubuntu9) 4.4.1

This kind of code is generated by SWIG for Java, so I can't change the source.

Curiously the bug disappears if you put a print statement before the 
assignment to baseptr.


-- 
           Summary: optimizer dereference
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: attardi at di dot unipi dot it
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45246