Re: sizeof an object that have same base class as member objects.

Note: This problem is related to gcc but after some back and forth in
group gnu.gcc.help it seems to have morph into more of a c++
specificiation question, hence the transplanting to this group.

The original post at gnu.gcc.help can be found at this link
http://groups.google.com/group/gnu.gcc.help/browse_thread/thread/ece5...

Here's the question:

class base {
public:
base(){};
~base(){};
};

class data : public base {
public:
data(){};
~data(){};
private:
int member;
}__attribute__((__packed__));

class group : public base {
public:
group(){};
~group(){};
private:
data d1;
data d2;
data d3;
} __attribute__((__packed__));

int main(int argc, char **argv) {
std::cout << "base = " << sizeof(base) << std::endl;
std::cout << "data = " << sizeof(data) << std::endl;
std::cout << "group = " << sizeof(group) << std::endl;
return (0);
}

The output of the program is:
base = 1
data = 4
group = 13

The result of sizeof(group) is puzzling as it should be 12
if EBO (empty base optimization) worked for both class data
and group. Apparently EBO kicked in for _ONLY_ one of them.
If EBO didn't work at all, sizeof(group) should be 16.

Removing the extension of class base from either class group
or data will cause sizeof(group) to return 12. It seems that
gcc is unable to fully apply EBO when a class and its member
inherits the same empty base class.

The same code had been tested on microsoft msvc compiler and
realview arm compiler, both correctly optimizes the code and
give the correct value as 12.

Is this a known bug with gcc 3.4.5? (Note: I'm using MinGW)