[Bug c++/54590] New: Incorrect multi-inheritence bases layout

[Bug c++/54590] New: Incorrect multi-inheritence bases layout

[wingfire at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54590

             Bug #: 54590
           Summary: Incorrect multi-inheritence bases layout
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: wingfire@gmail.com


the code:

#include <iostream>

struct A{ virtual void foo() {} };                                              

struct B { void * data; };

struct X : public B, A { };

int main(){

    X x;
    if((void*)&x == (void*)(&x.data))
        std::cout << "correct\n";
    else
        std::cout << "oops...\n";
    return 0;
}

Expect:
print "correct"

Result:
print "oops..."

Notes:
if A::foo is not virtual, the result is correct.

[Bug c++/54590] Incorrect multi-inheritence bases layout

[schwab@linux-m68k.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54590

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> 2012-09-15 15:02:12 UTC ---
The first word contains a pointer to the vtable for X.

[Bug c++/54590] Incorrect multi-inheritence bases layout

[wingfire at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54590

--- Comment #2 from Sun, Chaoyang <wingfire at gmail dot com> 2012-09-16 11:38:24 UTC ---
(In reply to comment #1)
> The first word contains a pointer to the vtable for X.

If the first word is the vtable of X, the memory layout should be:
{
vptr of x;
data of B;
vptr of A;
}?
but the actual layout is:
{
vptr of X and A
data of B;
}

g++ swapped the memory ordered of A and B.

also,if I have 
struct C{ virtual void foo() {} }; 
and X is defined:

struct X : public B, A,C { };

the actual layout will be:
{
vptr of X and A
data of B;
vptr of C;
}

Is it expected too? If so, it's too difficult to predicate the memory order of
each bases.

[Bug c++/54590] Incorrect multi-inheritence bases layout

[wingfire at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54590

Sun, Chaoyang <wingfire at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |

--- Comment #3 from Sun, Chaoyang <wingfire at gmail dot com> 2012-09-16 11:40:09 UTC ---
Back to unconfirmed. Maybe it's a "feature"?

[Bug c++/54590] Incorrect multi-inheritence bases layout

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54590

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |paolo.carlini at oracle dot
                   |                            |com
         Resolution|                            |INVALID

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-09-16 12:37:22 UTC ---
When you say "should be", are you referring to the Itanium ABI? Because that is
the reference, and unless you have solid evidence that we are misimplementing
the ABI, this is not a Bug.