c++/8979: Layout of class with virtual inheritance problem on IA64

c++/8979: Layout of class with virtual inheritance problem on IA64

[johnb]

>Number:         8979
>Category:       c++
>Synopsis:       Layout of class with virtual inheritance problem on IA64
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Dec 17 05:46:01 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Eugeny Belov
>Release:        gcc 3.2 on United Linux 1.0 (IA64) (no more info is there)
>Organization:
>Environment:
United Linux 1.0 on IA64 box
>Description:
  I have got a testcase from Codesourcery`s ABI validation suite named virtual2.cpp, it fails with error code 8 on IA64. Here is shorter example of code.

#include <iostream>

#ifdef __ia64__
#define INT_PADDED      8
#else
#define INT_PADDED      4
#endif

typedef const ptrdiff_t*  vptr_t;  /* The type of a pointer to a vtable.  */

template <typename DERIVED, typename BASE>
inline ptrdiff_t 
base_offset_of () {
  DERIVED d;
  return ((char*) (BASE*) (&d)) - (char*) &d;
}

struct S {
  int i;
  int j;
};

struct T : virtual public S {
  int k;
};

struct U3 : virtual public T, virtual public S {
};

int main ()
 {
    if (base_offset_of<U3,S>() != (2*sizeof(vptr_t) + INT_PADDED)) {
      std::cout << "FAILED\n";
      return 8;
    }
  
  std::cout << "PASSED\n";
  return 0;
 }

  What I`ve got for U3 layout using g++ with -fdump-class-hierarchy key:

Class U3
   size=32 align=8
U3 (0x2000000000372080) 0 nearly-empty
    vptridx=0 vptr=((&U3::_ZTV2U3) + 32)
  T (0x2000000000372100) 8 virtual canonical
      subvttidx=16 vptridx=8 vbaseoffset=-24 vptr=((&U3::_ZTV2U3) + 56)
    S (0x2000000000372180) 20 virtual canonical
        vbaseoffset=-32
  S (0x2000000000372200) 20 virtual non-canonical
      vbaseoffset=-32

  It looks that class S is wrong aligned by 4 bytes instead of 8 as expected. As I understood there should be 4 bytes padding after the T::k member.

>How-To-Repeat:
Compile testcase in Description field with g++ 3.2 and run it, it will print "FAILED".
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="virtual2.cpp"
Content-Disposition: inline; filename="virtual2.cpp"

/***********************************************************************

  File:    virtual2.cpp
  Author:  CodeSourcery, LLC
  Date:    8/22/2000

***********************************************************************/

// Check that, when a class with a virtual base is included as a base
// class, its non-virtual size is used.

#include "abi-test.hpp"

struct S {
  int i;
  int j;
};

struct T : virtual public S {
  int k;
};

struct U1 : virtual public S, public T {
};

struct U2 : virtual public S, virtual public T {
};

struct U3 : virtual public T, virtual public S {
};

int main (void)
{
  // T is allocated first.
  ASSERT_BASE_OFFSET (U1, T, 0, 1);
  // And then the virtual base S.
  ASSERT_BASE_OFFSET (U1, S, 
		      (sizeof (vptr_t) 
		       + pad_size_to_alignment<int, vptr_t>()), 
		      2);
  // And the total size should not include two copies of S.
  ABI_TEST (sizeof (U1) == sizeof (T), 3);

  // Finally, allocate any direct or indirect virtual base classes
  // (except the primary base clas or any indirect primary base
  // classes) as we did non-virtual base classes, in inheritance graph
  // order.

  // U2's vtable pointer is allocated first.
  // S is allocated next.
  ASSERT_BASE_OFFSET (U2, S, sizeof (vptr_t), 4);
  // And then the virtual base T without its virtual base.
  ASSERT_BASE_OFFSET (U2, T, sizeof (vptr_t) + sizeof (S), 5);
  // And the total size should not include two copies of S.
  ABI_TEST (sizeof (U2) == (pad_size_to_alignment<int, vptr_t>() + 2 * sizeof (int) + 2*sizeof (vptr_t)), 6);

  // U3's vtable pointer is allocated first.
  // T is allocated next.
  ASSERT_BASE_OFFSET (U3, T, sizeof (vptr_t), 7);
  // And then the virtual base T without its virtual base.
  ASSERT_BASE_OFFSET (U3, S, (2*sizeof (vptr_t) + pad_size_to_alignment<int, vptr_t>()), 8);
  // And the total size should not include two copies of S.
  ABI_TEST (sizeof (U3) == (pad_size_to_alignment<int, vptr_t>() + 2 * sizeof (int) + 2*sizeof (vptr_t)), 9);

  return EXIT_SUCCESS;
}