[C++ bug] Recent vtable regression

[Nathan Sidwell]

Mark,   
your recent reorganisation of the vtable code has broken multiple
inheritance :-(
        2000-01-31  Mark Mitchell  <mark@codesourcery.com>
I attach several files,
        nathan93.ii - a source file which lays out some vtables
        nathan93.s - the compilation of nathan93.ii
        nathan94.C - a test case

This occurs both on i86-linux and sparc-solaris (though the vtable
structure is different for those architectures, the same effect
occurs). These examples are from i86-linux

nathan93.s was generated with -O2, that just cuts down its size, the
vtable is the same as -O0. The vtables to look at are,
__vt_5Multi:
        .long   0
        .long   __tf5Multi
        .long   _._5Multi
        .long   Wibble__4Core
        .long   Wobble__4Core
        .long   Bogus__5Multi
        .long   Bink__4Base
        .long   Bonk__4Base
        .long   Stomped__5Multi
        .long   Bunk__5Multi

which has a Stomped__5Multi as the penultimate entry. A derivation
of this class is Trail, who's vtable is,
__vt_5Trail:
        .long   0
        .long   __tf5Trail
        .long   _._5Trail
        .long   Wibble__4Core
        .long   Wobble__4Core
        .long   Bogus__5Multi
        .long   Bink__4Base
        .long   Bonk__4Base
        .long   Bogus__5Multi
        .long   Bunk__5Multi

which, you'll notice has two instances of Bogus__5Multi. The latter one
having clobbered Stomped__5Multi. It's something about the multiple
inheritance of Multi, and changing the number of virtual functions in
Side moves where the second Bogus__5Multi appears in Trail's vtable.

nathan
-- 
Dr Nathan Sidwell :: Computer Science Department :: Bristol University
Never hand someone a gun unless you are sure where they will point it
nathan@acm.org  http://www.cs.bris.ac.uk/~nathan/  nathan@cs.bris.ac.uk
struct Core
{
  virtual ~Core ();
  virtual void Wibble ();
  virtual void Wobble ();
  virtual void Bogus  ();  
};
struct Side
{
  virtual ~Side ();
  virtual void Arfle ();
  virtual void Barfle ();
  virtual void Gloop ();
  virtual void Glorp ();
  virtual void Glump ();
  virtual void Bogus  ();  
};
struct Base : Core
{
  virtual ~Base ();
  virtual void Bink ();
  virtual void Bonk ();
  virtual void  Bogus ();  
};
struct Multi : Base, Side
{
  virtual ~Multi ();
  virtual void Stomped ();
  virtual void Bunk ();
  virtual void Bogus  ();  
};
struct Trail : Multi
{
  virtual ~Trail ();
};
Multi::~Multi () {}
Trail::~Trail () {}