Vcall offsets

[Mark Mitchell]

I apologize to everyone for being confusing via proxy last week.

Here was the example I was trying to get at:

  class A { virtual void f (); };
  class B1 : public A {};
  class B2 : public A {};
  class D : public B1, public B2 {};

Now, suppose that `D' is used as a virtual base.  The question is
whether or not there should be vcall offsets for both instances of
`f', even though they have the same signature.  This paragraph:

  If the above listing of vcall offsets includes more than one for a
  particular virtual function signature, only the first one (closest
  to the vtable address point) is allocated. That is, an offset from
  primary base P (and its non-virtual bases) eliminates any from A or
  its other bases, an offset from A eliminates any from the
  non-primary bases, and an offset from a non-primary base B of A
  eliminates any from the bases of B.

implies that there should be only one such offset, which I believe to
be reasonable.  But, what about:

  class A { virtual void f (); };
  class B1 : public A { virtual void f (); };
  class B2 : public A {};
  class D : public B1, public B2 {};

Here, do we get two offsets (for B1::f and A::f) or just one?

Frankly, I think we should just drop the paragraph quoted above.  I
challenge anyone to present a real-world program that suffers any
significant space or time penalty because of wasting a few extra vcall
offset entries in the vtable.  The implementation complexity, and the
time we have to spend thinking about the issues, is not worth it.  I
suggest we just replace that paragraph with:

  (Note: there may be more than one vcall offset entry for the same
   virtual function if that virtual function appears more than once
   in the various vtables considered.)

Thoughts?

--
Mark Mitchell                   mark at codesourcery.com
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