Pointers-to-members

[Mark Mitchell]

>>>>> "Jason" == Jason Merrill <jason at cygnus.com> writes:

    Jason> My guess would be that casting is more common than
    Jason> comparison, though I have no data for that. 

I agree.  That's in keeping with my second posting on this topic,
where I realized that making both fields be zero would penalize
casting:

  It's occurred to me that this imposes some overhead on casting
  pointers-to-members around: now when you convert from a base pointer
  to member to a derived version (or vice versa), you can't just adjust
  the `adj' member willy-nilly; instead, you have to check first whether
  or not the pointer is NULL.

So, it would be helpful if we were to add:

  (Note: the `adj' field is not necessarily zero even when the
   pointer-to-member is NULL.  Therefore, casting a
   pointer-to-derived-member to a pointer-to-base-member (or vice
   versa) requires only an adjustment to the `adj' field.  However,
   comparsion of two pointers-to-members requires more than
   a bitwise comparision.  Code equivalent to:
   
     p1.ptr == p2.ptr && (!p1.ptr || (p1.adj == p2.adj))

   is required since in the case that p1.ptr and p2.ptr are both 
   zero, there `adj' fields are irrelevant.)

to the ABI document.

   Jason> Also, we can
   Jason> optimize comparison to NULL by ignoring the adjustment
   Jason> entirely.

True -- in either scheme.  But an additional argument in favor of the
above resolution.

Thanks,

--
Mark Mitchell                   mark at codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com