[Bug tree-optimization/22488] [4.1 Regression] push_fields_onto_fieldstack calculates offset incorrectly

["mmitchel at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2005-09-28 15:57 -------
> If so, are those fields of "A in B" really *there*, at those offsets
> from the beginning of type B?

Sort of.  Because B would otherwise need a virtual table pointer, it just uses
the one in A.  So, there is a vptr at offset zero from the start of B.  However,
there's no actual A object at the same address as B because A is a virtual base.
 There's certainly no A::i at offset four from the start of B, for example.
 
> The layout you had posted in the bug matches the offsets when we process
> the type A field of C, but not the type A field of type B.

Ugh.

We create two versions of class types: the "complete" class type and the
"as-base" type.  (Look for CLASSTYPE_AS_BASE.)  The as-base type does not
include virtual bases, and the B field in C should have the B-as-base type, not
the complete B type.  The B-as-base type does not have the A parts.  In fact, if
you look at layout_class_type, and, in particular, build_base_field, you'll see
it does use the as-base type.

However, at the end of layout_class_type, we have:

  /* Now that we're done with layout, give the base fields the real types.  */

and that installs the complete type.  

Jason added this in 2004:

+ 2004-04-30  Jason Merrill  <jason@redhat.com>
+
+       Refer to base members using COMPONENT_REFs where possible.
+       * class.c (build_simple_base_path): New fn.
+       (build_base_path): Use it for non-virtual base references.
+       (layout_class_type): Change base fields to their real type
+       after layout is done.

I think this is a hack so that later, when we use the COMPONENT_REFs, we don't
have to cast from the as-base type back to the nominal C++ type.  (Of course,
this would also be cleaner with a real C++-lowering pass.)

However, I think the right thing is to have the cast; as you've discovered,
Jason's approach is lying about what's really there.  Jason?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22488