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

["mark at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From mark at codesourcery dot com  2005-09-28 20:28 -------
Subject: Re:  [4.1 Regression] push_fields_onto_fieldstack
 calculates offset incorrectly

jason at redhat dot com wrote:

> And besides, my approach isn't lying at all about this testcase; since 
> there's only one appearance of the virtual base, it appears exactly 
> where the field lists say it will.  Daniel agreed that the problem is in 
> his code.

What I meant by "lying" (an admittedly overly contentious choice of
word) was that the field for B-in-C is marked as having the complete B
type due to the code at the end of layout_class_type.  However, the A
virtual base isn't located in that B; it's located after the data
members of C.  So, the base field for B is incorrect.

I agree that using COMPONENT_REFs is good, but I think that the
FIELD_DECL for B should have the as-base type.  Then, when you actually
form the COMPONENT_REF, you should do the equivalent of:

  *(complete B*)&c.b_base

That will give the expression the correct type from the front-end's
point of view, but not result in inaccuracy with respect to the
TYPE_FIELDS for C.  Obviously, we need to make sure that B-as-base
complete-B are in the same alias set.

(It might be even better just to have the front end consider B-as-base
and complete-B as the same type, so that the expression could have the
more-accurate B-as-base type.  But, that would be a huge change to the
front end, so I think we have no choice but to use the complete-B type
for an expression like "*(B*)&c".)



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