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

["jason at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #42 from jason at redhat dot com  2005-10-04 21:05 -------
Subject: Re:  [4.1 Regression] push_fields_onto_fieldstack
 calculates offset incorrectly

mark at codesourcery dot com wrote:
> So, I guess maybe we agree on the right plan.  In particular:
> 
> 1. When we see "T*" or "T&", use the as-base version of T for the type.
>  Therefore, for expressions like "*p" where "p" has type "T*", we would
> give the expression the as-base version of T.  However, for expressions
> like "t" (where "t" is of type "T"), use the complete version of "t".

Yep.

> 2. Undo your change at the end of layout_class_type, so that base fields
> have the as-base type.

Agreed.

> We could implement (1) either by (a) doing that directly during parsing,
> etc., or (b) by handling it during genericization/gimplification.  I
> vote for the latter.  It should be relatively simple; do a pre-order
> walk of the expression tree, and adjust the types of things from the
> bottom up.
> 
> I'm not sure what else we would need to do.  We need to tell the
> optimizers that the as-base type can alias the complete type; I guess we
> just give them the same alias set.

I was suggesting that instead of making the complete type and base type 
essentially copies of one another, that we make the complete type a 
wrapper around the base type.  Proper alias semantics would fall out of 
that.

> We also need the optimizers to
> understand that "*p = *q" is going to copy TYPE_SIZE (complete type)
> bytes...

We don't do bitwise assignment if a class has a vptr, so whenever we 
generate something like that the complete type and base type should be 
the same.

Jason


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