[Bug c++/95976] [[no_unique_address]] on union members has the opposite-of-intended effect

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95976

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-06-30
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |ABI
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |redi at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
One way to fix this would be:
--- gcc/cp/class.c.jj   2020-06-27 12:35:11.935311848 +0200
+++ gcc/cp/class.c      2020-06-30 14:18:08.448067688 +0200
@@ -6575,7 +6575,9 @@ layout_class_type (tree t, tree *virtual
          /* We must also reset the DECL_MODE of the field.  */
          SET_DECL_MODE (field, TYPE_MODE (type));
        }
-      else if (might_overlap && is_empty_class (type))
+      else if (might_overlap
+              && is_empty_class (type)
+              && TREE_CODE (t) != UNION_TYPE)
        {
          DECL_FIELD_ABI_IGNORED (field) = 1;
          layout_empty_base_or_field (rli, field, empty_base_offsets);
Another might be not to change DECL_FIELD_OFFSET in layout_empty_base_or_field
for TREE_CODE (rli->t) == UNION_TYPE, etc.
The question is if [[no_unique_address]] members of a union should affect any
ABI decisions (such as passing the union by value) or not.
Looking at the config/*/* code, in most cases the targets ignore
DECL_FIELD_ABI_IGNORED on UNION_TYPE members, but ia64 does not, and I think
rs6000_special_round_type_align and darwin_rs6000_special_round_type_align
don't ignore it either.