[Bug libstdc++/77402] New: Use EBO for _Rb_tree_impl::_M_key_compare

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77402

            Bug ID: 77402
           Summary: Use EBO for _Rb_tree_impl::_M_key_compare
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: ABI
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

We could use the empty base-class optimization to reduce the size of RB trees
by combining the _Key_compare object with _M_node_count.

This would be an ABI break, so can only be enabled for the
gnu-versioned-namespace config.