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From: "kamkaz at windowslive dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102015] New: [missed optimization] Small memory overhead in
 _Rb_tree_impl (fix would require ABI break)
Date: Sun, 22 Aug 2021 17:36:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102015

            Bug ID: 102015
           Summary: [missed optimization] Small memory overhead in
                    _Rb_tree_impl (fix would require ABI break)
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kamkaz at windowslive dot com
  Target Milestone: ---

Current definition of _Rb_tree_key_compare causes size overhead for all
std::(multi)set/map-s:

  template<typename _Key_compare>
    struct _Rb_tree_key_compare
    {
      _Key_compare              _M_key_compare;
      ...
    };

If it were possible to change the ABI in the future, I think it can be impr=
oved
by empty-base-optimization for comparators that are not final classes - or =
by
adding  [[no_unique_address]] to _M_key_compare.=