[libc++] Add ABI flag to make __tree nodes more compact

Author: philnik777

libcxx/include/__configuration/abi.h

@@ -116,6 +116,7 @@
 // This setting disables the addition of such artificial padding, leading to a more optimal
 // representation for several types.
 #  define _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
+#  define _LIBCPP_ABI_TREE_POINTER_INT_PAIR
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support
@@ -135,6 +136,8 @@
 #  endif
 #endif
 
+#define _LIBCPP_ABI_TREE_POINTER_INT_PAIR
+
 // We had some bugs where we use [[no_unique_address]] together with construct_at,
 // which causes UB as the call on construct_at could write to overlapping subobjects
 //

libcxx/include/__tree

@@ -40,6 +40,7 @@
 #include <__utility/forward.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
+#include <__utility/pointer_int_pair.h>
 #include <__utility/swap.h>
 #include <limits>
 
@@ -654,6 +655,45 @@ public:
   __tree_node_base& operator=(__tree_node_base const&) = delete;
 };
 
+#ifdef _LIBCPP_ABI_TREE_POINTER_INT_PAIR
+template <>
+class __tree_node_base< void*> : public __tree_node_base_types<void*>::__end_node_type {
+  using _NodeBaseTypes = __tree_node_base_types<void*>;
+
+public:
+  using pointer          = typename _NodeBaseTypes::__node_base_pointer;
+  using __parent_pointer = typename _NodeBaseTypes::__parent_pointer;
+
+  pointer __right_;
+
+private:
+  using __pair_t = __pointer_int_pair<__parent_pointer, bool, __integer_width(1)>;
+
+  __pair_t __parent_and_color_;
+
+public:
+  _LIBCPP_HIDE_FROM_ABI pointer __parent_unsafe() const {
+    return static_cast<pointer>(__parent_and_color_.__get_ptr());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void __set_parent(pointer __ptr) { __set_parent(static_cast<__parent_pointer>(__ptr)); }
+
+  _LIBCPP_HIDE_FROM_ABI void __set_parent(__parent_pointer __ptr) {
+    __parent_and_color_ = __pair_t(__ptr, __parent_and_color_.__get_value());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI __parent_pointer __get_parent() const { return __parent_and_color_.__get_ptr(); }
+  _LIBCPP_HIDE_FROM_ABI __tree_color __get_color() const {
+    return static_cast<__tree_color>(__parent_and_color_.__get_value());
+  }
+  _LIBCPP_HIDE_FROM_ABI void __set_color(__tree_color __color) {
+    __parent_and_color_ = __pair_t(__parent_and_color_.__get_ptr(), __color == __tree_color::__black);
+  }
+};
+#endif // _LIBCPP_ABI_TREE_POINTER_INT_PAIR
+
+static_assert(sizeof(__tree_node_base<void*>) == 24);
+
 template <class _Tp, class _VoidPtr>
 class _LIBCPP_STANDALONE_DEBUG __tree_node : public __tree_node_base<_VoidPtr> {
 public: