[libcxx-commits] [libcxx] [libc++] Replace `__compressed_pair` with `[[no_unique_address]]` (PR #76756)

[Louis Dionne via libcxx-commits]

================
@@ -26,6 +27,128 @@
 
 #include "test_macros.h"
 
+namespace std {
+struct __default_init_tag {};
+struct __value_init_tag {};
+
+template <class _Tp, int _Idx, bool _CanBeEmptyBase = is_empty<_Tp>::value && !__libcpp_is_final<_Tp>::value>
+struct __compressed_pair_elem {
+  using _ParamT         = _Tp;
+  using reference       = _Tp&;
+  using const_reference = const _Tp&;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem(__default_init_tag) {}
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem(__value_init_tag) : __value_() {}
+
+  template <class _Up, class = __enable_if_t<!is_same<__compressed_pair_elem, __decay_t<_Up> >::value> >
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem(_Up&& __u)
+      : __value_(std::forward<_Up>(__u)) {}
+
+#ifndef _LIBCPP_CXX03_LANG
+  template <class... _Args, size_t... _Indices>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 explicit __compressed_pair_elem(
+      piecewise_construct_t, tuple<_Args...> __args, __tuple_indices<_Indices...>)
+      : __value_(std::forward<_Args>(std::get<_Indices>(__args))...) {}
+#endif
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 reference __get() _NOEXCEPT { return __value_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR const_reference __get() const _NOEXCEPT { return __value_; }
+
+private:
+  _Tp __value_;
+};
+
+template <class _Tp, int _Idx>
+struct __compressed_pair_elem<_Tp, _Idx, true> : private _Tp {
+  using _ParamT         = _Tp;
+  using reference       = _Tp&;
+  using const_reference = const _Tp&;
+  using __value_type    = _Tp;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem() = default;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem(__default_init_tag) {}
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem(__value_init_tag) : __value_type() {}
+
+  template <class _Up, class = __enable_if_t<!is_same<__compressed_pair_elem, __decay_t<_Up> >::value> >
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair_elem(_Up&& __u)
+      : __value_type(std::forward<_Up>(__u)) {}
+
+#ifndef _LIBCPP_CXX03_LANG
+  template <class... _Args, size_t... _Indices>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17
+  __compressed_pair_elem(piecewise_construct_t, tuple<_Args...> __args, __tuple_indices<_Indices...>)
+      : __value_type(std::forward<_Args>(std::get<_Indices>(__args))...) {}
+#endif
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 reference __get() _NOEXCEPT { return *this; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR const_reference __get() const _NOEXCEPT { return *this; }
+};
+
+template <class _T1, class _T2>
+class __compressed_pair : private __compressed_pair_elem<_T1, 0>, private __compressed_pair_elem<_T2, 1> {
+public:
+  // NOTE: This static assert should never fire because __compressed_pair
+  // is *almost never* used in a scenario where it's possible for T1 == T2.
+  // (The exception is std::function where it is possible that the function
+  //  object and the allocator have the same type).
+  static_assert(
+      (!is_same<_T1, _T2>::value),
+      "__compressed_pair cannot be instantiated when T1 and T2 are the same type; "
+      "The current implementation is NOT ABI-compatible with the previous implementation for this configuration");
+
+  using _Base1 _LIBCPP_NODEBUG = __compressed_pair_elem<_T1, 0>;
+  using _Base2 _LIBCPP_NODEBUG = __compressed_pair_elem<_T2, 1>;
+
+  template <bool _Dummy = true,
+            class       = __enable_if_t< __dependent_type<is_default_constructible<_T1>, _Dummy>::value &&
+                                   __dependent_type<is_default_constructible<_T2>, _Dummy>::value > >
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair()
+      : _Base1(__value_init_tag()), _Base2(__value_init_tag()) {}
+
+  template <class _U1, class _U2>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __compressed_pair(_U1&& __t1, _U2&& __t2)
+      : _Base1(std::forward<_U1>(__t1)), _Base2(std::forward<_U2>(__t2)) {}
+
+#ifndef _LIBCPP_CXX03_LANG
+  template <class... _Args1, class... _Args2>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 explicit __compressed_pair(
+      piecewise_construct_t __pc, tuple<_Args1...> __first_args, tuple<_Args2...> __second_args)
+      : _Base1(__pc, std::move(__first_args), typename __make_tuple_indices<sizeof...(_Args1)>::type()),
+        _Base2(__pc, std::move(__second_args), typename __make_tuple_indices<sizeof...(_Args2)>::type()) {}
+#endif
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 typename _Base1::reference first() _NOEXCEPT {
+    return static_cast<_Base1&>(*this).__get();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR typename _Base1::const_reference first() const _NOEXCEPT {
+    return static_cast<_Base1 const&>(*this).__get();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 typename _Base2::reference second() _NOEXCEPT {
+    return static_cast<_Base2&>(*this).__get();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR typename _Base2::const_reference second() const _NOEXCEPT {
+    return static_cast<_Base2 const&>(*this).__get();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR static _Base1* __get_first_base(__compressed_pair* __pair) _NOEXCEPT {
+    return static_cast<_Base1*>(__pair);
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR static _Base2* __get_second_base(__compressed_pair* __pair) _NOEXCEPT {
+    return static_cast<_Base2*>(__pair);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void swap(__compressed_pair& __x)
+      _NOEXCEPT_(__is_nothrow_swappable<_T1>::value&& __is_nothrow_swappable<_T2>::value) {
+    using std::swap;
+    swap(first(), __x.first());
+    swap(second(), __x.second());
+  }
+};
+} // namespace std
+
 // This is the pre-C++20 implementation of the control block used by non-array
 // std::allocate_shared and std::make_shared. We keep it here so that we can
 // make sure our implementation is backwards compatible with it forever.
----------------
ldionne wrote:

In `// Object types to store in the control block`, let's add a type that has a virtual function but is otherwise empty.

https://github.com/llvm/llvm-project/pull/76756