[libc++][atomic] P2835R7: Expose std::atomic_ref's object address (llvm#162236)

Implements https://wg21.link/P2835R7

Closes llvm#118377

# References

- https://wg21.link/atomics.ref.generic.general
- https://wg21.link/atomics.ref.int
- https://wg21.link/atomics.ref.float
- https://wg21.link/atomics.ref.pointer

---------

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/docs/FeatureTestMacroTable.rst

@@ -418,6 +418,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_atomic_min_max``                               *unimplemented*
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_atomic_ref``                                   ``202411L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_bind_front``                                   ``202306L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_bitset``                                       ``202306L``

libcxx/docs/ReleaseNotes/22.rst

@@ -43,6 +43,7 @@ Implemented Papers
 - P3044R2: sub-``string_view`` from ``string`` (`Github <https://llvm.org/PR148140>`__)
 - P3223R2: Making ``std::istream::ignore`` less surprising (`Github <https://llvm.org/PR148178>`__)
 - P3060R3: Add ``std::views::indices(n)`` (`Github <https://llvm.org/PR148175>`__)
+- P2835R7: Expose ``std::atomic_ref``'s object address (`Github <https://llvm.org/PR118377>`__)
 - P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
 
 Improvements and New Features

libcxx/docs/Status/Cxx2cPapers.csv

@@ -85,7 +85,7 @@
 "`P3222R0 <https://wg21.link/P3222R0>`__","Fix C++26 by adding transposed special cases for P2642 layouts","2024-11 (Wrocław)","","","`#118374 <https://github.com/llvm/llvm-project/issues/118374>`__",""
 "`P3050R2 <https://wg21.link/P3050R2>`__","Fix C++26 by optimizing ``linalg::conjugated`` for noncomplex value types","2024-11 (Wrocław)","","","`#118375 <https://github.com/llvm/llvm-project/issues/118375>`__",""
 "`P3396R1 <https://wg21.link/P3396R1>`__","``std::execution`` wording fixes","2024-11 (Wrocław)","","","`#118376 <https://github.com/llvm/llvm-project/issues/118376>`__",""
-"`P2835R7 <https://wg21.link/P2835R7>`__","Expose ``std::atomic_ref``'s object address","2024-11 (Wrocław)","","","`#118377 <https://github.com/llvm/llvm-project/issues/118377>`__",""
+"`P2835R7 <https://wg21.link/P2835R7>`__","Expose ``std::atomic_ref``'s object address","2024-11 (Wrocław)","|Complete|","22","`#118377 <https://github.com/llvm/llvm-project/issues/118377>`__",""
 "`P3323R1 <https://wg21.link/P3323R1>`__","cv-qualified types in ``atomic`` and ``atomic_ref``","2024-11 (Wrocław)","","","`#118378 <https://github.com/llvm/llvm-project/issues/118378>`__",""
 "`P3508R0 <https://wg21.link/P3508R0>`__","Wording for ""constexpr for specialized memory algorithms""","2024-11 (Wrocław)","","","`#118379 <https://github.com/llvm/llvm-project/issues/118379>`__",""
 "`P3369R0 <https://wg21.link/P3369R0>`__","constexpr for ``uninitialized_default_construct``","2024-11 (Wrocław)","","","`#118380 <https://github.com/llvm/llvm-project/issues/118380>`__",""

libcxx/include/__atomic/atomic_ref.h

@@ -220,6 +220,9 @@ struct __atomic_ref_base {
   }
   _LIBCPP_HIDE_FROM_ABI void notify_one() const noexcept { std::__atomic_notify_one(*this); }
   _LIBCPP_HIDE_FROM_ABI void notify_all() const noexcept { std::__atomic_notify_all(*this); }
+#  if _LIBCPP_STD_VER >= 26
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp* address() const noexcept { return __ptr_; }
+#  endif
 
 protected:
   using _Aligned_Tp [[__gnu__::__aligned__(required_alignment), __gnu__::__nodebug__]] = _Tp;

libcxx/include/version

@@ -37,7 +37,8 @@ __cpp_lib_atomic_float                                  201711L <atomic>
 __cpp_lib_atomic_is_always_lock_free                    201603L <atomic>
 __cpp_lib_atomic_lock_free_type_aliases                 201907L <atomic>
 __cpp_lib_atomic_min_max                                202403L <atomic>
-__cpp_lib_atomic_ref                                    201806L <atomic>
+__cpp_lib_atomic_ref                                    202411L <atomic>
+                                                        201806L // C++20
 __cpp_lib_atomic_shared_ptr                             201711L <atomic>
 __cpp_lib_atomic_value_initialization                   201911L <atomic> <memory>
 __cpp_lib_atomic_wait                                   201907L <atomic>
@@ -544,6 +545,8 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_aligned_accessor                     202411L
 // # define __cpp_lib_associative_heterogeneous_insertion  202306L
 // # define __cpp_lib_atomic_min_max                       202403L
+# undef  __cpp_lib_atomic_ref
+# define __cpp_lib_atomic_ref                           202411L
 # undef  __cpp_lib_bind_front
 # define __cpp_lib_bind_front                           202306L
 # define __cpp_lib_bitset                               202306L

libcxx/test/libcxx/diagnostics/atomic.nodiscard.verify.cpp

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// check that <atomic> functions are marked [[nodiscard]]
+
+#include <atomic>
+
+#include "atomic_helpers.h"
+#include "test_macros.h"
+
+template <typename T>
+struct TestAtomicRef {
+  void operator()() const {
+    T x(T(1));
+    const std::atomic_ref<T> a(x);
+
+    a.address(); // expected-warning 4 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  }
+};
+
+void test() {
+  TestAtomicRef<UserAtomicType>()();
+  TestAtomicRef<int>()();
+  TestAtomicRef<float>()();
+  TestAtomicRef<char*>()();
+}

libcxx/test/std/atomics/atomics.ref/address.pass.cpp

@@ -0,0 +1,37 @@
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// constexpr T* address() const noexcept;
+
+#include <atomic>
+#include <cassert>
+#include <concepts>
+#include <memory>
+
+#include "atomic_helpers.h"
+#include "test_macros.h"
+
+template <typename T>
+struct TestAddress {
+  void operator()() const {
+    T x(T(1));
+    const std::atomic_ref<T> a(x);
+
+    std::same_as<T*> decltype(auto) p = a.address();
+    assert(std::addressof(x) == p);
+
+    static_assert(noexcept((a.address())));
+  }
+};
+
+int main(int, char**) {
+  TestEachAtomicType<TestAddress>()();
+
+  return 0;
+}

libcxx/test/std/language.support/support.limits/support.limits.general/atomic.version.compile.pass.cpp

@@ -355,8 +355,8 @@
 #  ifndef __cpp_lib_atomic_ref
 #    error "__cpp_lib_atomic_ref should be defined in c++26"
 #  endif
-#  if __cpp_lib_atomic_ref != 201806L
-#    error "__cpp_lib_atomic_ref should have the value 201806L in c++26"
+#  if __cpp_lib_atomic_ref != 202411L
+#    error "__cpp_lib_atomic_ref should have the value 202411L in c++26"
 #  endif
 
 #  if !defined(_LIBCPP_VERSION)

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

@@ -6325,8 +6325,8 @@
 #  ifndef __cpp_lib_atomic_ref
 #    error "__cpp_lib_atomic_ref should be defined in c++26"
 #  endif
-#  if __cpp_lib_atomic_ref != 201806L
-#    error "__cpp_lib_atomic_ref should have the value 201806L in c++26"
+#  if __cpp_lib_atomic_ref != 202411L
+#    error "__cpp_lib_atomic_ref should have the value 202411L in c++26"
 #  endif
 
 #  if !defined(_LIBCPP_VERSION)

libcxx/utils/generate_feature_test_macro_components.py

@@ -195,7 +195,10 @@ def add_version_header(tc):
         },
         {
             "name": "__cpp_lib_atomic_ref",
-            "values": {"c++20": 201806},
+            "values": {
+                "c++20": 201806,
+                "c++26": 202411,  # P2835R7: Expose std::atomic_ref 's object address
+            },
             "headers": ["atomic"],
         },
         {