[libcxx-commits] [libcxx] [libc++][atomic] P2835R7: Expose `std::atomic_ref`'s object address (PR #162236)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Tue Oct 7 07:51:38 PDT 2025
https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/162236
>From 037a1cbe1392d6fa7ec2b44605970809dd348975 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Tue, 7 Oct 2025 10:30:01 +0300
Subject: [PATCH 1/2] [libc++][atomic] P2835R7: Expose `std::atomic_ref`'s
object address
Implements https://wg21.link/P2835R7
Closes #118377
- 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
---
libcxx/docs/ReleaseNotes/22.rst | 1 +
libcxx/docs/Status/Cxx2cPapers.csv | 2 +-
libcxx/include/__atomic/atomic_ref.h | 3 ++
.../std/atomics/atomics.ref/address.pass.cpp | 39 +++++++++++++++++++
4 files changed, 44 insertions(+), 1 deletion(-)
create mode 100644 libcxx/test/std/atomics/atomics.ref/address.pass.cpp
diff --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
index ec23ba9d1e3a1..1a450218cb988 100644
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/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
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index 69b9984f43ccf..9b83047f4b983 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/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>`__",""
diff --git a/libcxx/include/__atomic/atomic_ref.h b/libcxx/include/__atomic/atomic_ref.h
index 9bdc6b1160d2c..1b5191ebb31cc 100644
--- a/libcxx/include/__atomic/atomic_ref.h
+++ b/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
+ _LIBCPP_HIDE_FROM_ABI constexpr _Tp* address() const noexcept { return __ptr_; }
+# endif
protected:
using _Aligned_Tp [[__gnu__::__aligned__(required_alignment), __gnu__::__nodebug__]] = _Tp;
diff --git a/libcxx/test/std/atomics/atomics.ref/address.pass.cpp b/libcxx/test/std/atomics/atomics.ref/address.pass.cpp
new file mode 100644
index 0000000000000..5b00e13234dbf
--- /dev/null
+++ b/libcxx/test/std/atomics/atomics.ref/address.pass.cpp
@@ -0,0 +1,39 @@
+//
+// 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
+
+// XFAIL: !has-64-bit-atomics
+// XFAIL: !has-1024-bit-atomics
+
+// constexpr T* address() const noexcept;
+
+#include <atomic>
+#include <cassert>
+#include <concepts>
+
+#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;
+}
>From 8b61107727d62c7dc2cbe8d1cb49326b7985605c Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Tue, 7 Oct 2025 17:50:37 +0300
Subject: [PATCH 2/2] Addressed review comments
---
libcxx/include/__atomic/atomic_ref.h | 2 +-
.../diagnostics/atomic.nodiscard.verify.cpp | 37 +++++++++++++++++++
.../std/atomics/atomics.ref/address.pass.cpp | 1 +
3 files changed, 39 insertions(+), 1 deletion(-)
create mode 100644 libcxx/test/libcxx/diagnostics/atomic.nodiscard.verify.cpp
diff --git a/libcxx/include/__atomic/atomic_ref.h b/libcxx/include/__atomic/atomic_ref.h
index 1b5191ebb31cc..ec5ae2c28c310 100644
--- a/libcxx/include/__atomic/atomic_ref.h
+++ b/libcxx/include/__atomic/atomic_ref.h
@@ -221,7 +221,7 @@ 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
- _LIBCPP_HIDE_FROM_ABI constexpr _Tp* address() const noexcept { return __ptr_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp* address() const noexcept { return __ptr_; }
# endif
protected:
diff --git a/libcxx/test/libcxx/diagnostics/atomic.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/atomic.nodiscard.verify.cpp
new file mode 100644
index 0000000000000..c1e8aeb97eb8a
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/atomic.nodiscard.verify.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// check that <atomic> functions are marked [[nodiscard]]
+
+// clang-format off
+
+#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);
+
+#if TEST_STD_VER >= 26
+ a.address(); // expected-warning 4 {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+ }
+};
+
+void test() {
+ TestAtomicRef<UserAtomicType>()();
+ TestAtomicRef<int>()();
+ TestAtomicRef<float>()();
+ TestAtomicRef<char*>()();
+}
diff --git a/libcxx/test/std/atomics/atomics.ref/address.pass.cpp b/libcxx/test/std/atomics/atomics.ref/address.pass.cpp
index 5b00e13234dbf..23c6088f474eb 100644
--- a/libcxx/test/std/atomics/atomics.ref/address.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.ref/address.pass.cpp
@@ -15,6 +15,7 @@
#include <atomic>
#include <cassert>
#include <concepts>
+#include <memory>
#include "atomic_helpers.h"
#include "test_macros.h"
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