[libc++] Implement C++20 atomic_ref (#76647)

Implement the std::atomic_ref class template by reusing atomic_base_impl. Based on the work from https://reviews.llvm.org/D72240
llvm · May 21, 2024 · 42ba740 · 42ba740
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diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
@@ -53,6 +53,7 @@ Implemented Papers
 - P2387R3 - Pipe support for user-defined range adaptors
 - P2713R1 - Escaping improvements in ``std::format``
 - P2231R1 - Missing ``constexpr`` in ``std::optional`` and ``std::variant``
+- P0019R8 - ``std::atomic_ref``
 
 Improvements and New Features
 -----------------------------

diff --git a/libcxx/docs/Status/Cxx20Papers.csv b/libcxx/docs/Status/Cxx20Papers.csv
@@ -26,7 +26,7 @@
 "`P0905R1 <https://wg21.link/P0905R1>`__","CWG","Symmetry for spaceship","Jacksonville","|Complete|","7.0","|spaceship|"
 "`P0966R1 <https://wg21.link/P0966R1>`__","LWG","``string::reserve``\  Should Not Shrink","Jacksonville","|Complete| [#note-P0966]_","12.0"
 "","","","","","",""
-"`P0019R8 <https://wg21.link/P0019R8>`__","LWG","Atomic Ref","Rapperswil","",""
+"`P0019R8 <https://wg21.link/P0019R8>`__","LWG","Atomic Ref","Rapperswil","|Complete|","19.0"
 "`P0458R2 <https://wg21.link/P0458R2>`__","LWG","Checking for Existence of an Element in Associative Containers","Rapperswil","|Complete|","13.0"
 "`P0475R1 <https://wg21.link/P0475R1>`__","LWG","LWG 2511: guaranteed copy elision for piecewise construction","Rapperswil","|Complete|",""
 "`P0476R2 <https://wg21.link/P0476R2>`__","LWG","Bit-casting object representations","Rapperswil","|Complete|","14.0"
@@ -125,7 +125,7 @@
 "`P1612R1 <https://wg21.link/P1612R1>`__","LWG","Relocate Endian's Specification","Cologne","|Complete|","10.0"
 "`P1614R2 <https://wg21.link/P1614R2>`__","LWG","The Mothership has Landed","Cologne","|In Progress|",""
 "`P1638R1 <https://wg21.link/P1638R1>`__","LWG","basic_istream_view::iterator should not be copyable","Cologne","|Complete|","16.0","|ranges|"
-"`P1643R1 <https://wg21.link/P1643R1>`__","LWG","Add wait/notify to atomic_ref","Cologne","",""
+"`P1643R1 <https://wg21.link/P1643R1>`__","LWG","Add wait/notify to atomic_ref","Cologne","|Complete|","19.0"
 "`P1644R0 <https://wg21.link/P1644R0>`__","LWG","Add wait/notify to atomic<shared_ptr>","Cologne","",""
 "`P1650R0 <https://wg21.link/P1650R0>`__","LWG","Output std::chrono::days with 'd' suffix","Cologne","|Complete|","16.0"
 "`P1651R0 <https://wg21.link/P1651R0>`__","LWG","bind_front should not unwrap reference_wrapper","Cologne","|Complete|","13.0"

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
@@ -224,6 +224,7 @@ set(files
   __atomic/atomic_flag.h
   __atomic/atomic_init.h
   __atomic/atomic_lock_free.h
+  __atomic/atomic_ref.h
   __atomic/atomic_sync.h
   __atomic/check_memory_order.h
   __atomic/contention_t.h
@@ -232,6 +233,7 @@ set(files
   __atomic/is_always_lock_free.h
   __atomic/kill_dependency.h
   __atomic/memory_order.h
+  __atomic/to_gcc_order.h
   __availability
   __bit/bit_cast.h
   __bit/bit_ceil.h

diff --git a/libcxx/include/__atomic/atomic_ref.h b/libcxx/include/__atomic/atomic_ref.h
diff --git a/libcxx/include/__atomic/atomic_sync.h b/libcxx/include/__atomic/atomic_sync.h
@@ -12,6 +12,7 @@
 #include <__atomic/contention_t.h>
 #include <__atomic/cxx_atomic_impl.h>
 #include <__atomic/memory_order.h>
+#include <__atomic/to_gcc_order.h>
 #include <__availability>
 #include <__chrono/duration.h>
 #include <__config>

diff --git a/libcxx/include/__atomic/check_memory_order.h b/libcxx/include/__atomic/check_memory_order.h
@@ -27,4 +27,8 @@
   _LIBCPP_DIAGNOSE_WARNING(__f == memory_order_release || __f == memory_order_acq_rel,                                 \
                            "memory order argument to atomic operation is invalid")
 
+#define _LIBCPP_CHECK_WAIT_MEMORY_ORDER(__m)                                                                           \
+  _LIBCPP_DIAGNOSE_WARNING(__m == memory_order_release || __m == memory_order_acq_rel,                                 \
+                           "memory order argument to atomic operation is invalid")
+
 #endif // _LIBCPP___ATOMIC_CHECK_MEMORY_ORDER_H
diff --git a/libcxx/include/__atomic/cxx_atomic_impl.h b/libcxx/include/__atomic/cxx_atomic_impl.h
@@ -10,6 +10,7 @@
 #define _LIBCPP___ATOMIC_CXX_ATOMIC_IMPL_H
 
 #include <__atomic/memory_order.h>
+#include <__atomic/to_gcc_order.h>
 #include <__config>
 #include <__memory/addressof.h>
 #include <__type_traits/is_assignable.h>
@@ -54,32 +55,6 @@ struct __cxx_atomic_base_impl {
   _Tp __a_value;
 };
 
-_LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR int __to_gcc_order(memory_order __order) {
-  // Avoid switch statement to make this a constexpr.
-  return __order == memory_order_relaxed
-           ? __ATOMIC_RELAXED
-           : (__order == memory_order_acquire
-                  ? __ATOMIC_ACQUIRE
-                  : (__order == memory_order_release
-                         ? __ATOMIC_RELEASE
-                         : (__order == memory_order_seq_cst
-                                ? __ATOMIC_SEQ_CST
-                                : (__order == memory_order_acq_rel ? __ATOMIC_ACQ_REL : __ATOMIC_CONSUME))));
-}
-
-_LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR int __to_gcc_failure_order(memory_order __order) {
-  // Avoid switch statement to make this a constexpr.
-  return __order == memory_order_relaxed
-           ? __ATOMIC_RELAXED
-           : (__order == memory_order_acquire
-                  ? __ATOMIC_ACQUIRE
-                  : (__order == memory_order_release
-                         ? __ATOMIC_RELAXED
-                         : (__order == memory_order_seq_cst
-                                ? __ATOMIC_SEQ_CST
-                                : (__order == memory_order_acq_rel ? __ATOMIC_ACQUIRE : __ATOMIC_CONSUME))));
-}
-
 template <typename _Tp>
 _LIBCPP_HIDE_FROM_ABI void __cxx_atomic_init(volatile __cxx_atomic_base_impl<_Tp>* __a, _Tp __val) {
   __cxx_atomic_assign_volatile(__a->__a_value, __val);

diff --git a/libcxx/include/__atomic/to_gcc_order.h b/libcxx/include/__atomic/to_gcc_order.h
@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ATOMIC_TO_GCC_ORDER_H
+#define _LIBCPP___ATOMIC_TO_GCC_ORDER_H
+
+#include <__atomic/memory_order.h>
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if defined(__ATOMIC_RELAXED) && defined(__ATOMIC_CONSUME) && defined(__ATOMIC_ACQUIRE) &&                             \
+    defined(__ATOMIC_RELEASE) && defined(__ATOMIC_ACQ_REL) && defined(__ATOMIC_SEQ_CST)
+
+_LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR int __to_gcc_order(memory_order __order) {
+  // Avoid switch statement to make this a constexpr.
+  return __order == memory_order_relaxed
+           ? __ATOMIC_RELAXED
+           : (__order == memory_order_acquire
+                  ? __ATOMIC_ACQUIRE
+                  : (__order == memory_order_release
+                         ? __ATOMIC_RELEASE
+                         : (__order == memory_order_seq_cst
+                                ? __ATOMIC_SEQ_CST
+                                : (__order == memory_order_acq_rel ? __ATOMIC_ACQ_REL : __ATOMIC_CONSUME))));
+}
+
+_LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR int __to_gcc_failure_order(memory_order __order) {
+  // Avoid switch statement to make this a constexpr.
+  return __order == memory_order_relaxed
+           ? __ATOMIC_RELAXED
+           : (__order == memory_order_acquire
+                  ? __ATOMIC_ACQUIRE
+                  : (__order == memory_order_release
+                         ? __ATOMIC_RELAXED
+                         : (__order == memory_order_seq_cst
+                                ? __ATOMIC_SEQ_CST
+                                : (__order == memory_order_acq_rel ? __ATOMIC_ACQUIRE : __ATOMIC_CONSUME))));
+}
+
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ATOMIC_TO_GCC_ORDER_H
diff --git a/libcxx/include/atomic b/libcxx/include/atomic
@@ -599,6 +599,7 @@ template <class T>
 #include <__atomic/atomic_flag.h>
 #include <__atomic/atomic_init.h>
 #include <__atomic/atomic_lock_free.h>
+#include <__atomic/atomic_ref.h>
 #include <__atomic/atomic_sync.h>
 #include <__atomic/check_memory_order.h>
 #include <__atomic/contention_t.h>