[libc++] [C++2b] [P0943] Add stdatomic.h header.

* https://wg21.link/P0943
* https://eel.is/c++draft/stdatomic.h.syn

Differential Revision: https://reviews.llvm.org/D97044

libcxx/docs/FeatureTestMacroTable.rst

@@ -342,7 +342,7 @@ Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_stacktrace``                          *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_stdatomic_h``                         *unimplemented*
+    ``__cpp_lib_stdatomic_h``                         ``202011L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_string_contains``                     ``202011L``
     ------------------------------------------------- -----------------

libcxx/docs/Status/Cxx2bPapers.csv

@@ -1,6 +1,6 @@
 "Paper #","Group","Paper Name","Meeting","Status","First released version"
 "`P0881R7 <https://wg21.link/P0881R7>`__","LWG","A Proposal to add stacktrace library","Autumn 2020","",""
-"`P0943R6 <https://wg21.link/P0943R6>`__","LWG","Support C atomics in C++","Autumn 2020","",""
+"`P0943R6 <https://wg21.link/P0943R6>`__","LWG","Support C atomics in C++","Autumn 2020","|Complete|","15.0"
 "`P1048R1 <https://wg21.link/P1048R1>`__","LWG","A proposal for a type trait to detect scoped enumerations","Autumn 2020","|Complete|","12.0"
 "`P1679R3 <https://wg21.link/P1679R3>`__","LWG","string contains function","Autumn 2020","|Complete|","12.0"
 "","","","","",""

libcxx/include/CMakeLists.txt

@@ -517,6 +517,7 @@ set(files
   span
   sstream
   stack
+  stdatomic.h
   stdbool.h
   stddef.h
   stdexcept

libcxx/include/atomic

@@ -540,7 +540,7 @@ template <class T>
 # error <atomic> is not implemented
 #endif
 #ifdef kill_dependency
-# error C++ standard library is incompatible with <stdatomic.h>
+# error C++ standard library is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
 #endif
 
 #define _LIBCPP_CHECK_STORE_MEMORY_ORDER(__m) \

libcxx/include/module.modulemap

@@ -50,6 +50,10 @@ module std [system] {
       header "setjmp.h"
       export *
     }
+    module stdatomic_h {
+      header "stdatomic.h"
+      export *
+    }
     // FIXME: <stdalign.h> is missing.
     // <signal.h> provided by C library.
     // <stdarg.h> provided by compiler.

libcxx/include/stdatomic.h

@@ -0,0 +1,235 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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_STDATOMIC_H
+#define _LIBCPP_STDATOMIC_H
+
+/*
+    stdatomic.h synopsis
+
+template<class T>
+  using std-atomic = std::atomic<T>;        // exposition only
+
+#define _Atomic(T) std-atomic<T>
+
+#define ATOMIC_BOOL_LOCK_FREE see below
+#define ATOMIC_CHAR_LOCK_FREE see below
+#define ATOMIC_CHAR16_T_LOCK_FREE see below
+#define ATOMIC_CHAR32_T_LOCK_FREE see below
+#define ATOMIC_WCHAR_T_LOCK_FREE see below
+#define ATOMIC_SHORT_LOCK_FREE see below
+#define ATOMIC_INT_LOCK_FREE see below
+#define ATOMIC_LONG_LOCK_FREE see below
+#define ATOMIC_LLONG_LOCK_FREE see below
+#define ATOMIC_POINTER_LOCK_FREE see below
+
+using std::memory_order                // see below
+using std::memory_order_relaxed        // see below
+using std::memory_order_consume        // see below
+using std::memory_order_acquire        // see below
+using std::memory_order_release        // see below
+using std::memory_order_acq_rel        // see below
+using std::memory_order_seq_cst        // see below
+
+using std::atomic_flag                 // see below
+
+using std::atomic_bool                 // see below
+using std::atomic_char                 // see below
+using std::atomic_schar                // see below
+using std::atomic_uchar                // see below
+using std::atomic_short                // see below
+using std::atomic_ushort               // see below
+using std::atomic_int                  // see below
+using std::atomic_uint                 // see below
+using std::atomic_long                 // see below
+using std::atomic_ulong                // see below
+using std::atomic_llong                // see below
+using std::atomic_ullong               // see below
+using std::atomic_char8_t              // see below
+using std::atomic_char16_t             // see below
+using std::atomic_char32_t             // see below
+using std::atomic_wchar_t              // see below
+using std::atomic_int8_t               // see below
+using std::atomic_uint8_t              // see below
+using std::atomic_int16_t              // see below
+using std::atomic_uint16_t             // see below
+using std::atomic_int32_t              // see below
+using std::atomic_uint32_t             // see below
+using std::atomic_int64_t              // see below
+using std::atomic_uint64_t             // see below
+using std::atomic_int_least8_t         // see below
+using std::atomic_uint_least8_t        // see below
+using std::atomic_int_least16_t        // see below
+using std::atomic_uint_least16_t       // see below
+using std::atomic_int_least32_t        // see below
+using std::atomic_uint_least32_t       // see below
+using std::atomic_int_least64_t        // see below
+using std::atomic_uint_least64_t       // see below
+using std::atomic_int_fast8_t          // see below
+using std::atomic_uint_fast8_t         // see below
+using std::atomic_int_fast16_t         // see below
+using std::atomic_uint_fast16_t        // see below
+using std::atomic_int_fast32_t         // see below
+using std::atomic_uint_fast32_t        // see below
+using std::atomic_int_fast64_t         // see below
+using std::atomic_uint_fast64_t        // see below
+using std::atomic_intptr_t             // see below
+using std::atomic_uintptr_t            // see below
+using std::atomic_size_t               // see below
+using std::atomic_ptrdiff_t            // see below
+using std::atomic_intmax_t             // see below
+using std::atomic_uintmax_t            // see below
+
+using std::atomic_is_lock_free                         // see below
+using std::atomic_load                                 // see below
+using std::atomic_load_explicit                        // see below
+using std::atomic_store                                // see below
+using std::atomic_store_explicit                       // see below
+using std::atomic_exchange                             // see below
+using std::atomic_exchange_explicit                    // see below
+using std::atomic_compare_exchange_strong              // see below
+using std::atomic_compare_exchange_strong_explicit     // see below
+using std::atomic_compare_exchange_weak                // see below
+using std::atomic_compare_exchange_weak_explicit       // see below
+using std::atomic_fetch_add                            // see below
+using std::atomic_fetch_add_explicit                   // see below
+using std::atomic_fetch_sub                            // see below
+using std::atomic_fetch_sub_explicit                   // see below
+using std::atomic_fetch_or                             // see below
+using std::atomic_fetch_or_explicit                    // see below
+using std::atomic_fetch_and                            // see below
+using std::atomic_fetch_and_explicit                   // see below
+using std::atomic_flag_test_and_set                    // see below
+using std::atomic_flag_test_and_set_explicit           // see below
+using std::atomic_flag_clear                           // see below
+using std::atomic_flag_clear_explicit                  // see below
+
+using std::atomic_thread_fence                         // see below
+using std::atomic_signal_fence                         // see below
+
+*/
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 20
+
+#include <atomic>
+#include <version>
+
+#ifdef _Atomic
+# undef _Atomic
+#endif
+
+#define _Atomic(_Tp) ::std::atomic<_Tp>
+
+using std::memory_order _LIBCPP_USING_IF_EXISTS;
+using std::memory_order_relaxed _LIBCPP_USING_IF_EXISTS;
+using std::memory_order_consume _LIBCPP_USING_IF_EXISTS;
+using std::memory_order_acquire _LIBCPP_USING_IF_EXISTS;
+using std::memory_order_release _LIBCPP_USING_IF_EXISTS;
+using std::memory_order_acq_rel _LIBCPP_USING_IF_EXISTS;
+using std::memory_order_seq_cst _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_flag _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_bool _LIBCPP_USING_IF_EXISTS;
+using std::atomic_char _LIBCPP_USING_IF_EXISTS;
+using std::atomic_schar _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uchar _LIBCPP_USING_IF_EXISTS;
+using std::atomic_short _LIBCPP_USING_IF_EXISTS;
+using std::atomic_ushort _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint _LIBCPP_USING_IF_EXISTS;
+using std::atomic_long _LIBCPP_USING_IF_EXISTS;
+using std::atomic_ulong _LIBCPP_USING_IF_EXISTS;
+using std::atomic_llong _LIBCPP_USING_IF_EXISTS;
+using std::atomic_ullong _LIBCPP_USING_IF_EXISTS;
+using std::atomic_char8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_char16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_char32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_wchar_t _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_int8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int64_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint64_t _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_int_least8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_least8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int_least16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_least16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int_least32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_least32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int_least64_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_least64_t _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_int_fast8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_fast8_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int_fast16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_fast16_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int_fast32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_fast32_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_int_fast64_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uint_fast64_t _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_intptr_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uintptr_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_size_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_ptrdiff_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_intmax_t _LIBCPP_USING_IF_EXISTS;
+using std::atomic_uintmax_t _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_compare_exchange_strong _LIBCPP_USING_IF_EXISTS;
+using std::atomic_compare_exchange_strong_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_compare_exchange_weak _LIBCPP_USING_IF_EXISTS;
+using std::atomic_compare_exchange_weak_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_exchange _LIBCPP_USING_IF_EXISTS;
+using std::atomic_exchange_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_add _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_add_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_and _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_and_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_or _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_or_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_sub _LIBCPP_USING_IF_EXISTS;
+using std::atomic_fetch_sub_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_flag_clear _LIBCPP_USING_IF_EXISTS;
+using std::atomic_flag_clear_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_flag_test_and_set _LIBCPP_USING_IF_EXISTS;
+using std::atomic_flag_test_and_set_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_is_lock_free _LIBCPP_USING_IF_EXISTS;
+using std::atomic_load _LIBCPP_USING_IF_EXISTS;
+using std::atomic_load_explicit _LIBCPP_USING_IF_EXISTS;
+using std::atomic_store _LIBCPP_USING_IF_EXISTS;
+using std::atomic_store_explicit _LIBCPP_USING_IF_EXISTS;
+
+using std::atomic_signal_fence _LIBCPP_USING_IF_EXISTS;
+using std::atomic_thread_fence _LIBCPP_USING_IF_EXISTS;
+
+#elif defined(_LIBCPP_COMPILER_CLANG_BASED)
+
+// Before C++23, we include the next <stdatomic.h> on the path to avoid hijacking
+// the header. We do this because Clang has historically shipped a <stdatomic.h>
+// header that would be available in all Standard modes, and we don't want to
+// break that use case.
+# if __has_include_next(<stdatomic.h>)
+#   include_next <stdatomic.h>
+# endif
+
+#endif // _LIBCPP_STD_VER > 20
+
+#endif // _LIBCPP_STDATOMIC_H

libcxx/include/version

@@ -396,7 +396,7 @@ __cpp_lib_void_t                                        201411L <type_traits>
 // # define __cpp_lib_reference_from_temporary             202202L
 // # define __cpp_lib_spanstream                           202106L
 // # define __cpp_lib_stacktrace                           202011L
-// # define __cpp_lib_stdatomic_h                          202011L
+# define __cpp_lib_stdatomic_h                          202011L
 # define __cpp_lib_string_contains                      202011L
 # define __cpp_lib_string_resize_and_overwrite          202110L
 # define __cpp_lib_to_underlying                        202102L

libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.compile.pass.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: libcpp-has-no-threads
+
+// This test ensures that we don't hijack the <stdatomic.h> header even when compiling
+// before C++23, since Clang used to provide that header before libc++ provided one.
+
+// On GCC, the compiler-provided <stdatomic.h> is not C++ friendly, so including <stdatomic.h>
+// doesn't work at all if we don't use the <stdatomic.h> provided by libc++ in C++23 and above.
+// XFAIL: (c++11 || c++14 || c++17 || c++20) && gcc
+
+#include <stdatomic.h>
+
+void f() {
+  atomic_int i; // just make sure the header isn't empty
+  (void)i;
+}

libcxx/test/libcxx/double_include.sh.cpp

@@ -165,6 +165,9 @@
 #    include <sstream>
 #endif
 #include <stack>
+#ifndef _LIBCPP_HAS_NO_THREADS
+#    include <stdatomic.h>
+#endif
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdexcept>

libcxx/test/libcxx/min_max_macros.compile.pass.cpp

@@ -254,6 +254,10 @@ TEST_MACROS();
 #endif
 #include <stack>
 TEST_MACROS();
+#ifndef _LIBCPP_HAS_NO_THREADS
+#    include <stdatomic.h>
+TEST_MACROS();
+#endif
 #include <stdbool.h>
 TEST_MACROS();
 #include <stddef.h>

libcxx/test/libcxx/nasty_macros.compile.pass.cpp

@@ -275,6 +275,9 @@
 #    include <sstream>
 #endif
 #include <stack>
+#ifndef _LIBCPP_HAS_NO_THREADS
+#    include <stdatomic.h>
+#endif
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdexcept>

libcxx/test/libcxx/no_assert_include.compile.pass.cpp

@@ -160,6 +160,9 @@
 #    include <sstream>
 #endif
 #include <stack>
+#ifndef _LIBCPP_HAS_NO_THREADS
+#    include <stdatomic.h>
+#endif
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdexcept>

libcxx/test/std/atomics/stdatomic.h.syn/types.compile.pass.cpp

@@ -0,0 +1,237 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++11, c++14, c++17, c++20
+// UNSUPPORTED: libcpp-has-no-threads
+
+// <stdatomic.h>
+
+// template<class T>
+//   using std-atomic = std::atomic<T>;        // exposition only
+//
+// #define _Atomic(T) std-atomic<T>
+//
+// #define ATOMIC_BOOL_LOCK_FREE see below
+// #define ATOMIC_CHAR_LOCK_FREE see below
+// #define ATOMIC_CHAR16_T_LOCK_FREE see below
+// #define ATOMIC_CHAR32_T_LOCK_FREE see below
+// #define ATOMIC_WCHAR_T_LOCK_FREE see below
+// #define ATOMIC_SHORT_LOCK_FREE see below
+// #define ATOMIC_INT_LOCK_FREE see below
+// #define ATOMIC_LONG_LOCK_FREE see below
+// #define ATOMIC_LLONG_LOCK_FREE see below
+// #define ATOMIC_POINTER_LOCK_FREE see below
+//
+// using std::memory_order                // see below
+// using std::memory_order_relaxed        // see below
+// using std::memory_order_consume        // see below
+// using std::memory_order_acquire        // see below
+// using std::memory_order_release        // see below
+// using std::memory_order_acq_rel        // see below
+// using std::memory_order_seq_cst        // see below
+//
+// using std::atomic_flag                 // see below
+//
+// using std::atomic_bool                 // see below
+// using std::atomic_char                 // see below
+// using std::atomic_schar                // see below
+// using std::atomic_uchar                // see below
+// using std::atomic_short                // see below
+// using std::atomic_ushort               // see below
+// using std::atomic_int                  // see below
+// using std::atomic_uint                 // see below
+// using std::atomic_long                 // see below
+// using std::atomic_ulong                // see below
+// using std::atomic_llong                // see below
+// using std::atomic_ullong               // see below
+// using std::atomic_char8_t              // see below
+// using std::atomic_char16_t             // see below
+// using std::atomic_char32_t             // see below
+// using std::atomic_wchar_t              // see below
+// using std::atomic_int8_t               // see below
+// using std::atomic_uint8_t              // see below
+// using std::atomic_int16_t              // see below
+// using std::atomic_uint16_t             // see below
+// using std::atomic_int32_t              // see below
+// using std::atomic_uint32_t             // see below
+// using std::atomic_int64_t              // see below
+// using std::atomic_uint64_t             // see below
+// using std::atomic_int_least8_t         // see below
+// using std::atomic_uint_least8_t        // see below
+// using std::atomic_int_least16_t        // see below
+// using std::atomic_uint_least16_t       // see below
+// using std::atomic_int_least32_t        // see below
+// using std::atomic_uint_least32_t       // see below
+// using std::atomic_int_least64_t        // see below
+// using std::atomic_uint_least64_t       // see below
+// using std::atomic_int_fast8_t          // see below
+// using std::atomic_uint_fast8_t         // see below
+// using std::atomic_int_fast16_t         // see below
+// using std::atomic_uint_fast16_t        // see below
+// using std::atomic_int_fast32_t         // see below
+// using std::atomic_uint_fast32_t        // see below
+// using std::atomic_int_fast64_t         // see below
+// using std::atomic_uint_fast64_t        // see below
+// using std::atomic_intptr_t             // see below
+// using std::atomic_uintptr_t            // see below
+// using std::atomic_size_t               // see below
+// using std::atomic_ptrdiff_t            // see below
+// using std::atomic_intmax_t             // see below
+// using std::atomic_uintmax_t            // see below
+//
+// using std::atomic_is_lock_free                         // see below
+// using std::atomic_load                                 // see below
+// using std::atomic_load_explicit                        // see below
+// using std::atomic_store                                // see below
+// using std::atomic_store_explicit                       // see below
+// using std::atomic_exchange                             // see below
+// using std::atomic_exchange_explicit                    // see below
+// using std::atomic_compare_exchange_strong              // see below
+// using std::atomic_compare_exchange_strong_explicit     // see below
+// using std::atomic_compare_exchange_weak                // see below
+// using std::atomic_compare_exchange_weak_explicit       // see below
+// using std::atomic_fetch_add                            // see below
+// using std::atomic_fetch_add_explicit                   // see below
+// using std::atomic_fetch_sub                            // see below
+// using std::atomic_fetch_sub_explicit                   // see below
+// using std::atomic_fetch_or                             // see below
+// using std::atomic_fetch_or_explicit                    // see below
+// using std::atomic_fetch_and                            // see below
+// using std::atomic_fetch_and_explicit                   // see below
+// using std::atomic_flag_test_and_set                    // see below
+// using std::atomic_flag_test_and_set_explicit           // see below
+// using std::atomic_flag_clear                           // see below
+// using std::atomic_flag_clear_explicit                  // see below
+//
+// using std::atomic_thread_fence                         // see below
+// using std::atomic_signal_fence                         // see below
+
+#include <stdatomic.h>
+#include <type_traits>
+
+#include "test_macros.h"
+
+static_assert(std::atomic<bool>::is_always_lock_free == (2 == ATOMIC_BOOL_LOCK_FREE));
+static_assert(std::atomic<char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE));
+static_assert(std::atomic<signed char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE));
+static_assert(std::atomic<unsigned char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE));
+static_assert(std::atomic<char16_t>::is_always_lock_free == (2 == ATOMIC_CHAR16_T_LOCK_FREE));
+static_assert(std::atomic<char32_t>::is_always_lock_free == (2 == ATOMIC_CHAR32_T_LOCK_FREE));
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+static_assert(std::atomic<wchar_t>::is_always_lock_free == (2 == ATOMIC_WCHAR_T_LOCK_FREE));
+#endif
+static_assert(std::atomic<short>::is_always_lock_free == (2 == ATOMIC_SHORT_LOCK_FREE));
+static_assert(std::atomic<unsigned short>::is_always_lock_free == (2 == ATOMIC_SHORT_LOCK_FREE));
+static_assert(std::atomic<int>::is_always_lock_free == (2 == ATOMIC_INT_LOCK_FREE));
+static_assert(std::atomic<unsigned int>::is_always_lock_free == (2 == ATOMIC_INT_LOCK_FREE));
+static_assert(std::atomic<long>::is_always_lock_free == (2 == ATOMIC_LONG_LOCK_FREE));
+static_assert(std::atomic<unsigned long>::is_always_lock_free == (2 == ATOMIC_LONG_LOCK_FREE));
+static_assert(std::atomic<long long>::is_always_lock_free == (2 == ATOMIC_LLONG_LOCK_FREE));
+static_assert(std::atomic<unsigned long long>::is_always_lock_free == (2 == ATOMIC_LLONG_LOCK_FREE));
+static_assert(std::atomic<void*>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE));
+static_assert(std::atomic<std::nullptr_t>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE));
+
+void f() {
+  static_assert(std::is_same_v<std::atomic<char>, _Atomic(char)>);
+  static_assert(std::is_same_v<std::atomic<int>, _Atomic(int)>);
+  static_assert(std::is_same_v<std::atomic<const long>, _Atomic(const long)>);
+
+  static_assert(std::is_same_v<std::memory_order, ::memory_order>);
+  static_assert(&std::memory_order_relaxed == &::memory_order_relaxed);
+  static_assert(&std::memory_order_consume == &::memory_order_consume);
+  static_assert(&std::memory_order_acquire == &::memory_order_acquire);
+  static_assert(&std::memory_order_release == &::memory_order_release);
+  static_assert(&std::memory_order_acq_rel == &::memory_order_acq_rel);
+  static_assert(&std::memory_order_seq_cst == &::memory_order_seq_cst);
+
+  static_assert(std::is_same_v<std::atomic_flag, ::atomic_flag>);
+
+  static_assert(std::is_same_v<std::atomic<bool>, ::atomic_bool>);
+  static_assert(std::is_same_v<std::atomic<char>, ::atomic_char>);
+  static_assert(std::is_same_v<std::atomic<signed char>, ::atomic_schar>);
+  static_assert(std::is_same_v<std::atomic<unsigned char>, ::atomic_uchar>);
+  static_assert(std::is_same_v<std::atomic<short>, ::atomic_short>);
+  static_assert(std::is_same_v<std::atomic<unsigned short>, ::atomic_ushort>);
+  static_assert(std::is_same_v<std::atomic<int>, ::atomic_int>);
+  static_assert(std::is_same_v<std::atomic<unsigned int>, ::atomic_uint>);
+  static_assert(std::is_same_v<std::atomic<long>, ::atomic_long>);
+  static_assert(std::is_same_v<std::atomic<unsigned long>, ::atomic_ulong>);
+  static_assert(std::is_same_v<std::atomic<long long>, ::atomic_llong>);
+  static_assert(std::is_same_v<std::atomic<unsigned long long>, ::atomic_ullong>);
+
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+  static_assert(std::is_same_v<std::atomic<char8_t>,  ::atomic_char8_t>);
+#endif
+  static_assert(std::is_same_v<std::atomic<char16_t>, ::atomic_char16_t>);
+  static_assert(std::is_same_v<std::atomic<char32_t>, ::atomic_char32_t>);
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  static_assert(std::is_same_v<std::atomic<wchar_t>,  ::atomic_wchar_t>);
+#endif
+
+  static_assert(std::is_same_v<std::atomic<int8_t>,   ::atomic_int8_t>);
+  static_assert(std::is_same_v<std::atomic<uint8_t>,  ::atomic_uint8_t>);
+  static_assert(std::is_same_v<std::atomic<int16_t>,  ::atomic_int16_t>);
+  static_assert(std::is_same_v<std::atomic<uint16_t>, ::atomic_uint16_t>);
+  static_assert(std::is_same_v<std::atomic<int32_t>,  ::atomic_int32_t>);
+  static_assert(std::is_same_v<std::atomic<uint32_t>, ::atomic_uint32_t>);
+  static_assert(std::is_same_v<std::atomic<int64_t>,  ::atomic_int64_t>);
+  static_assert(std::is_same_v<std::atomic<uint64_t>, ::atomic_uint64_t>);
+
+  static_assert(std::is_same_v<std::atomic<int_least8_t>,   ::atomic_int_least8_t>);
+  static_assert(std::is_same_v<std::atomic<uint_least8_t>,  ::atomic_uint_least8_t>);
+  static_assert(std::is_same_v<std::atomic<int_least16_t>,  ::atomic_int_least16_t>);
+  static_assert(std::is_same_v<std::atomic<uint_least16_t>, ::atomic_uint_least16_t>);
+  static_assert(std::is_same_v<std::atomic<int_least32_t>,  ::atomic_int_least32_t>);
+  static_assert(std::is_same_v<std::atomic<uint_least32_t>, ::atomic_uint_least32_t>);
+  static_assert(std::is_same_v<std::atomic<int_least64_t>,  ::atomic_int_least64_t>);
+  static_assert(std::is_same_v<std::atomic<uint_least64_t>, ::atomic_uint_least64_t>);
+
+  static_assert(std::is_same_v<std::atomic<int_fast8_t>,    ::atomic_int_fast8_t>);
+  static_assert(std::is_same_v<std::atomic<uint_fast8_t>,   ::atomic_uint_fast8_t>);
+  static_assert(std::is_same_v<std::atomic<int_fast16_t>,   ::atomic_int_fast16_t>);
+  static_assert(std::is_same_v<std::atomic<uint_fast16_t>,  ::atomic_uint_fast16_t>);
+  static_assert(std::is_same_v<std::atomic<int_fast32_t>,   ::atomic_int_fast32_t>);
+  static_assert(std::is_same_v<std::atomic<uint_fast32_t>,  ::atomic_uint_fast32_t>);
+  static_assert(std::is_same_v<std::atomic<int_fast64_t>,   ::atomic_int_fast64_t>);
+  static_assert(std::is_same_v<std::atomic<uint_fast64_t>,  ::atomic_uint_fast64_t>);
+
+  static_assert(std::is_same_v<std::atomic<intptr_t>,  ::atomic_intptr_t>);
+  static_assert(std::is_same_v<std::atomic<uintptr_t>, ::atomic_uintptr_t>);
+  static_assert(std::is_same_v<std::atomic<size_t>,    ::atomic_size_t>);
+  static_assert(std::is_same_v<std::atomic<ptrdiff_t>, ::atomic_ptrdiff_t>);
+  static_assert(std::is_same_v<std::atomic<intmax_t>,  ::atomic_intmax_t>);
+  static_assert(std::is_same_v<std::atomic<uintmax_t>, ::atomic_uintmax_t>);
+
+  // Just check that the symbols in the global namespace are visible.
+  using ::atomic_compare_exchange_strong;
+  using ::atomic_compare_exchange_strong_explicit;
+  using ::atomic_compare_exchange_weak;
+  using ::atomic_compare_exchange_weak_explicit;
+  using ::atomic_exchange;
+  using ::atomic_exchange_explicit;
+  using ::atomic_fetch_add;
+  using ::atomic_fetch_add_explicit;
+  using ::atomic_fetch_and;
+  using ::atomic_fetch_and_explicit;
+  using ::atomic_fetch_or;
+  using ::atomic_fetch_or_explicit;
+  using ::atomic_fetch_sub;
+  using ::atomic_fetch_sub_explicit;
+  using ::atomic_flag_clear;
+  using ::atomic_flag_clear_explicit;
+  using ::atomic_flag_test_and_set;
+  using ::atomic_flag_test_and_set_explicit;
+  using ::atomic_is_lock_free;
+  using ::atomic_load;
+  using ::atomic_load_explicit;
+  using ::atomic_store;
+  using ::atomic_store_explicit;
+
+  using ::atomic_signal_fence;
+  using ::atomic_thread_fence;
+}

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

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// WARNING: This test was generated by generate_feature_test_macro_components.py
+// and should not be edited manually.
+//
+// clang-format off
+
+// UNSUPPORTED: libcpp-has-no-threads
+
+// <stdatomic.h>
+
+// Test the feature test macros defined by <stdatomic.h>
+
+/*  Constant                 Value
+    __cpp_lib_stdatomic_h    202011L [C++2b]
+*/
+
+#include <stdatomic.h>
+#include "test_macros.h"
+
+#if TEST_STD_VER < 14
+
+# ifdef __cpp_lib_stdatomic_h
+#   error "__cpp_lib_stdatomic_h should not be defined before c++2b"
+# endif
+
+#elif TEST_STD_VER == 14
+
+# ifdef __cpp_lib_stdatomic_h
+#   error "__cpp_lib_stdatomic_h should not be defined before c++2b"
+# endif
+
+#elif TEST_STD_VER == 17
+
+# ifdef __cpp_lib_stdatomic_h
+#   error "__cpp_lib_stdatomic_h should not be defined before c++2b"
+# endif
+
+#elif TEST_STD_VER == 20
+
+# ifdef __cpp_lib_stdatomic_h
+#   error "__cpp_lib_stdatomic_h should not be defined before c++2b"
+# endif
+
+#elif TEST_STD_VER > 20
+
+# ifndef __cpp_lib_stdatomic_h
+#   error "__cpp_lib_stdatomic_h should be defined in c++2b"
+# endif
+# if __cpp_lib_stdatomic_h != 202011L
+#   error "__cpp_lib_stdatomic_h should have the value 202011L in c++2b"
+# endif
+
+#endif // TEST_STD_VER > 20
+
+int main(int, char**) { return 0; }

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

@@ -4848,17 +4848,11 @@
 #   error "__cpp_lib_starts_ends_with should have the value 201711L in c++2b"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_stdatomic_h
-#     error "__cpp_lib_stdatomic_h should be defined in c++2b"
-#   endif
-#   if __cpp_lib_stdatomic_h != 202011L
-#     error "__cpp_lib_stdatomic_h should have the value 202011L in c++2b"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_stdatomic_h
-#     error "__cpp_lib_stdatomic_h should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_stdatomic_h
+#   error "__cpp_lib_stdatomic_h should be defined in c++2b"
+# endif
+# if __cpp_lib_stdatomic_h != 202011L
+#   error "__cpp_lib_stdatomic_h should have the value 202011L in c++2b"
 # endif
 
 # ifndef __cpp_lib_string_contains

libcxx/utils/generate_feature_test_macro_components.py

@@ -664,7 +664,6 @@ feature_test_macros = [ add_version_header(x) for x in [
     "name": "__cpp_lib_stdatomic_h",
     "values": { "c++2b": 202011 },
     "headers": ["stdatomic.h"],
-    "unimplemented": True,
   }, {
     "name": "__cpp_lib_string_contains",
     "values": { "c++2b": 202011 },
@@ -786,7 +785,8 @@ lit_markup = {
   "regex": ["UNSUPPORTED: libcpp-has-no-localization"],
   "semaphore": ["UNSUPPORTED: libcpp-has-no-threads"],
   "shared_mutex": ["UNSUPPORTED: libcpp-has-no-threads"],
-  "thread": ["UNSUPPORTED: libcpp-has-no-threads"]
+  "stdatomic.h": ["UNSUPPORTED: libcpp-has-no-threads"],
+  "thread": ["UNSUPPORTED: libcpp-has-no-threads"],
 }
 
 def get_std_dialects():

libcxx/utils/generate_header_tests.py

@@ -27,6 +27,7 @@ header_markup = {
     "mutex": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "semaphore": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "shared_mutex": ["ifndef _LIBCPP_HAS_NO_THREADS"],
+    "stdatomic.h": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "thread": ["ifndef _LIBCPP_HAS_NO_THREADS"],
 
     "filesystem": ["ifndef _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY"],