[libc++] Enable <atomic> when threads are disabled

[ldionne]

Summary

std::atomic is, for the most part, just a thin veneer on top of compiler
builtins. Hence, it should be available even when threads are not available
on the system, and in fact there has been requests for such support.

This patch:

• Moves __libcpp_thread_poll_with_backoff to its own header so it can be used in <atomic> when threads are disabled.
• Adds a dummy backoff policy for atomic polling that doesn't know about threads.
• Adjusts the <atomic> feature-test macros so they are provided even when threads are disabled.
• Runs the <atomic> tests when threads are disabled.

rdar://77873569

[Mordante]

Nice change! LGTM modulo some nits. (Obviously assuming the CI is happy.)

libcxx/include/__thread/poll_with_backoff.h

60

Should this be marked with _LIBCPP_TYPE_VIS?

libcxx/include/atomic

1524

libcxx/test/std/atomics/types.pass.cpp

21

[nilayvaish]

libcxx/include/__config

1196

Do we know why this constraint on having atomics only when threads are enabled was put in place?

libcxx/include/__thread/poll_with_backoff.h

40–45

I am feeling slightly strange about this code. Do you think following would be better?

while (true) {

while (count < polling_count) {
// check __f
 count++;
}
// check elapsed time
// check backoff policy.

}

Or the following two separate loop version:

while (count < polling_count) {

// check __f
count++;

}
while (!__f()) {

// check elapsed time
// check backoff policy.

}

This is just for discussion since the code here has been moved from another file.

[curdeius]

libcxx/utils/generate_feature_test_macro_components.py

674

I think you need to do the same modification in other generate_* scripts. E.g. https://github.com/llvm/llvm-project/blob/main/libcxx/utils/generate_header_inclusion_tests.py#L93, https://github.com/llvm/llvm-project/blob/main/libcxx/utils/generate_header_tests.py#L24.

[ldionne]

Address review comments

[ldionne]

It seems I forgot to submit my reply to review comments. Doing this now. I'm also going to ship this since I addressed everything so far and CI is green, however please feel free to comment if you have further observations/requests and I will honor them post-commit.

libcxx/include/__config

1196

I suspect there's a couple of reasons:

1. One could argue that without threads, there is no need for atomic operations since there's no concurrent accesses. In practice, however, it can still be useful if there are threads but just not ones with an API that can be used to make libc++'s <thread> work.
2. Some large atomics require taking locks, which are most likely unavailable when threads are not available. I suspect a system that doesn't provide threads wouldn't provide something akin to libatomic, and so non-lockfree atomics wouldn't work.
3. We were including <__threading_support>, which requires threading to be available. Perhaps it was just out of "laziness" that it was disabled.

This is all speculation, I don't know for sure.

libcxx/include/__thread/poll_with_backoff.h

40–45

Oh my, actually the current implementation looks like a bug to me. I don't think the continue should be there at all, since it means we're never going to back off until we've polled __libcpp_polling_count times back-to-back (or __f() has been satisfied). Are we on the same page? If so, I'll create a patch once this is landed. *Great* catch.

60

It doesn't matter because the type doesn't have a vtable, and hopefully nobody is ever going to use its RTTI. And its only member function is marked as having hidden visibility. I would rather not use the attribute since it adds an unnecessary annotation.

libcxx/include/atomic

1524

Actually I will unindent the whole thing since IMO it increases readability most. While it's technically nested inside another #if block, it's so far away that I think it's OK to *not* indent the inner one at all.

libcxx/utils/generate_feature_test_macro_components.py

674

You're right, thanks!

[androm3da]

Questions about platforms w/limited 'atomic' support.

libcxx/include/atomic

2697

After this commit landed downstream we encountered a failure building for an arm baremetal target that defines the __CLANG_ATOMIC_*_LOCK_FREE vals to '1' ("sometimes lock free").

This results in the atomic_{,un}signed_lock_free typedefs here referencing an undefined _libcpp_{,un}signed_lock_free.

I suspect that before this commit _LIBCPP_HAS_NO_ATOMIC_HEADER was false.

I think maybe we could consider an #error: in the else case above that might make it clearer?

I could use some advice as to how to handle this -- does it make sense to have the atomic header in the case where these types are only designated 'sometimes lock free'? If not I think the fix would be to change the conditional for _LIBCPP_HAS_NO_ATOMIC_HEADER to consider these? Otherwise, if it does make sense to have an atomic header, can I just suppress these typedefs? That does seem to address the build failure so far.

[efriedma]

libcxx/include/atomic

2697

The standard suggests we should just suppress the typedefs, not the entire header.

From the current C++ standard, [compliance]p3:

The supplied version of the header <atomic> shall meet the same requirements as for a hosted implementation except that support for always lock-free integral atomic types ([atomics.lockfree]) is implementation-defined, and whether or not the type aliases atomic_­signed_­lock_­free and atomic_­unsigned_­lock_­free are defined ([atomics.alias]) is implementation-defined. The other headers listed in this table shall meet the same requirements as for a hosted implementation.

[androm3da]

libcxx/include/atomic

2697

The standard suggests we should just suppress the typedefs, not the entire header.

From the current C++ standard, [compliance]p3:

The supplied version of the header <atomic> shall meet the same requirements as for a hosted implementation except that support for always lock-free integral atomic types ([atomics.lockfree]) is implementation-defined, and whether or not the type aliases atomic_­signed_­lock_­free and atomic_­unsigned_­lock_­free are defined ([atomics.alias]) is implementation-defined. The other headers listed in this table shall meet the same requirements as for a hosted implementation.

Ok, thanks, that clears it up.

@efriedma @ldionne should I post a change like this?

--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -2691,10 +2691,13 @@ typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, char>::typ
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, unsigned char>::type      __libcpp_unsigned_lock_free;
 #else
     // No signed/unsigned lock-free types
+#define _LIBCPP_NO_LOCK_FREE_TYPES
 #endif
 
+#if !defined(_LIBCPP_NO_LOCK_FREE_TYPES)
 typedef atomic<__libcpp_signed_lock_free> atomic_signed_lock_free;
 typedef atomic<__libcpp_unsigned_lock_free> atomic_unsigned_lock_free;
+#endif
 
 #define ATOMIC_FLAG_INIT {false}
 #define ATOMIC_VAR_INIT(__v) {__v}

[efriedma]

libcxx/include/atomic

2697

Looks reasonable to me.

[ldionne]

libcxx/include/atomic

2697

Sorry for missing this thread, but I think I saw this change go by and approved it, so I guess there's no additional action needed here. Please LMK if you're still looking for an answer to this.

libcxx/include/CMakeLists.txt

set(files
   __support/solaris/floatingpoint.h
   __support/solaris/wchar.h
   __support/solaris/xlocale.h
   __support/win32/limits_msvc_win32.h
   __support/win32/locale_win32.h
   __support/xlocale/__nop_locale_mgmt.h
   __support/xlocale/__posix_l_fallback.h
   __support/xlocale/__strtonum_fallback.h
+  __thread/poll_with_backoff.h
   __threading_support
   __tree
   __tuple
   __undef_macros
   __utility/as_const.h
   __utility/cmp.h
   __utility/decay_copy.h
   __utility/declval.h

libcxx/include/__config

 #endif
 
 #if __has_feature(cxx_atomic) || __has_extension(c_atomic) || __has_keyword(_Atomic)
 #  define _LIBCPP_HAS_C_ATOMIC_IMP
 #elif defined(_LIBCPP_COMPILER_GCC)
 #  define _LIBCPP_HAS_GCC_ATOMIC_IMP
 #endif
 
-#if (!defined(_LIBCPP_HAS_C_ATOMIC_IMP) && \
-     !defined(_LIBCPP_HAS_GCC_ATOMIC_IMP) && \
-     !defined(_LIBCPP_HAS_EXTERNAL_ATOMIC_IMP)) \
-     || defined(_LIBCPP_HAS_NO_THREADS)
+#if !defined(_LIBCPP_HAS_C_ATOMIC_IMP) && \
+    !defined(_LIBCPP_HAS_EXTERNAL_ATOMIC_IMP)
 #  define _LIBCPP_HAS_NO_ATOMIC_HEADER
 #else
 #  ifndef _LIBCPP_ATOMIC_FLAG_TYPE
 #    define _LIBCPP_ATOMIC_FLAG_TYPE bool
 #  endif
 #  ifdef _LIBCPP_FREESTANDING
 #    define _LIBCPP_ATOMIC_ONLY_USE_BUILTINS
 #  endif

libcxx/include/__memory/shared_ptr.h

 };
 
 template<class _CharT, class _Traits, class _Yp>
 inline _LIBCPP_INLINE_VISIBILITY
 basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os, shared_ptr<_Yp> const& __p);
 
 
-#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#if !defined(_LIBCPP_HAS_NO_THREADS)
 
 class _LIBCPP_TYPE_VIS __sp_mut
 {
     void* __lx;
 public:
     void lock() _NOEXCEPT;
     void unlock() _NOEXCEPT;
 

 _LIBCPP_AVAILABILITY_ATOMIC_SHARED_PTR
 bool
 atomic_compare_exchange_weak_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v,
                                       shared_ptr<_Tp> __w, memory_order, memory_order)
 {
     return atomic_compare_exchange_weak(__p, __v, __w);
 }
 
-#endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#endif // !defined(_LIBCPP_HAS_NO_THREADS)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___MEMORY_SHARED_PTR_H

libcxx/include/__thread/poll_with_backoff.h

+// -*- 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___THREAD_POLL_WITH_BACKOFF_H
+#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
+
+#include <__config>
+#include <chrono>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
+
+// Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
+// before polling again.
+//
+// - __f is the "test function" that should return true if polling succeeded, and false if it failed.
+//
+// - __bf is the "backoff policy", which is called with the duration since we started polling. It should
+//   return false in order to resume polling, and true if polling should stop entirely for some reason.
+//   In general, backoff policies sleep for some time before returning control to the polling loop.
+//
+// - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
+//   the polling loop will return false to report a timeout.
+template<class _Fn, class _BFn>
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
+bool __libcpp_thread_poll_with_backoff(_Fn&& __f, _BFn&& __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {
+    auto const __start = chrono::high_resolution_clock::now();
+    for (int __count = 0;;) {
+      if (__f())
+        return true; // _Fn completion means success
+      if (__count < __libcpp_polling_count) {
+        __count += 1;
+        continue;
+      }
+      chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
+      if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
+          return false; // timeout failure
+      if (__bf(__elapsed))
+        return false; // _BFn completion means failure
+    }
+}
+
+// A trivial backoff policy that always immediately returns the control to
+// the polling loop.
+//
+// This is not very well-behaved since it will cause the polling loop to spin,
+// so this should most likely only be used on single-threaded systems where there
+// are no other threads to compete with.
+struct __spinning_backoff_policy {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  bool operator()(chrono::nanoseconds const&) const {
+      return false;
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H

libcxx/include/__threading_support

 // -*- 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_THREADING_SUPPORT
 #define _LIBCPP_THREADING_SUPPORT
 
 #include <__availability>
 #include <__config>
+#include <__thread/poll_with_backoff.h>
 #include <chrono>
 #include <errno.h>
 #include <iosfwd>
 #include <limits>
 
 #ifdef __MVS__
 # include <__support/ibm/nanosleep.h>
 #endif
bool operator()(chrono::nanoseconds __elapsed) const
       else if(__elapsed > chrono::microseconds(4))
         __libcpp_thread_yield();
       else
         {} // poll
       return false;
   }
 };
 
-static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
-
-template<class _Fn, class _BFn>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
-bool __libcpp_thread_poll_with_backoff(
-  _Fn && __f, _BFn && __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero())
-{
-    auto const __start = chrono::high_resolution_clock::now();
-    for(int __count = 0;;) {
-      if(__f())
-        return true; // _Fn completion means success
-      if(__count < __libcpp_polling_count) {
-        __count += 1;
-        continue;
-      }
-      chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
-      if(__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
-          return false; // timeout failure
-      if(__bf(__elapsed))
-        return false; // _BFn completion means failure
-    }
-}
-
 #if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
      defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL))
 
 
 namespace __thread_detail {
 
 inline __libcpp_timespec_t __convert_to_timespec(const chrono::nanoseconds& __ns)
 {

libcxx/include/atomic

 #define ATOMIC_FLAG_INIT see below
 
 }  // std
 
 */
 
 #include <__availability>
 #include <__config>
-#include <__threading_support>
+#include <__thread/poll_with_backoff.h>
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
 #include <type_traits>
 #include <version>
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+# include <__threading_support>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-# error <atomic> is not supported on this single threaded system
-#endif
 #ifdef _LIBCPP_HAS_NO_ATOMIC_HEADER
 # error <atomic> is not implemented
 #endif
 #ifdef kill_dependency
 # error C++ standard library is incompatible with <stdatomic.h>
 #endif
 
 #define _LIBCPP_CHECK_STORE_MEMORY_ORDER(__m) \

 #ifdef __linux__
     using __cxx_contention_t = int32_t;
 #else
     using __cxx_contention_t = int64_t;
 #endif //__linux__
 
 using __cxx_atomic_contention_t = __cxx_atomic_impl<__cxx_contention_t>;
 
+#if defined(_LIBCPP_HAS_NO_THREADS)
+#   define _LIBCPP_HAS_NO_PLATFORM_WAIT
+#endif
+
+// TODO:
+// _LIBCPP_HAS_NO_PLATFORM_WAIT is currently a "dead" macro, in the sense that
+// it is not tied anywhere into the build system or even documented. We should
+// clean it up because it is technically never defined except when threads are
+// disabled. We should clean it up in its own changeset in case we break "bad"
+// users.
 #ifndef _LIBCPP_HAS_NO_PLATFORM_WAIT
 
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(void const volatile*);
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(void const volatile*);
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(void const volatile*);
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __libcpp_atomic_wait(void const volatile*, __cxx_contention_t);
 
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile*);

 
 template <class _Tp>
 _LIBCPP_INLINE_VISIBILITY void __cxx_atomic_notify_all(__cxx_atomic_impl<_Tp> const volatile*) { }
 template <class _Tp>
 _LIBCPP_INLINE_VISIBILITY void __cxx_atomic_notify_one(__cxx_atomic_impl<_Tp> const volatile*) { }
 template <class _Atp, class _Fn>
 _LIBCPP_INLINE_VISIBILITY bool __cxx_atomic_wait(_Atp*, _Fn && __test_fn)
 {
-    return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
+#if defined(_LIBCPP_HAS_NO_THREADS)
+    using _Policy = __spinning_backoff_policy;
+#else
+    using _Policy = __libcpp_timed_backoff_policy;
+#endif
+    return __libcpp_thread_poll_with_backoff(__test_fn, _Policy());
 }
 
 #endif // _LIBCPP_HAS_NO_PLATFORM_WAIT
 
 template <class _Atp, class _Tp>
 struct __cxx_atomic_wait_test_fn_impl {
     _Atp* __a;
     _Tp __val;

 #elif ATOMIC_CHAR_LOCK_FREE == 2
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, char>::type               __libcpp_signed_lock_free;
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, unsigned char>::type      __libcpp_unsigned_lock_free;
 #else
     // No signed/unsigned lock-free types
 #endif
 
 typedef atomic<__libcpp_signed_lock_free> atomic_signed_lock_free;
 typedef atomic<__libcpp_unsigned_lock_free> atomic_unsigned_lock_free;
 
 #define ATOMIC_FLAG_INIT {false}
 #define ATOMIC_VAR_INIT(__v) {__v}
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_ATOMIC

libcxx/include/module.modulemap

module std [system] {
   }
   module system_error {
     header "system_error"
     export *
   }
   module thread {
     header "thread"
     export *
+
+    module __thread {
+      module poll_with_backoff { private header "__thread/poll_with_backoff.h" }
+    }
   }
   module tuple {
     header "tuple"
     export *
   }
   module type_traits {
     header "type_traits"
     export functional.__functional.unwrap_ref

libcxx/include/thread

 }  // std
 
 */
 
 #include <__config>
 #include <__debug>
 #include <__functional_base>
 #include <__mutex_base>
+#include <__thread/poll_with_backoff.h>
 #include <__threading_support>
 #include <__utility/decay_copy.h>
 #include <__utility/forward.h>
 #include <chrono>
 #include <cstddef>
 #include <functional>
 #include <iosfwd>
 #include <memory>

libcxx/include/version

 
 #if _LIBCPP_STD_VER > 14
 # define __cpp_lib_addressof_constexpr                  201603L
 # define __cpp_lib_allocator_traits_is_always_equal     201411L
 # define __cpp_lib_any                                  201606L
 # define __cpp_lib_apply                                201603L
 # define __cpp_lib_array_constexpr                      201603L
 # define __cpp_lib_as_const                             201510L
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   define __cpp_lib_atomic_is_always_lock_free         201603L
-# endif
+# define __cpp_lib_atomic_is_always_lock_free           201603L
 # define __cpp_lib_bool_constant                        201505L
 // # define __cpp_lib_boyer_moore_searcher                 201603L
 # define __cpp_lib_byte                                 201603L
 # define __cpp_lib_chrono                               201611L
 # define __cpp_lib_clamp                                201603L
 # define __cpp_lib_enable_shared_from_this              201603L
 // # define __cpp_lib_execution                            201603L
 # if !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_filesystem)

 # define __cpp_lib_variant                              202102L
 # define __cpp_lib_void_t                               201411L
 #endif
 
 #if _LIBCPP_STD_VER > 17
 # undef  __cpp_lib_array_constexpr
 # define __cpp_lib_array_constexpr                      201811L
 // # define __cpp_lib_assume_aligned                       201811L
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   define __cpp_lib_atomic_flag_test                   201907L
-# endif
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-// #   define __cpp_lib_atomic_float                       201711L
-# endif
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   define __cpp_lib_atomic_lock_free_type_aliases      201907L
-# endif
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-// #   define __cpp_lib_atomic_ref                         201806L
-# endif
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-// #   define __cpp_lib_atomic_shared_ptr                  201711L
-# endif
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   define __cpp_lib_atomic_value_initialization        201911L
-# endif
-# if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
+# define __cpp_lib_atomic_flag_test                     201907L
+// # define __cpp_lib_atomic_float                         201711L
+# define __cpp_lib_atomic_lock_free_type_aliases        201907L
+// # define __cpp_lib_atomic_ref                           201806L
+// # define __cpp_lib_atomic_shared_ptr                    201711L
+# define __cpp_lib_atomic_value_initialization          201911L
+# if !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
 #   define __cpp_lib_atomic_wait                        201907L
 # endif
 # if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier)
 #   define __cpp_lib_barrier                            201907L
 # endif
 # define __cpp_lib_bind_front                           201907L
 # define __cpp_lib_bit_cast                             201806L
 // # define __cpp_lib_bitops                               201907L

libcxx/src/memory.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #include "memory"
 #ifndef _LIBCPP_HAS_NO_THREADS
-#include "mutex"
-#include "thread"
-#if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
-#pragma comment(lib, "pthread")
-#endif
+#   include "mutex"
+#   include "thread"
+#   if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB)
+#       pragma comment(lib, "pthread")
+#   endif
 #endif
 #include "include/atomic_support.h"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 const allocator_arg_t allocator_arg = allocator_arg_t();
 
 bad_weak_ptr::~bad_weak_ptr() noexcept {}

 }
 
 const void*
 __shared_weak_count::__get_deleter(const type_info&) const noexcept
 {
     return nullptr;
 }
 
-#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#if !defined(_LIBCPP_HAS_NO_THREADS)
 
 _LIBCPP_SAFE_STATIC static const std::size_t __sp_mut_count = 16;
 _LIBCPP_SAFE_STATIC static __libcpp_mutex_t mut_back[__sp_mut_count] =
 {
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER
static __sp_mut muts[__sp_mut_count]
         &mut_back[ 0], &mut_back[ 1], &mut_back[ 2], &mut_back[ 3],
         &mut_back[ 4], &mut_back[ 5], &mut_back[ 6], &mut_back[ 7],
         &mut_back[ 8], &mut_back[ 9], &mut_back[10], &mut_back[11],
         &mut_back[12], &mut_back[13], &mut_back[14], &mut_back[15]
     };
     return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
 }
 
-#endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#endif // !defined(_LIBCPP_HAS_NO_THREADS)
 
 void*
 align(size_t alignment, size_t size, void*& ptr, size_t& space)
 {
     void* r = nullptr;
     if (size <= space)
     {
         char* p1 = static_cast<char*>(ptr);

libcxx/test/libcxx/atomics/atomics.align/align.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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, c++03
+// UNSUPPORTED: c++03
 // REQUIRES: is-lockfree-runtime-function
 // ADDITIONAL_COMPILE_FLAGS: -Wno-psabi
 // ... since C++20 std::__atomic_base initializes, so we get a warning about an
 // ABI change for vector variants since the constructor code for that is
 // different if one were to compile with architecture-specific vector
 // extensions enabled.
 // This however isn't ABI breaking as it was impossible for any code to trigger
 // this without using libc++ internals.

libcxx/test/libcxx/atomics/atomics.flag/init_bool.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // TESTING EXTENSION atomic_flag(bool)
 
 #include <atomic>

libcxx/test/libcxx/atomics/atomics.order/memory_order.underlying_type.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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 std::memory_order has the same size under all
 // standard versions to make sure we're not breaking the ABI. This is
 // relevant because std::memory_order is a scoped enumeration in C++20,
 // but an unscoped enumeration pre-C++20.
 //
 // See PR40977 for details.
 
 #include <atomic>

libcxx/test/libcxx/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // template <class T>
 //     T* atomic_fetch_add(volatile atomic<T*>* obj, ptrdiff_t op)
 // template <class T>
 //     T* atomic_fetch_add(atomic<T*>* obj, ptrdiff_t op);
 
 #include <atomic>

libcxx/test/libcxx/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // template <class T>
 //     T*
 //     atomic_fetch_add_explicit(volatile atomic<T*>* obj, ptrdiff_t op,
 //                               memory_order m);
 // template <class T>
 //     T*

libcxx/test/libcxx/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // template <class T>
 //     T* atomic_fetch_sub(volatile atomic<T*>* obj, ptrdiff_t op)
 // template <class T>
 //     T* atomic_fetch_sub(atomic<T*>* obj, ptrdiff_t op);
 
 #include <atomic>

libcxx/test/libcxx/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // template <class T>
 //     T*
 //     atomic_fetch_sub_explicit(volatile atomic<T*>* obj, ptrdiff_t op,
 //                               memory_order m);
 // template <class T>
 //     T*

libcxx/test/libcxx/atomics/diagnose_invalid_memory_order.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // This test fails because diagnose_if doesn't emit all of the diagnostics
 // when -fdelayed-template-parsing is enabled, like it is in MSVC mode.
 // XFAIL: msvc
 
 // REQUIRES: diagnose-if-support
-// UNSUPPORTED: libcpp-has-no-threads
 
 // <atomic>
 
 // Test that invalid memory order arguments are diagnosed where possible.
 
 #include <atomic>
 
 int main(int, char**) {

libcxx/test/libcxx/atomics/ext-int.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <atomic>
 
 // Make sure that `std::atomic` doesn't work with `_ExtInt`. The intent is to
 // disable them for now until their behavior can be designed better later.
 // See https://reviews.llvm.org/D84049 for details.
 
 // UNSUPPORTED: apple-clang-12
 
-// UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++03
 
 #include <atomic>
 
 int main(int, char**)
 {
   // expected-error@atomic:*1 {{_Atomic cannot be applied to integer type '_ExtInt(32)'}}
   std::atomic<_ExtInt(32)> x(42);
 
   return 0;
 }

libcxx/test/libcxx/atomics/libcpp-has-no-threads.pass.cpp

-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-// XFAIL: libcpp-has-no-threads
-
-#ifdef _LIBCPP_HAS_NO_THREADS
-#error This should be XFAILed for the purpose of detecting that the LIT feature\
-   'libcpp-has-no-threads' is available iff _LIBCPP_HAS_NO_THREADS is defined
-#endif
-
-int main(int, char**)
-{
-
-  return 0;
-}

libcxx/test/libcxx/atomics/version.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 #include <atomic>
 
 #include "test_macros.h"
 
 #ifndef _LIBCPP_VERSION
 #error _LIBCPP_VERSION not defined
 #endif
 
 int main(int, char**)
 {
 
   return 0;
 }

libcxx/test/libcxx/diagnostics/detail.headers/thread/poll_with_backoff.module.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
-// <atomic>
+// REQUIRES: modules-build
 
-// Test that including <atomic> fails to compile when _LIBCPP_HAS_NO_THREADS
-// is defined.
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_HAS_NO_THREADS
-
-#include <atomic>
-
-int main(int, char**)
-{
-
-  return 0;
-}
+// expected-error@*:* {{use of private header from outside its module: '__thread/poll_with_backoff.h'}}
+#include <__thread/poll_with_backoff.h>

libcxx/test/libcxx/double_include.sh.cpp

 
 // WARNING: This test was generated by generate_header_tests.py
 // and should not be edited manually.
 
 // Top level headers
 #include <algorithm>
 #include <any>
 #include <array>
-#ifndef _LIBCPP_HAS_NO_THREADS
-#    include <atomic>
-#endif
+#include <atomic>
 #ifndef _LIBCPP_HAS_NO_THREADS
 #    include <barrier>
 #endif
 #include <bit>
 #include <bitset>
 #include <cassert>
 #include <ccomplex>
 #include <cctype>

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

 
 // Top level headers
 #include <algorithm>
 TEST_MACROS();
 #include <any>
 TEST_MACROS();
 #include <array>
 TEST_MACROS();
-#ifndef _LIBCPP_HAS_NO_THREADS
-#    include <atomic>
+#include <atomic>
 TEST_MACROS();
-#endif
 #ifndef _LIBCPP_HAS_NO_THREADS
 #    include <barrier>
 TEST_MACROS();
 #endif
 #include <bit>
 TEST_MACROS();
 #include <bitset>
 TEST_MACROS();

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

 
 // WARNING: This test was generated by generate_header_tests.py
 // and should not be edited manually.
 
 // Top level headers
 #include <algorithm>
 #include <any>
 #include <array>
-#ifndef _LIBCPP_HAS_NO_THREADS
-#    include <atomic>
-#endif
+#include <atomic>
 #ifndef _LIBCPP_HAS_NO_THREADS
 #    include <barrier>
 #endif
 #include <bit>
 #include <bitset>
 #include <ccomplex>
 #include <cctype>
 #include <cerrno>

libcxx/test/std/atomics/atomics.fences/atomic_signal_fence.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // void atomic_signal_fence(memory_order m);
 
 #include <atomic>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     std::atomic_signal_fence(std::memory_order_seq_cst);
 
   return 0;
 }

libcxx/test/std/atomics/atomics.fences/atomic_thread_fence.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // void atomic_thread_fence(memory_order m);
 
 #include <atomic>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     std::atomic_thread_fence(std::memory_order_seq_cst);
 
   return 0;
 }

libcxx/test/std/atomics/atomics.flag/atomic_flag_clear.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // void atomic_flag_clear(volatile atomic_flag*);
 // void atomic_flag_clear(atomic_flag*);
 

libcxx/test/std/atomics/atomics.flag/atomic_flag_clear_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // void atomic_flag_clear_explicit(volatile atomic_flag*, memory_order);
 // void atomic_flag_clear_explicit(atomic_flag*, memory_order);
 

libcxx/test/std/atomics/atomics.flag/atomic_flag_test.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // bool atomic_flag_test(const volatile atomic_flag*);
 // bool atomic_flag_test(const atomic_flag*);
 

libcxx/test/std/atomics/atomics.flag/atomic_flag_test_and_set.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // bool atomic_flag_test_and_set(volatile atomic_flag*);
 // bool atomic_flag_test_and_set(atomic_flag*);
 

libcxx/test/std/atomics/atomics.flag/atomic_flag_test_and_set_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // bool atomic_flag_test_and_set_explicit(volatile atomic_flag*, memory_order);
 // bool atomic_flag_test_and_set_explicit(atomic_flag*, memory_order);
 

libcxx/test/std/atomics/atomics.flag/atomic_flag_test_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // bool atomic_flag_test_explicit(volatile atomic_flag*, memory_order);
 // bool atomic_flag_test_explicit(atomic_flag*, memory_order);
 

libcxx/test/std/atomics/atomics.flag/clear.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // void clear(memory_order = memory_order_seq_cst);
 // void clear(memory_order = memory_order_seq_cst) volatile;
 

libcxx/test/std/atomics/atomics.flag/default.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // atomic_flag() = default;
 
 #include <atomic>

libcxx/test/std/atomics/atomics.flag/init.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: c++03
 
 // <atomic>
 
 // struct atomic_flag
 
 // atomic_flag() = ATOMIC_FLAG_INIT;
 

libcxx/test/std/atomics/atomics.flag/test_and_set.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // struct atomic_flag
 
 // bool test_and_set(memory_order = memory_order_seq_cst);
 // bool test_and_set(memory_order = memory_order_seq_cst) volatile;
 

libcxx/test/std/atomics/atomics.general/replace_failure_order.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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 verifies behavior specified by [atomics.types.operations.req]/21:
 //
 //     When only one memory_order argument is supplied, the value of success is
 //     order, and the value of failure is order except that a value of
 //     memory_order_acq_rel shall be replaced by the value memory_order_acquire
 //     and a value of memory_order_release shall be replaced by the value
 //     memory_order_relaxed.
 //

libcxx/test/std/atomics/atomics.general/replace_failure_order_codegen.sh.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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: clang
-// UNSUPPORTED: libcpp-has-no-threads
 
 // Adding "-fsanitize=thread" directly causes many platforms to fail (because
 // they don't support tsan), and causes other sanitizer builds to fail (e.g.
 // asan and tsan don't mix). Instead, require the tsan feature.
 // REQUIRES: tsan
 
 // This test verifies behavior specified by [atomics.types.operations.req]/21:
 //

libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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, c++03, c++11, c++14
+// UNSUPPORTED: c++03, c++11, c++14
 
 // Fails for 32-bit builds.
 // UNSUPPORTED: LIBCXX-AIX-FIXME
 
 // <atomic>
 
 // static constexpr bool is_always_lock_free;
 

libcxx/test/std/atomics/atomics.lockfree/lockfree.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // #define ATOMIC_BOOL_LOCK_FREE unspecified
 // #define ATOMIC_CHAR_LOCK_FREE unspecified
 // #define ATOMIC_CHAR8_T_LOCK_FREE unspecified // C++20
 // #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
 // #define ATOMIC_CHAR32_T_LOCK_FREE unspecified

libcxx/test/std/atomics/atomics.order/kill_dependency.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template <class T> T kill_dependency(T y);
 
 #include <atomic>
 #include <cassert>
 

libcxx/test/std/atomics/atomics.order/memory_order.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // typedef enum memory_order
 // {
 //     memory_order_relaxed, memory_order_consume, memory_order_acquire,
 //     memory_order_release, memory_order_acq_rel, memory_order_seq_cst
 // } memory_order;

libcxx/test/std/atomics/atomics.order/memory_order_new.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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, c++03, c++11, c++14, c++17
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
 
 #include <atomic>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     static_assert(std::memory_order_relaxed == std::memory_order::relaxed);
     static_assert(std::memory_order_consume == std::memory_order::consume);
     static_assert(std::memory_order_acquire == std::memory_order::acquire);
     static_assert(std::memory_order_release == std::memory_order::release);
     static_assert(std::memory_order_acq_rel == std::memory_order::acq_rel);
     static_assert(std::memory_order_seq_cst == std::memory_order::seq_cst);
 
     return 0;
 }

libcxx/test/std/atomics/atomics.types.generic/address.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template <class T>
 // struct atomic<T*>
 // {
 //     bool is_lock_free() const volatile;
 //     bool is_lock_free() const;

libcxx/test/std/atomics/atomics.types.generic/bool.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template <class T>
 // struct atomic
 // {
 //     bool is_lock_free() const volatile;
 //     bool is_lock_free() const;

libcxx/test/std/atomics/atomics.types.generic/constexpr_noexcept.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-// UNSUPPORTED: libcpp-has-no-threads
 
 #include <atomic>
 
 #include "test_macros.h"
 
 template <typename T>
 constexpr bool test() {
   [[maybe_unused]] constexpr T a;

libcxx/test/std/atomics/atomics.types.generic/copy_semantics_traits.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-// UNSUPPORTED: libcpp-has-no-threads
 
 // <atomic>
 
 // template <class T>
 // struct atomic
 // {
 //     atomic(const atomic&) = delete;
 //     atomic& operator=(const atomic&) = delete;
 //     atomic& operator=(const atomic&) volatile = delete;
 // };
 
 // template <class T>
 // struct atomic<T*>
 // {
 //     atomic(const atomic&) = delete;
 //     atomic& operator=(const atomic&) = delete;
 //     atomic& operator=(const atomic&) volatile = delete;
 // };
 
 #include <atomic>
-
 #include <type_traits>
 
 template <typename T>
 using is_volatile_copy_assignable = std::is_assignable<volatile T&, const T&>;
 
 int main(int, char**)
 {
     static_assert(!std::is_copy_constructible<std::atomic<int> >::value, "");
     static_assert(!std::is_copy_assignable<std::atomic<int> >::value, "");
     static_assert(!is_volatile_copy_assignable<std::atomic<int> >::value, "");
     static_assert(!std::is_copy_constructible<std::atomic<int*> >::value, "");
     static_assert(!std::is_copy_assignable<std::atomic<int*> >::value, "");
     static_assert(!is_volatile_copy_assignable<std::atomic<int*> >::value, "");
 
     return 0;
 }

libcxx/test/std/atomics/atomics.types.generic/cstdint_typedefs.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // typedef atomic<int_least8_t>   atomic_int_least8_t;
 // typedef atomic<uint_least8_t>  atomic_uint_least8_t;
 // typedef atomic<int_least16_t>  atomic_int_least16_t;
 // typedef atomic<uint_least16_t> atomic_uint_least16_t;
 // typedef atomic<int_least32_t>  atomic_int_least32_t;

libcxx/test/std/atomics/atomics.types.generic/integral.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template <>
 // struct atomic<integral>
 // {
 //     bool is_lock_free() const volatile;
 //     bool is_lock_free() const;

libcxx/test/std/atomics/atomics.types.generic/integral_typedefs.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // typedef atomic<char>               atomic_char;
 // typedef atomic<signed char>        atomic_schar;
 // typedef atomic<unsigned char>      atomic_uchar;
 // typedef atomic<short>              atomic_short;
 // typedef atomic<unsigned short>     atomic_ushort;

libcxx/test/std/atomics/atomics.types.generic/standard_layout.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // UNSUPPORTED: c++03
 
 // <atomic>
 
 #include <atomic>
 #include <type_traits>
 #include <cassert>
 

libcxx/test/std/atomics/atomics.types.generic/throw.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-// UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: no-exceptions
 
 // <atomic>
 
 #include <atomic>
 #include <cassert>
 
 struct throwing {

libcxx/test/std/atomics/atomics.types.generic/trivially_copyable.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template <class T>
 // struct atomic;
 
 // Make sure atomic<TriviallyCopyable> can be instantiated.
 
 #include <atomic>
 #include <new>
 #include <cassert>
-#include <thread> // for thread_id
 #include <chrono> // for nanoseconds
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+#   include <thread> // for thread_id
+#endif
+
 struct TriviallyCopyable {
   explicit TriviallyCopyable(int i) : i_(i) { }
   int i_;
 };
 
 template <class T>
 void test(T t) {
   std::atomic<T> t0(t);
 }
 
 int main(int, char**) {
   test(TriviallyCopyable(42));
-  test(std::this_thread::get_id());
   test(std::chrono::nanoseconds(2));
+#ifndef _LIBCPP_HAS_NO_THREADS
+  test(std::this_thread::get_id());
+#endif
 
   return 0;
 }

libcxx/test/std/atomics/atomics.types.generic/trivially_copyable.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // template <class T>
 // struct atomic;
 
 // This test checks that we static_assert inside std::atomic<T> when T
 // is not trivially copyable, however Clang will sometimes emit additional
 // errors while trying to instantiate the rest of std::atomic<T>.

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_strong.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     bool
 //     atomic_compare_exchange_strong(volatile atomic<T>*,
 //                                    atomic<T>::value_type*,

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_strong_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     bool
 //     atomic_compare_exchange_strong_explicit(volatile atomic<T>*,
 //                                             atomic<T>::value_type*,

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_weak.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     bool
 //     atomic_compare_exchange_weak(volatile atomic<T>*,
 //                                  atomic<T>::value_type*,

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_weak_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     bool
 //     atomic_compare_exchange_weak_explicit(volatile atomic<T>*,
 //                                           atomic<T>::value_type*,

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_exchange.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     T
 //     atomic_exchange(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_exchange_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     T
 //     atomic_exchange_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                              memory_order) noexcept;

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_add(volatile atomic<T>* obj, atomic<T>::difference_type) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_add_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_add_explicit(volatile atomic<T>*, atomic<T>::difference_type,
 //                               memory_order) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_and(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_and_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_and_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                               memory_order) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_or(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_or_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_or_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                              memory_order) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_sub(volatile atomic<T>*, atomic<T>::difference_type) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_sub_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_sub_explicit(volatile atomic<T>*, atomic<T>::difference_type,
 //                               memory_order) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_xor(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //
 // template<class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_fetch_xor_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <atomic>
 
 // template<class T>
 //     T
 //     atomic_fetch_xor_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                               memory_order) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_init.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
 
 // <atomic>
 
 // template <class T>
 //     void
 //     atomic_init(volatile atomic<T>* obj, atomic<T>::value_type desr) noexcept;

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_is_lock_free.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !is-lockfree-runtime-function
 
 // <atomic>
 
 // template <class T>
 // bool atomic_is_lock_free(const volatile atomic<T>* obj) noexcept;
 //
 // template <class T>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_load.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     T
 //     atomic_load(const volatile atomic<T>*) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_load_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     T
 //     atomic_load_explicit(const volatile atomic<T>*, memory_order) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_store.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     void
 //     atomic_store(volatile atomic<T>*, atomic<T>::value_type) noexcept;
 //

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_store_explicit.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: !non-lockfree-atomics
 
 // <atomic>
 
 // template <class T>
 //     void
 //     atomic_store_explicit(volatile atomic<T>*, atomic<T>::value_type,
 //                           memory_order) noexcept;

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_var_init.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // XFAIL: c++03
 
 // <atomic>
 
 // #define ATOMIC_VAR_INIT(value)
 
 #include <atomic>
 #include <type_traits>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/copy.assign.ptr.volatile.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // std::atomic
 
 // atomic& operator=( const atomic& ) volatile = delete;
 
 #include <atomic>
 
 int main(int, char**)
 {
     volatile std::atomic<int*> obj1;
     std::atomic<int*> obj2;
     obj1 = obj2; // expected-error {{overload resolution selected deleted operator '='}}
 }

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/copy.assign.volatile.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // std::atomic
 
 // atomic& operator=( const atomic& ) volatile = delete;
 
 #include <atomic>
 
 int main(int, char**)
 {
   volatile std::atomic<int> obj1;
   std::atomic<int> obj2;
   obj1 = obj2; // expected-error {{overload resolution selected deleted operator '='}}
 }

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/ctor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // UNSUPPORTED: c++03
 
 // <atomic>
 
 // constexpr atomic<T>::atomic(T value)
 
 #include <atomic>
 #include <type_traits>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/dtor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
+
 // UNSUPPORTED: c++03
 
 // <atomic>
 
 // constexpr atomic<T>::~atomic()
 
 #include <atomic>
 #include <type_traits>

libcxx/test/std/atomics/types.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // Test nested types
 
 // template <class T>
 // class atomic
 // {
 // public:
-//     typedef T                                        value_type;
+//     typedef T value_type;
 // };
 
 #include <atomic>
+#include <chrono>
+#include <memory>
 #include <type_traits>
 
-#include <thread>
-#include <chrono>
+#ifndef _LIBCPP_HAS_NO_THREADS
+#   include <thread>
+#endif
 
 #include "test_macros.h"
 
-#if TEST_STD_VER >= 20
-# include <memory>
-#endif
-
 template <class A, bool Integral>
 struct test_atomic
 {
     test_atomic()
     {
         A a; (void)a;
 #if TEST_STD_VER >= 17
     static_assert((std::is_same_v<typename A::value_type, decltype(a.load())>), "");
#ifndef __APPLE__ // Apple doesn't ship libatomic
     /*
         These aren't going to be lock-free,
         so some libatomic.a is necessary.
     */
     test<WeirdTriviallyCopyable>();
     test<LargeTriviallyCopyable>();
 #endif
 
+#ifndef _LIBCPP_HAS_NO_THREADS
     test<std::thread::id>();
+#endif
     test<std::chrono::nanoseconds>();
     test<float>();
 
 #if TEST_STD_VER >= 20
     test<std::atomic_signed_lock_free::value_type>();
     test<std::atomic_unsigned_lock_free::value_type>();
 /*
     test<std::shared_ptr<int>>();
 */
 #endif
 
     return 0;
 }

libcxx/test/std/depr/depr.atomics/depr.atomics.nonmembers/atomic_init.depr_in_cxx20.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-// UNSUPPORTED: libcpp-has-no-threads
 
 // <atomic>
 //
 // atomic_init
 
 #include <atomic>
 
 void test() {
   std::atomic<int> a;
   std::atomic_init(&a, 1); // expected-warning {{'atomic_init<int>' is deprecated}}
 
   volatile std::atomic<int> b;
   std::atomic_init(&b, 1); // expected-warning {{'atomic_init<int>' is deprecated}}
 }

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

 //===----------------------------------------------------------------------===//
 //
 // 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
-
 // <atomic>
 
 // Test the feature test macros defined by <atomic>
 
 /*  Constant                                   Value
     __cpp_lib_atomic_flag_test                 201907L [C++20]
     __cpp_lib_atomic_float                     201711L [C++20]
     __cpp_lib_atomic_is_always_lock_free       201603L [C++17]

 # ifdef __cpp_lib_atomic_flag_test
 #   error "__cpp_lib_atomic_flag_test should not be defined before c++20"
 # endif
 
 # ifdef __cpp_lib_atomic_float
 #   error "__cpp_lib_atomic_float should not be defined before c++20"
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should be defined in c++17"
-#   endif
-#   if __cpp_lib_atomic_is_always_lock_free != 201603L
-#     error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++17"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_is_always_lock_free
+#   error "__cpp_lib_atomic_is_always_lock_free should be defined in c++17"
+# endif
+# if __cpp_lib_atomic_is_always_lock_free != 201603L
+#   error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++17"
+# endif
 
 # ifdef __cpp_lib_atomic_lock_free_type_aliases
 #   error "__cpp_lib_atomic_lock_free_type_aliases should not be defined before c++20"
 # endif
 
 # ifdef __cpp_lib_atomic_ref
 #   error "__cpp_lib_atomic_ref should not be defined before c++20"
 # endif

 # endif
 
 # ifdef __cpp_lib_char8_t
 #   error "__cpp_lib_char8_t should not be defined before c++20"
 # endif
 
 #elif TEST_STD_VER == 20
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_flag_test != 201907L
-#     error "__cpp_lib_atomic_flag_test should have the value 201907L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_flag_test
+#   error "__cpp_lib_atomic_flag_test should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_flag_test != 201907L
+#   error "__cpp_lib_atomic_flag_test should have the value 201907L in c++20"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should be defined in c++20"
 #   endif
 #   if __cpp_lib_atomic_float != 201711L
 #     error "__cpp_lib_atomic_float should have the value 201711L in c++20"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_is_always_lock_free != 201603L
-#     error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_is_always_lock_free
+#   error "__cpp_lib_atomic_is_always_lock_free should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_is_always_lock_free != 201603L
+#   error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++20"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_lock_free_type_aliases != 201907L
-#     error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_lock_free_type_aliases
+#   error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_lock_free_type_aliases != 201907L
+#   error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++20"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_ref
 #     error "__cpp_lib_atomic_ref should be defined in c++20"
 #   endif
 #   if __cpp_lib_atomic_ref != 201806L
 #     error "__cpp_lib_atomic_ref should have the value 201806L in c++20"
 #   endif

 #     error "__cpp_lib_atomic_shared_ptr should have the value 201711L in c++20"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_shared_ptr
 #     error "__cpp_lib_atomic_shared_ptr should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_value_initialization != 201911L
-#     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_value_initialization
+#   error "__cpp_lib_atomic_value_initialization should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_value_initialization != 201911L
+#   error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++20"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
+# if !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
 #   ifndef __cpp_lib_atomic_wait
 #     error "__cpp_lib_atomic_wait should be defined in c++20"
 #   endif
 #   if __cpp_lib_atomic_wait != 201907L
 #     error "__cpp_lib_atomic_wait should have the value 201907L in c++20"
 #   endif
 # else
 #   ifdef __cpp_lib_atomic_wait
-#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
+#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
 #   endif
 # endif
 
 # if defined(__cpp_char8_t)
 #   ifndef __cpp_lib_char8_t
 #     error "__cpp_lib_char8_t should be defined in c++20"
 #   endif
 #   if __cpp_lib_char8_t != 201811L
 #     error "__cpp_lib_char8_t should have the value 201811L in c++20"
 #   endif
 # else
 #   ifdef __cpp_lib_char8_t
 #     error "__cpp_lib_char8_t should not be defined when defined(__cpp_char8_t) is not defined!"
 #   endif
 # endif
 
 #elif TEST_STD_VER > 20
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_flag_test != 201907L
-#     error "__cpp_lib_atomic_flag_test should have the value 201907L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_flag_test
+#   error "__cpp_lib_atomic_flag_test should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_flag_test != 201907L
+#   error "__cpp_lib_atomic_flag_test should have the value 201907L in c++2b"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should be defined in c++2b"
 #   endif
 #   if __cpp_lib_atomic_float != 201711L
 #     error "__cpp_lib_atomic_float should have the value 201711L in c++2b"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_is_always_lock_free != 201603L
-#     error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_is_always_lock_free
+#   error "__cpp_lib_atomic_is_always_lock_free should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_is_always_lock_free != 201603L
+#   error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++2b"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_lock_free_type_aliases != 201907L
-#     error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_lock_free_type_aliases
+#   error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_lock_free_type_aliases != 201907L
+#   error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++2b"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_ref
 #     error "__cpp_lib_atomic_ref should be defined in c++2b"
 #   endif
 #   if __cpp_lib_atomic_ref != 201806L
 #     error "__cpp_lib_atomic_ref should have the value 201806L in c++2b"
 #   endif

 #     error "__cpp_lib_atomic_shared_ptr should have the value 201711L in c++2b"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_shared_ptr
 #     error "__cpp_lib_atomic_shared_ptr should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_value_initialization != 201911L
-#     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_value_initialization
+#   error "__cpp_lib_atomic_value_initialization should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_value_initialization != 201911L
+#   error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2b"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
+# if !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
 #   ifndef __cpp_lib_atomic_wait
 #     error "__cpp_lib_atomic_wait should be defined in c++2b"
 #   endif
 #   if __cpp_lib_atomic_wait != 201907L
 #     error "__cpp_lib_atomic_wait should have the value 201907L in c++2b"
 #   endif
 # else
 #   ifdef __cpp_lib_atomic_wait
-#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
+#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
 #   endif
 # endif
 
 # if defined(__cpp_char8_t)
 #   ifndef __cpp_lib_char8_t
 #     error "__cpp_lib_char8_t should be defined in c++2b"
 #   endif
 #   if __cpp_lib_char8_t != 201811L

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

 #     error "__cpp_lib_assume_aligned should have the value 201811L in c++20"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_assume_aligned
 #     error "__cpp_lib_assume_aligned should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_value_initialization != 201911L
-#     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_value_initialization
+#   error "__cpp_lib_atomic_value_initialization should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_value_initialization != 201911L
+#   error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++20"
+# endif
 
 # ifndef __cpp_lib_constexpr_dynamic_alloc
 #   error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++20"
 # endif
 # if __cpp_lib_constexpr_dynamic_alloc != 201907L
 #   error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++20"
 # endif
 

 #     error "__cpp_lib_assume_aligned should have the value 201811L in c++2b"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_assume_aligned
 #     error "__cpp_lib_assume_aligned should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_value_initialization != 201911L
-#     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_value_initialization
+#   error "__cpp_lib_atomic_value_initialization should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_value_initialization != 201911L
+#   error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2b"
+# endif
 
 # ifndef __cpp_lib_constexpr_dynamic_alloc
 #   error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2b"
 # endif
 # if __cpp_lib_constexpr_dynamic_alloc != 201907L
 #   error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2b"
 # endif
 

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

 # ifdef __cpp_lib_atomic_flag_test
 #   error "__cpp_lib_atomic_flag_test should not be defined before c++20"
 # endif
 
 # ifdef __cpp_lib_atomic_float
 #   error "__cpp_lib_atomic_float should not be defined before c++20"
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should be defined in c++17"
-#   endif
-#   if __cpp_lib_atomic_is_always_lock_free != 201603L
-#     error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++17"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_is_always_lock_free
+#   error "__cpp_lib_atomic_is_always_lock_free should be defined in c++17"
+# endif
+# if __cpp_lib_atomic_is_always_lock_free != 201603L
+#   error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++17"
+# endif
 
 # ifdef __cpp_lib_atomic_lock_free_type_aliases
 #   error "__cpp_lib_atomic_lock_free_type_aliases should not be defined before c++20"
 # endif
 
 # ifdef __cpp_lib_atomic_ref
 #   error "__cpp_lib_atomic_ref should not be defined before c++20"
 # endif

 #     error "__cpp_lib_assume_aligned should have the value 201811L in c++20"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_assume_aligned
 #     error "__cpp_lib_assume_aligned should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_flag_test != 201907L
-#     error "__cpp_lib_atomic_flag_test should have the value 201907L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_flag_test
+#   error "__cpp_lib_atomic_flag_test should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_flag_test != 201907L
+#   error "__cpp_lib_atomic_flag_test should have the value 201907L in c++20"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should be defined in c++20"
 #   endif
 #   if __cpp_lib_atomic_float != 201711L
 #     error "__cpp_lib_atomic_float should have the value 201711L in c++20"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_is_always_lock_free != 201603L
-#     error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_is_always_lock_free
+#   error "__cpp_lib_atomic_is_always_lock_free should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_is_always_lock_free != 201603L
+#   error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++20"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_lock_free_type_aliases != 201907L
-#     error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_lock_free_type_aliases
+#   error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_lock_free_type_aliases != 201907L
+#   error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++20"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_ref
 #     error "__cpp_lib_atomic_ref should be defined in c++20"
 #   endif
 #   if __cpp_lib_atomic_ref != 201806L
 #     error "__cpp_lib_atomic_ref should have the value 201806L in c++20"
 #   endif

 #     error "__cpp_lib_atomic_shared_ptr should have the value 201711L in c++20"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_shared_ptr
 #     error "__cpp_lib_atomic_shared_ptr should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should be defined in c++20"
-#   endif
-#   if __cpp_lib_atomic_value_initialization != 201911L
-#     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++20"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_value_initialization
+#   error "__cpp_lib_atomic_value_initialization should be defined in c++20"
+# endif
+# if __cpp_lib_atomic_value_initialization != 201911L
+#   error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++20"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
+# if !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
 #   ifndef __cpp_lib_atomic_wait
 #     error "__cpp_lib_atomic_wait should be defined in c++20"
 #   endif
 #   if __cpp_lib_atomic_wait != 201907L
 #     error "__cpp_lib_atomic_wait should have the value 201907L in c++20"
 #   endif
 # else
 #   ifdef __cpp_lib_atomic_wait
-#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
+#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
 #   endif
 # endif
 
 # if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier)
 #   ifndef __cpp_lib_barrier
 #     error "__cpp_lib_barrier should be defined in c++20"
 #   endif
 #   if __cpp_lib_barrier != 201907L

 #     error "__cpp_lib_assume_aligned should have the value 201811L in c++2b"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_assume_aligned
 #     error "__cpp_lib_assume_aligned should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_flag_test != 201907L
-#     error "__cpp_lib_atomic_flag_test should have the value 201907L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_flag_test
-#     error "__cpp_lib_atomic_flag_test should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_flag_test
+#   error "__cpp_lib_atomic_flag_test should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_flag_test != 201907L
+#   error "__cpp_lib_atomic_flag_test should have the value 201907L in c++2b"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should be defined in c++2b"
 #   endif
 #   if __cpp_lib_atomic_float != 201711L
 #     error "__cpp_lib_atomic_float should have the value 201711L in c++2b"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_float
 #     error "__cpp_lib_atomic_float should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_is_always_lock_free != 201603L
-#     error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_is_always_lock_free
-#     error "__cpp_lib_atomic_is_always_lock_free should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_is_always_lock_free
+#   error "__cpp_lib_atomic_is_always_lock_free should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_is_always_lock_free != 201603L
+#   error "__cpp_lib_atomic_is_always_lock_free should have the value 201603L in c++2b"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_lock_free_type_aliases != 201907L
-#     error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_lock_free_type_aliases
-#     error "__cpp_lib_atomic_lock_free_type_aliases should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_lock_free_type_aliases
+#   error "__cpp_lib_atomic_lock_free_type_aliases should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_lock_free_type_aliases != 201907L
+#   error "__cpp_lib_atomic_lock_free_type_aliases should have the value 201907L in c++2b"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_atomic_ref
 #     error "__cpp_lib_atomic_ref should be defined in c++2b"
 #   endif
 #   if __cpp_lib_atomic_ref != 201806L
 #     error "__cpp_lib_atomic_ref should have the value 201806L in c++2b"
 #   endif

 #     error "__cpp_lib_atomic_shared_ptr should have the value 201711L in c++2b"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_shared_ptr
 #     error "__cpp_lib_atomic_shared_ptr should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS)
-#   ifndef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should be defined in c++2b"
-#   endif
-#   if __cpp_lib_atomic_value_initialization != 201911L
-#     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2b"
-#   endif
-# else
-#   ifdef __cpp_lib_atomic_value_initialization
-#     error "__cpp_lib_atomic_value_initialization should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) is not defined!"
-#   endif
-# endif
+# ifndef __cpp_lib_atomic_value_initialization
+#   error "__cpp_lib_atomic_value_initialization should be defined in c++2b"
+# endif
+# if __cpp_lib_atomic_value_initialization != 201911L
+#   error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2b"
+# endif
 
-# if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
+# if !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)
 #   ifndef __cpp_lib_atomic_wait
 #     error "__cpp_lib_atomic_wait should be defined in c++2b"
 #   endif
 #   if __cpp_lib_atomic_wait != 201907L
 #     error "__cpp_lib_atomic_wait should have the value 201907L in c++2b"
 #   endif
 # else
 #   ifdef __cpp_lib_atomic_wait
-#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
+#     error "__cpp_lib_atomic_wait should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait) is not defined!"
 #   endif
 # endif
 
 # if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier)
 #   ifndef __cpp_lib_barrier
 #     error "__cpp_lib_barrier should be defined in c++2b"
 #   endif
 #   if __cpp_lib_barrier != 201907L

libcxx/utils/generate_feature_test_macro_components.py

feature_test_macros = [ add_version_header(x) for x in [
     "name": "__cpp_lib_assume_aligned",
     "values": { "c++20": 201811 },
     "headers": ["memory"],
     "unimplemented": True,
   }, {
     "name": "__cpp_lib_atomic_flag_test",
     "values": { "c++20": 201907 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
   }, {
     "name": "__cpp_lib_atomic_float",
     "values": { "c++20": 201711 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
     "unimplemented": True,
   }, {
     "name": "__cpp_lib_atomic_is_always_lock_free",
     "values": { "c++17": 201603 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
   }, {
     "name": "__cpp_lib_atomic_lock_free_type_aliases",
     "values": { "c++20": 201907 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
   }, {
     "name": "__cpp_lib_atomic_ref",
     "values": { "c++20": 201806 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
     "unimplemented": True,
   }, {
     "name": "__cpp_lib_atomic_shared_ptr",
     "values": { "c++20": 201711 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
     "unimplemented": True,
   }, {
     "name": "__cpp_lib_atomic_value_initialization",
     "values": { "c++20": 201911 },
     "headers": ["atomic", "memory"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS)",
   }, {
     "name": "__cpp_lib_atomic_wait",
     "values": { "c++20": 201907 },
     "headers": ["atomic"],
-    "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)",
-    "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)",
+    "test_suite_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)",
+    "libcxx_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_atomic_wait)",
   }, {
     "name": "__cpp_lib_barrier",
     "values": { "c++20": 201907 },
     "headers": ["barrier"],
     "test_suite_guard": "!defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier)",
     "libcxx_guard": "!defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier)",
   }, {
     "name": "__cpp_lib_bind_front",

 
 # Map from each header to the Lit annotations that should be used for
 # tests that include that header.
 #
 # For example, when threads are not supported, any feature-test-macro test
 # that includes <thread> should be marked as UNSUPPORTED, because including
 # <thread> is a hard error in that case.
 lit_markup = {
-  "atomic": ["UNSUPPORTED: libcpp-has-no-threads"],
   "barrier": ["UNSUPPORTED: libcpp-has-no-threads"],
   "filesystem": ["UNSUPPORTED: libcpp-has-no-filesystem-library"],
   "format": ["UNSUPPORTED: libcpp-has-no-incomplete-format"],
   "iomanip": ["UNSUPPORTED: libcpp-has-no-localization"],
   "istream": ["UNSUPPORTED: libcpp-has-no-localization"],
   "latch": ["UNSUPPORTED: libcpp-has-no-threads"],
   "locale": ["UNSUPPORTED: libcpp-has-no-localization"],
   "ostream": ["UNSUPPORTED: libcpp-has-no-localization"],

libcxx/utils/generate_header_inclusion_tests.py

 
 # Map from each header to the Lit annotations that should be used for
 # tests that include that header.
 #
 # For example, when threads are not supported, any test
 # that includes <thread> should be marked as UNSUPPORTED, because including
 # <thread> is a hard error in that case.
 lit_markup = {
-  "atomic": ["UNSUPPORTED: libcpp-has-no-threads"],
   "barrier": ["UNSUPPORTED: libcpp-has-no-threads"],
   "filesystem": ["UNSUPPORTED: libcpp-has-no-filesystem-library"],
   "format": ["UNSUPPORTED: libcpp-has-no-incomplete-format"],
   "iomanip": ["UNSUPPORTED: libcpp-has-no-localization"],
   "ios": ["UNSUPPORTED: libcpp-has-no-localization"],
   "iostream": ["UNSUPPORTED: libcpp-has-no-localization"],
   "istream": ["UNSUPPORTED: libcpp-has-no-localization"],
   "latch": ["UNSUPPORTED: libcpp-has-no-threads"],

libcxx/utils/generate_header_tests.py

def get_libcxx_paths():
     libcxx_test_path = os.path.join(src_root, 'test', 'libcxx')
     assert os.path.exists(libcxx_test_path)
     return script_name, src_root, include_path, libcxx_test_path
 
 
 script_name, source_root, include_path, libcxx_test_path = get_libcxx_paths()
 
 header_markup = {
-    "atomic": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "barrier": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "future": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "latch": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "mutex": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "semaphore": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "shared_mutex": ["ifndef _LIBCPP_HAS_NO_THREADS"],
     "thread": ["ifndef _LIBCPP_HAS_NO_THREADS"],