[libc++] Drop support for the C++20 Synchronization Library before C++20

When we initially implemented the C++20 synchronization library, we
reluctantly accepted for the implementation to be backported to C++03
upon request from the person who provided the patch. This was when we
were only starting to have experience with the issues this can create,
so we flinched. Nowadays, we have a much stricter stance about not
backporting features to previous standards.

We have recently started fixing several bugs (and near bugs) in our
implementation of the synchronization library. A recurring theme during
these reviews has been how difficult to undertand the current code is,
and upon inspection it becomes clear that being able to use a few recent
C++ features (in particular lambdas) would help a great deal. The code
would still be pretty intricate, but it would be a lot easier to reason
about the flow of callbacks through things like __thread_poll_with_backoff.

As a result, this patch drops support for the synchronization library
before C++20. This makes us more strictly conforming and opens the door
to major simplifications, in particular around atomic_wait which was
supported all the way to C++03.

This change will probably have some impact on downstream users, however
since the C++20 synchronization library was added only in LLVM 10 (~3 years
ago) and it's quite a niche feature, the set of people trying to use this
part of the library before C++20 should be reasonably small.

Author: ldionne

libcxx/docs/ReleaseNotes/19.rst

@@ -61,6 +61,10 @@ Deprecations and Removals
   the LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation
   <using-hardening-modes>` for more details.
 
+- Support for the C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been
+  removed in language modes prior to C++20. If you are using these features prior to C++20, you will need to
+  update to ``-std=c++20``.
+
 - The base template for ``std::char_traits`` has been removed in LLVM 19. If you are using ``std::char_traits`` with
   types other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``, ``char32_t`` or a custom character type for which you
   specialized ``std::char_traits``, your code will stop working. The Standard does not mandate that a base template is

libcxx/include/__atomic/atomic.h

@@ -429,6 +429,8 @@ _LIBCPP_HIDE_FROM_ABI bool atomic_compare_exchange_strong_explicit(
   return __o->compare_exchange_strong(*__e, __d, __s, __f);
 }
 
+#if _LIBCPP_STD_VER >= 20
+
 // atomic_wait
 
 template <class _Tp>
@@ -481,6 +483,8 @@ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(atomic<_T
   __o->notify_all();
 }
 
+#endif // _LIBCPP_STD_VER >= 20
+
 // atomic_fetch_add
 
 template <class _Tp>

libcxx/include/__atomic/atomic_base.h

@@ -102,6 +102,7 @@ struct __atomic_base // false
     return std::__cxx_atomic_compare_exchange_strong(std::addressof(__a_), std::addressof(__e), __d, __m, __m);
   }
 
+#if _LIBCPP_STD_VER >= 20
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait(_Tp __v, memory_order __m = memory_order_seq_cst) const
       volatile _NOEXCEPT {
     std::__atomic_wait(*this, __v, __m);
@@ -114,10 +115,8 @@ struct __atomic_base // false
     std::__atomic_notify_one(*this);
   }
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() _NOEXCEPT { std::__atomic_notify_one(*this); }
-  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() volatile _NOEXCEPT {
-    std::__atomic_notify_all(*this);
-  }
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT { std::__atomic_notify_all(*this); }
+#endif //  _LIBCPP_STD_VER >= 20
 
 #if _LIBCPP_STD_VER >= 20
   _LIBCPP_HIDE_FROM_ABI constexpr __atomic_base() noexcept(is_nothrow_default_constructible_v<_Tp>) : __a_(_Tp()) {}

libcxx/include/__atomic/atomic_flag.h

@@ -49,6 +49,7 @@ struct atomic_flag {
     __cxx_atomic_store(&__a_, _LIBCPP_ATOMIC_FLAG_TYPE(false), __m);
   }
 
+#if _LIBCPP_STD_VER >= 20
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait(bool __v, memory_order __m = memory_order_seq_cst) const
       volatile _NOEXCEPT {
     std::__atomic_wait(*this, _LIBCPP_ATOMIC_FLAG_TYPE(__v), __m);
@@ -65,6 +66,7 @@ struct atomic_flag {
     std::__atomic_notify_all(*this);
   }
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT { std::__atomic_notify_all(*this); }
+#endif
 
 #if _LIBCPP_STD_VER >= 20
   _LIBCPP_HIDE_FROM_ABI constexpr atomic_flag() _NOEXCEPT : __a_(false) {}
@@ -141,6 +143,7 @@ inline _LIBCPP_HIDE_FROM_ABI void atomic_flag_clear_explicit(atomic_flag* __o, m
   __o->clear(__m);
 }
 
+#if _LIBCPP_STD_VER >= 20
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait(const volatile atomic_flag* __o, bool __v) _NOEXCEPT {
   __o->wait(__v);
@@ -178,6 +181,7 @@ atomic_flag_notify_all(volatile atomic_flag* __o) _NOEXCEPT {
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void atomic_flag_notify_all(atomic_flag* __o) _NOEXCEPT {
   __o->notify_all();
 }
+#endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
 

libcxx/include/atomic

@@ -101,12 +101,12 @@ struct atomic
     bool compare_exchange_strong(T& expc, T desr,
                                  memory_order m = memory_order_seq_cst) noexcept;
 
-    void wait(T, memory_order = memory_order::seq_cst) const volatile noexcept;
-    void wait(T, memory_order = memory_order::seq_cst) const noexcept;
-    void notify_one() volatile noexcept;
-    void notify_one() noexcept;
-    void notify_all() volatile noexcept;
-    void notify_all() noexcept;
+    void wait(T, memory_order = memory_order::seq_cst) const volatile noexcept; // since C++20
+    void wait(T, memory_order = memory_order::seq_cst) const noexcept;          // since C++20
+    void notify_one() volatile noexcept;                                        // since C++20
+    void notify_one() noexcept;                                                 // since C++20
+    void notify_all() volatile noexcept;                                        // since C++20
+    void notify_all() noexcept;                                                 // since C++20
 };
 
 template <>
@@ -184,12 +184,12 @@ struct atomic<integral>
     integral operator^=(integral op) volatile noexcept;
     integral operator^=(integral op) noexcept;
 
-    void wait(integral, memory_order = memory_order::seq_cst) const volatile noexcept;
-    void wait(integral, memory_order = memory_order::seq_cst) const noexcept;
-    void notify_one() volatile noexcept;
-    void notify_one() noexcept;
-    void notify_all() volatile noexcept;
-    void notify_all() noexcept;
+    void wait(integral, memory_order = memory_order::seq_cst) const volatile noexcept; // since C++20
+    void wait(integral, memory_order = memory_order::seq_cst) const noexcept;          // since C++20
+    void notify_one() volatile noexcept;                                               // since C++20
+    void notify_one() noexcept;                                                        // since C++20
+    void notify_all() volatile noexcept;                                               // since C++20
+    void notify_all() noexcept;                                                        // since C++20
 };
 
 template <class T>
@@ -254,12 +254,12 @@ struct atomic<T*>
     T* operator-=(ptrdiff_t op) volatile noexcept;
     T* operator-=(ptrdiff_t op) noexcept;
 
-    void wait(T*, memory_order = memory_order::seq_cst) const volatile noexcept;
-    void wait(T*, memory_order = memory_order::seq_cst) const noexcept;
-    void notify_one() volatile noexcept;
-    void notify_one() noexcept;
-    void notify_all() volatile noexcept;
-    void notify_all() noexcept;
+    void wait(T*, memory_order = memory_order::seq_cst) const volatile noexcept; // since C++20
+    void wait(T*, memory_order = memory_order::seq_cst) const noexcept;          // since C++20
+    void notify_one() volatile noexcept;                                         // since C++20
+    void notify_one() noexcept;                                                  // since C++20
+    void notify_all() volatile noexcept;                                         // since C++20
+    void notify_all() noexcept;                                                  // since C++20
 };
 
 template<>
@@ -321,12 +321,12 @@ struct atomic<floating-point-type> {  // since C++20
   floating-point-type operator-=(floating-point-type) volatile noexcept;
   floating-point-type operator-=(floating-point-type) noexcept;
 
-  void wait(floating-point-type, memory_order = memory_order::seq_cst) const volatile noexcept;
-  void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;
-  void notify_one() volatile noexcept;
-  void notify_one() noexcept;
-  void notify_all() volatile noexcept;
-  void notify_all() noexcept;
+  void wait(floating-point-type, memory_order = memory_order::seq_cst) const volatile noexcept; // since C++20
+  void wait(floating-point-type, memory_order = memory_order::seq_cst) const noexcept;          // since C++20
+  void notify_one() volatile noexcept;                                                          // since C++20
+  void notify_one() noexcept;                                                                   // since C++20
+  void notify_all() volatile noexcept;                                                          // since C++20
+  void notify_all() noexcept;                                                                   // since C++20
 };
 
 // [atomics.nonmembers], non-member functions
@@ -443,23 +443,23 @@ template<class T>
                               memory_order) noexcept;
 
 template<class T>
-  void atomic_wait(const volatile atomic<T>*, atomic<T>::value_type) noexcept;
+  void atomic_wait(const volatile atomic<T>*, atomic<T>::value_type) noexcept; // since C++20
 template<class T>
-  void atomic_wait(const atomic<T>*, atomic<T>::value_type) noexcept;
+  void atomic_wait(const atomic<T>*, atomic<T>::value_type) noexcept;          // since C++20
 template<class T>
-  void atomic_wait_explicit(const volatile atomic<T>*, atomic<T>::value_type,
+  void atomic_wait_explicit(const volatile atomic<T>*, atomic<T>::value_type,  // since C++20
                             memory_order) noexcept;
 template<class T>
-  void atomic_wait_explicit(const atomic<T>*, atomic<T>::value_type,
+  void atomic_wait_explicit(const atomic<T>*, atomic<T>::value_type,           // since C++20
                             memory_order) noexcept;
 template<class T>
-  void atomic_notify_one(volatile atomic<T>*) noexcept;
+  void atomic_notify_one(volatile atomic<T>*) noexcept;                        // since C++20
 template<class T>
-  void atomic_notify_one(atomic<T>*) noexcept;
+  void atomic_notify_one(atomic<T>*) noexcept;                                 // since C++20
 template<class T>
-  void atomic_notify_all(volatile atomic<T>*) noexcept;
+  void atomic_notify_all(volatile atomic<T>*) noexcept;                        // since C++20
 template<class T>
-  void atomic_notify_all(atomic<T>*) noexcept;
+  void atomic_notify_all(atomic<T>*) noexcept;                                 // since C++20
 
 // Atomics for standard typedef types
 
@@ -534,12 +534,12 @@ typedef struct atomic_flag
     void clear(memory_order m = memory_order_seq_cst) volatile noexcept;
     void clear(memory_order m = memory_order_seq_cst) noexcept;
 
-    void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept;
-    void wait(bool, memory_order = memory_order::seq_cst) const noexcept;
-    void notify_one() volatile noexcept;
-    void notify_one() noexcept;
-    void notify_all() volatile noexcept;
-    void notify_all() noexcept;
+    void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept; // since C++20
+    void wait(bool, memory_order = memory_order::seq_cst) const noexcept;          // since C++20
+    void notify_one() volatile noexcept;                                           // since C++20
+    void notify_one() noexcept;                                                    // since C++20
+    void notify_all() volatile noexcept;                                           // since C++20
+    void notify_all() noexcept;                                                    // since C++20
 } atomic_flag;
 
 bool atomic_flag_test(volatile atomic_flag* obj) noexcept;
@@ -557,14 +557,14 @@ void atomic_flag_clear(atomic_flag* obj) noexcept;
 void atomic_flag_clear_explicit(volatile atomic_flag* obj, memory_order m) noexcept;
 void atomic_flag_clear_explicit(atomic_flag* obj, memory_order m) noexcept;
 
-void atomic_wait(const volatile atomic_flag* obj, T old) noexcept;
-void atomic_wait(const atomic_flag* obj, T old) noexcept;
-void atomic_wait_explicit(const volatile atomic_flag* obj, T old, memory_order m) noexcept;
-void atomic_wait_explicit(const atomic_flag* obj, T old, memory_order m) noexcept;
-void atomic_one(volatile atomic_flag* obj) noexcept;
-void atomic_one(atomic_flag* obj) noexcept;
-void atomic_all(volatile atomic_flag* obj) noexcept;
-void atomic_all(atomic_flag* obj) noexcept;
+void atomic_wait(const volatile atomic_flag* obj, T old) noexcept;                          // since C++20
+void atomic_wait(const atomic_flag* obj, T old) noexcept;                                   // since C++20
+void atomic_wait_explicit(const volatile atomic_flag* obj, T old, memory_order m) noexcept; // since C++20
+void atomic_wait_explicit(const atomic_flag* obj, T old, memory_order m) noexcept;          // since C++20
+void atomic_one(volatile atomic_flag* obj) noexcept;                                        // since C++20
+void atomic_one(atomic_flag* obj) noexcept;                                                 // since C++20
+void atomic_all(volatile atomic_flag* obj) noexcept;                                        // since C++20
+void atomic_all(atomic_flag* obj) noexcept;                                                 // since C++20
 
 // fences
 

libcxx/include/barrier

@@ -17,7 +17,7 @@ namespace std
 {
 
   template<class CompletionFunction = see below>
-  class barrier
+  class barrier                                   // since C++20
   {
   public:
     using arrival_token = see below;
@@ -71,7 +71,7 @@ namespace std
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#if _LIBCPP_STD_VER >= 14
+#if _LIBCPP_STD_VER >= 20
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -293,7 +293,7 @@ public:
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP_STD_VER >= 14
+#endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_POP_MACROS
 
@@ -306,4 +306,4 @@ _LIBCPP_POP_MACROS
 #  include <variant>
 #endif
 
-#endif //_LIBCPP_BARRIER
+#endif // _LIBCPP_BARRIER

libcxx/include/latch

@@ -16,7 +16,7 @@
 namespace std
 {
 
-  class latch
+  class latch                                     // since C++20
   {
   public:
     static constexpr ptrdiff_t max() noexcept;
@@ -62,7 +62,7 @@ namespace std
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#if _LIBCPP_STD_VER >= 14
+#if _LIBCPP_STD_VER >= 20
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -119,12 +119,12 @@ private:
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP_STD_VER >= 14
+#endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_POP_MACROS
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <atomic>
 #endif
 
-#endif //_LIBCPP_LATCH
+#endif // _LIBCPP_LATCH

libcxx/include/semaphore

@@ -16,7 +16,7 @@
 namespace std {
 
 template<ptrdiff_t least_max_value = implementation-defined>
-class counting_semaphore
+class counting_semaphore                          // since C++20
 {
 public:
 static constexpr ptrdiff_t max() noexcept;
@@ -39,7 +39,7 @@ private:
 ptrdiff_t counter; // exposition only
 };
 
-using binary_semaphore = counting_semaphore<1>;
+using binary_semaphore = counting_semaphore<1>; // since C++20
 
 }
 
@@ -71,7 +71,7 @@ using binary_semaphore = counting_semaphore<1>;
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
-#if _LIBCPP_STD_VER >= 14
+#if _LIBCPP_STD_VER >= 20
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -176,12 +176,12 @@ using binary_semaphore = counting_semaphore<1>;
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP_STD_VER >= 14
+#endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_POP_MACROS
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <atomic>
 #endif
 
-#endif //_LIBCPP_SEMAPHORE
+#endif // _LIBCPP_SEMAPHORE