[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/.clang-format

@@ -24,7 +24,6 @@ AttributeMacros: [
                   '_LIBCPP_CONSTEXPR_SINCE_CXX23',
                   '_LIBCPP_CONSTEXPR',
                   '_LIBCPP_CONSTINIT',
-                  '_LIBCPP_DEPRECATED_ATOMIC_SYNC',
                   '_LIBCPP_DEPRECATED_IN_CXX11',
                   '_LIBCPP_DEPRECATED_IN_CXX14',
                   '_LIBCPP_DEPRECATED_IN_CXX17',

libcxx/docs/ReleaseNotes/20.rst

@@ -53,7 +53,9 @@ Deprecations and Removals
 - TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to enable
   the safe mode will be removed in LLVM 20.
 
-- TODO: The C++20 synchronization library will be removed entirely in language modes prior to C++20 in LLVM 20.
+- 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``.
 
 - TODO: The relational operators for ``std::chrono::weekday`` will be removed entirely, and the
   ``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro that was used to re-enable this extension will be

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>
@@ -462,29 +464,27 @@ atomic_wait_explicit(const atomic<_Tp>* __o, typename atomic<_Tp>::value_type __
 // atomic_notify_one
 
 template <class _Tp>
-_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
-atomic_notify_one(volatile atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_one(volatile atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_one();
 }
 template <class _Tp>
-_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
-atomic_notify_one(atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_one(atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_one();
 }
 
 // atomic_notify_all
 
 template <class _Tp>
-_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
-atomic_notify_all(volatile atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(volatile atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_all();
 }
 template <class _Tp>
-_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
-atomic_notify_all(atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(atomic<_Tp>* __o) _NOEXCEPT {
   __o->notify_all();
 }
 
+#endif // _LIBCPP_STD_VER >= 20
+
 // atomic_fetch_add
 
 template <class _Tp>

libcxx/include/__atomic/atomic_base.h

@@ -101,6 +101,7 @@ 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);
@@ -117,6 +118,7 @@ struct __atomic_base // false
     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

@@ -48,26 +48,24 @@ struct atomic_flag {
     __cxx_atomic_store(&__a_, _LIBCPP_ATOMIC_FLAG_TYPE(false), __m);
   }
 
-  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
-  wait(bool __v, memory_order __m = memory_order_seq_cst) const volatile _NOEXCEPT {
+#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);
   }
-  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
   wait(bool __v, memory_order __m = memory_order_seq_cst) const _NOEXCEPT {
     std::__atomic_wait(*this, _LIBCPP_ATOMIC_FLAG_TYPE(__v), __m);
   }
-  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() volatile _NOEXCEPT {
-    std::__atomic_notify_one(*this);
-  }
-  _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() _NOEXCEPT {
+  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() volatile _NOEXCEPT {
     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_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT {
-    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) {}
@@ -144,45 +142,45 @@ inline _LIBCPP_HIDE_FROM_ABI void atomic_flag_clear_explicit(atomic_flag* __o, m
   __o->clear(__m);
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+#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);
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait(const atomic_flag* __o, bool __v) _NOEXCEPT {
   __o->wait(__v);
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait_explicit(const volatile atomic_flag* __o, bool __v, memory_order __m) _NOEXCEPT {
   __o->wait(__v, __m);
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_wait_explicit(const atomic_flag* __o, bool __v, memory_order __m) _NOEXCEPT {
   __o->wait(__v, __m);
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_notify_one(volatile atomic_flag* __o) _NOEXCEPT {
   __o->notify_one();
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
-atomic_flag_notify_one(atomic_flag* __o) _NOEXCEPT {
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void atomic_flag_notify_one(atomic_flag* __o) _NOEXCEPT {
   __o->notify_one();
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
 atomic_flag_notify_all(volatile atomic_flag* __o) _NOEXCEPT {
   __o->notify_all();
 }
 
-inline _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_SYNC void
-atomic_flag_notify_all(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/__config

@@ -699,14 +699,6 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_DEPRECATED_(m)
 #  endif
 
-#  if _LIBCPP_STD_VER < 20
-#    define _LIBCPP_DEPRECATED_ATOMIC_SYNC                                                                             \
-      _LIBCPP_DEPRECATED_("The C++20 synchronization library has been deprecated prior to C++20. Please update to "    \
-                          "using -std=c++20 if you need to use these facilities.")
-#  else
-#    define _LIBCPP_DEPRECATED_ATOMIC_SYNC /* nothing */
-#  endif
-
 #  if !defined(_LIBCPP_CXX03_LANG)
 #    define _LIBCPP_DEPRECATED_IN_CXX11 _LIBCPP_DEPRECATED
 #  else

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;
@@ -68,7 +68,7 @@ namespace std
 _LIBCPP_PUSH_MACROS
 #  include <__undef_macros>
 
-#  if _LIBCPP_STD_VER >= 14
+#  if _LIBCPP_STD_VER >= 20
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -254,7 +254,7 @@ public:
 #    endif // !_LIBCPP_HAS_NO_TREE_BARRIER
 
 template <class _CompletionF = __empty_completion>
-class _LIBCPP_DEPRECATED_ATOMIC_SYNC barrier {
+class barrier {
   __barrier_base<_CompletionF> __b_;
 
 public:
@@ -290,7 +290,7 @@ public:
 
 _LIBCPP_END_NAMESPACE_STD
 
-#  endif // _LIBCPP_STD_VER >= 14
+#  endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_POP_MACROS
 
@@ -305,4 +305,4 @@ _LIBCPP_POP_MACROS
 #  include <variant>
 #endif
 
-#endif //_LIBCPP_BARRIER
+#endif // _LIBCPP_BARRIER