[libc++] Make __atomic_base into an implementation detail of std::atomic (#115764)

The __atomic_base base class is only useful to conditionalize the
operations we provide inside std::atomic. It shouldn't be used directly
from other places in the library which can use std::atomic directly
instead.

Since we've granularized our includes, using std::atomic directly should
not make much of a difference compile-time wise.

This patch starts using std::atomic directly from other classes like
std::barrier and std::latch. Changing this shouldn't be an ABI break
since both classes have the same size and layout.

The benefits of this patch are isolating other parts of the code base
from implementation details of std::atomic and simplifying the mental
model for std::atomic's layers of implementation by making it clear that
__atomic_base is only an implementation detail of std::atomic.

Author: ldionne

libcxx/include/CMakeLists.txt

@@ -205,7 +205,6 @@ set(files
   __assert
   __atomic/aliases.h
   __atomic/atomic.h
-  __atomic/atomic_base.h
   __atomic/atomic_flag.h
   __atomic/atomic_init.h
   __atomic/atomic_lock_free.h

libcxx/include/__atomic/atomic.h

@@ -9,9 +9,10 @@
 #ifndef _LIBCPP___ATOMIC_ATOMIC_H
 #define _LIBCPP___ATOMIC_ATOMIC_H
 
-#include <__atomic/atomic_base.h>
+#include <__atomic/atomic_sync.h>
 #include <__atomic/check_memory_order.h>
 #include <__atomic/cxx_atomic_impl.h>
+#include <__atomic/is_always_lock_free.h>
 #include <__atomic/memory_order.h>
 #include <__config>
 #include <__cstddef/ptrdiff_t.h>
@@ -21,6 +22,7 @@
 #include <__type_traits/is_floating_point.h>
 #include <__type_traits/is_function.h>
 #include <__type_traits/is_integral.h>
+#include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/remove_const.h>
 #include <__type_traits/remove_pointer.h>
@@ -34,6 +36,197 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Tp, bool = is_integral<_Tp>::value && !is_same<_Tp, bool>::value>
+struct __atomic_base // false
+{
+  mutable __cxx_atomic_impl<_Tp> __a_;
+
+#if _LIBCPP_STD_VER >= 17
+  static constexpr bool is_always_lock_free = __libcpp_is_always_lock_free<__cxx_atomic_impl<_Tp> >::__value;
+#endif
+
+  _LIBCPP_HIDE_FROM_ABI bool is_lock_free() const volatile _NOEXCEPT {
+    return __cxx_atomic_is_lock_free(sizeof(__cxx_atomic_impl<_Tp>));
+  }
+  _LIBCPP_HIDE_FROM_ABI bool is_lock_free() const _NOEXCEPT {
+    return static_cast<__atomic_base const volatile*>(this)->is_lock_free();
+  }
+  _LIBCPP_HIDE_FROM_ABI void store(_Tp __d, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT
+      _LIBCPP_CHECK_STORE_MEMORY_ORDER(__m) {
+    std::__cxx_atomic_store(std::addressof(__a_), __d, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI void store(_Tp __d, memory_order __m = memory_order_seq_cst) _NOEXCEPT
+      _LIBCPP_CHECK_STORE_MEMORY_ORDER(__m) {
+    std::__cxx_atomic_store(std::addressof(__a_), __d, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp load(memory_order __m = memory_order_seq_cst) const volatile _NOEXCEPT
+      _LIBCPP_CHECK_LOAD_MEMORY_ORDER(__m) {
+    return std::__cxx_atomic_load(std::addressof(__a_), __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp load(memory_order __m = memory_order_seq_cst) const _NOEXCEPT
+      _LIBCPP_CHECK_LOAD_MEMORY_ORDER(__m) {
+    return std::__cxx_atomic_load(std::addressof(__a_), __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI operator _Tp() const volatile _NOEXCEPT { return load(); }
+  _LIBCPP_HIDE_FROM_ABI operator _Tp() const _NOEXCEPT { return load(); }
+  _LIBCPP_HIDE_FROM_ABI _Tp exchange(_Tp __d, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_exchange(std::addressof(__a_), __d, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp exchange(_Tp __d, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_exchange(std::addressof(__a_), __d, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool
+  compare_exchange_weak(_Tp& __e, _Tp __d, memory_order __s, memory_order __f) volatile _NOEXCEPT
+      _LIBCPP_CHECK_EXCHANGE_MEMORY_ORDER(__s, __f) {
+    return std::__cxx_atomic_compare_exchange_weak(std::addressof(__a_), std::addressof(__e), __d, __s, __f);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool compare_exchange_weak(_Tp& __e, _Tp __d, memory_order __s, memory_order __f) _NOEXCEPT
+      _LIBCPP_CHECK_EXCHANGE_MEMORY_ORDER(__s, __f) {
+    return std::__cxx_atomic_compare_exchange_weak(std::addressof(__a_), std::addressof(__e), __d, __s, __f);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool
+  compare_exchange_strong(_Tp& __e, _Tp __d, memory_order __s, memory_order __f) volatile _NOEXCEPT
+      _LIBCPP_CHECK_EXCHANGE_MEMORY_ORDER(__s, __f) {
+    return std::__cxx_atomic_compare_exchange_strong(std::addressof(__a_), std::addressof(__e), __d, __s, __f);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool compare_exchange_strong(_Tp& __e, _Tp __d, memory_order __s, memory_order __f) _NOEXCEPT
+      _LIBCPP_CHECK_EXCHANGE_MEMORY_ORDER(__s, __f) {
+    return std::__cxx_atomic_compare_exchange_strong(std::addressof(__a_), std::addressof(__e), __d, __s, __f);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool
+  compare_exchange_weak(_Tp& __e, _Tp __d, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_compare_exchange_weak(std::addressof(__a_), std::addressof(__e), __d, __m, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool
+  compare_exchange_weak(_Tp& __e, _Tp __d, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_compare_exchange_weak(std::addressof(__a_), std::addressof(__e), __d, __m, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool
+  compare_exchange_strong(_Tp& __e, _Tp __d, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_compare_exchange_strong(std::addressof(__a_), std::addressof(__e), __d, __m, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool
+  compare_exchange_strong(_Tp& __e, _Tp __d, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    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);
+  }
+  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+  wait(_Tp __v, memory_order __m = memory_order_seq_cst) const _NOEXCEPT {
+    std::__atomic_wait(*this, __v, __m);
+  }
+  _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_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()) {}
+#else
+  _LIBCPP_HIDE_FROM_ABI __atomic_base() _NOEXCEPT = default;
+#endif
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __atomic_base(_Tp __d) _NOEXCEPT : __a_(__d) {}
+
+  __atomic_base(const __atomic_base&) = delete;
+};
+
+// atomic<Integral>
+
+template <class _Tp>
+struct __atomic_base<_Tp, true> : public __atomic_base<_Tp, false> {
+  using __base = __atomic_base<_Tp, false>;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __atomic_base() _NOEXCEPT = default;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __atomic_base(_Tp __d) _NOEXCEPT : __base(__d) {}
+
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_add(_Tp __op, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_fetch_add(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_add(_Tp __op, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_fetch_add(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_sub(_Tp __op, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_fetch_sub(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_sub(_Tp __op, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_fetch_sub(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_and(_Tp __op, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_fetch_and(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_and(_Tp __op, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_fetch_and(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_or(_Tp __op, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_fetch_or(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_or(_Tp __op, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_fetch_or(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_xor(_Tp __op, memory_order __m = memory_order_seq_cst) volatile _NOEXCEPT {
+    return std::__cxx_atomic_fetch_xor(std::addressof(this->__a_), __op, __m);
+  }
+  _LIBCPP_HIDE_FROM_ABI _Tp fetch_xor(_Tp __op, memory_order __m = memory_order_seq_cst) _NOEXCEPT {
+    return std::__cxx_atomic_fetch_xor(std::addressof(this->__a_), __op, __m);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _Tp operator++(int) volatile _NOEXCEPT { return fetch_add(_Tp(1)); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator++(int) _NOEXCEPT { return fetch_add(_Tp(1)); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator--(int) volatile _NOEXCEPT { return fetch_sub(_Tp(1)); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator--(int) _NOEXCEPT { return fetch_sub(_Tp(1)); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator++() volatile _NOEXCEPT { return fetch_add(_Tp(1)) + _Tp(1); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator++() _NOEXCEPT { return fetch_add(_Tp(1)) + _Tp(1); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator--() volatile _NOEXCEPT { return fetch_sub(_Tp(1)) - _Tp(1); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator--() _NOEXCEPT { return fetch_sub(_Tp(1)) - _Tp(1); }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator+=(_Tp __op) volatile _NOEXCEPT { return fetch_add(__op) + __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator+=(_Tp __op) _NOEXCEPT { return fetch_add(__op) + __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator-=(_Tp __op) volatile _NOEXCEPT { return fetch_sub(__op) - __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator-=(_Tp __op) _NOEXCEPT { return fetch_sub(__op) - __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator&=(_Tp __op) volatile _NOEXCEPT { return fetch_and(__op) & __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator&=(_Tp __op) _NOEXCEPT { return fetch_and(__op) & __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator|=(_Tp __op) volatile _NOEXCEPT { return fetch_or(__op) | __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator|=(_Tp __op) _NOEXCEPT { return fetch_or(__op) | __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator^=(_Tp __op) volatile _NOEXCEPT { return fetch_xor(__op) ^ __op; }
+  _LIBCPP_HIDE_FROM_ABI _Tp operator^=(_Tp __op) _NOEXCEPT { return fetch_xor(__op) ^ __op; }
+};
+
+// Here we need _IsIntegral because the default template argument is not enough
+// e.g  __atomic_base<int> is __atomic_base<int, true>, which inherits from
+// __atomic_base<int, false> and the caller of the wait function is
+// __atomic_base<int, false>. So specializing __atomic_base<_Tp> does not work
+template <class _Tp, bool _IsIntegral>
+struct __atomic_waitable_traits<__atomic_base<_Tp, _IsIntegral> > {
+  static _LIBCPP_HIDE_FROM_ABI _Tp __atomic_load(const __atomic_base<_Tp, _IsIntegral>& __a, memory_order __order) {
+    return __a.load(__order);
+  }
+
+  static _LIBCPP_HIDE_FROM_ABI _Tp
+  __atomic_load(const volatile __atomic_base<_Tp, _IsIntegral>& __this, memory_order __order) {
+    return __this.load(__order);
+  }
+
+  static _LIBCPP_HIDE_FROM_ABI const __cxx_atomic_impl<_Tp>*
+  __atomic_contention_address(const __atomic_base<_Tp, _IsIntegral>& __a) {
+    return std::addressof(__a.__a_);
+  }
+
+  static _LIBCPP_HIDE_FROM_ABI const volatile __cxx_atomic_impl<_Tp>*
+  __atomic_contention_address(const volatile __atomic_base<_Tp, _IsIntegral>& __this) {
+    return std::addressof(__this.__a_);
+  }
+};
+
 template <class _Tp>
 struct atomic : public __atomic_base<_Tp> {
   using __base          = __atomic_base<_Tp>;
@@ -123,6 +316,9 @@ struct atomic<_Tp*> : public __atomic_base<_Tp*> {
   atomic& operator=(const atomic&) volatile = delete;
 };
 
+template <class _Tp>
+struct __atomic_waitable_traits<atomic<_Tp> > : __atomic_waitable_traits<__atomic_base<_Tp> > {};
+
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
   requires is_floating_point_v<_Tp>