Upgrade to 2022-era LLVM LIBCXX

third_party/libcxx/latch

@@ -0,0 +1,121 @@
+// -*- 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_LATCH
+#define _LIBCPP_LATCH
+
+/*
+    latch synopsis
+
+namespace std
+{
+
+  class latch
+  {
+  public:
+    static constexpr ptrdiff_t max() noexcept;
+
+    constexpr explicit latch(ptrdiff_t __expected);
+    ~latch();
+
+    latch(const latch&) = delete;
+    latch& operator=(const latch&) = delete;
+
+    void count_down(ptrdiff_t __update = 1);
+    bool try_wait() const noexcept;
+    void wait() const;
+    void arrive_and_wait(ptrdiff_t __update = 1);
+
+  private:
+    ptrdiff_t __counter; // exposition only
+  };
+
+}
+
+*/
+
+#include <__assert> // all public C++ headers provide the assertion handler
+#include <__atomic/atomic_base.h>
+#include <__atomic/atomic_sync.h>
+#include <__atomic/memory_order.h>
+#include <__availability>
+#include <__config>
+#include <cstddef>
+#include <limits>
+#include <version>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+# error "<latch> is not supported since libc++ has been configured without support for threads."
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 14
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+class latch
+{
+    __atomic_base<ptrdiff_t> __a_;
+
+public:
+    static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept {
+        return numeric_limits<ptrdiff_t>::max();
+    }
+
+    inline _LIBCPP_INLINE_VISIBILITY
+    constexpr explicit latch(ptrdiff_t __expected) : __a_(__expected) { }
+
+    _LIBCPP_HIDE_FROM_ABI ~latch() = default;
+    latch(const latch&) = delete;
+    latch& operator=(const latch&) = delete;
+
+    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
+    void count_down(ptrdiff_t __update = 1)
+    {
+        auto const __old = __a_.fetch_sub(__update, memory_order_release);
+        if(__old == __update)
+            __a_.notify_all();
+    }
+    inline _LIBCPP_INLINE_VISIBILITY
+    bool try_wait() const noexcept
+    {
+        return 0 == __a_.load(memory_order_acquire);
+    }
+    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
+    void wait() const
+    {
+        __cxx_atomic_wait(&__a_.__a_, [&]() -> bool {
+            return try_wait();
+        });
+    }
+    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
+    void arrive_and_wait(ptrdiff_t __update = 1)
+    {
+        count_down(__update);
+        wait();
+    }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 14
+
+_LIBCPP_POP_MACROS
+
+#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
+#  include <atomic>
+#endif
+
+#endif //_LIBCPP_LATCH