From bf182818f4c55f254c254c5bbaf07f0be952d2cb Mon Sep 17 00:00:00 2001
From: Andrey Semashev <andrey.semashev@gmail.com>
Date: Sat, 30 May 2020 14:47:28 +0300
Subject: [PATCH] Added futex-based implementation for waiting/notifying
 operations.

---
 include/boost/atomic/detail/platform.hpp      |  9 +++
 .../boost/atomic/detail/wait_ops_futex.hpp    | 64 +++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 include/boost/atomic/detail/wait_ops_futex.hpp

diff --git a/include/boost/atomic/detail/platform.hpp b/include/boost/atomic/detail/platform.hpp
index b8955d7..6f1916f 100644
--- a/include/boost/atomic/detail/platform.hpp
+++ b/include/boost/atomic/detail/platform.hpp
@@ -133,6 +133,15 @@
 
 #endif // !defined(BOOST_ATOMIC_DETAIL_CORE_BACKEND)
 
+// Waiting and notifying operations backends
+#if !defined(BOOST_WINDOWS)
+#include <boost/atomic/detail/futex.hpp>
+
+#if defined(BOOST_ATOMIC_DETAIL_HAS_FUTEX)
+#define BOOST_ATOMIC_DETAIL_WAIT_BACKEND futex
+#endif
+#endif // !defined(BOOST_WINDOWS)
+
 #endif // !defined(BOOST_ATOMIC_FORCE_FALLBACK)
 
 #if !defined(BOOST_ATOMIC_DETAIL_CORE_BACKEND)
diff --git a/include/boost/atomic/detail/wait_ops_futex.hpp b/include/boost/atomic/detail/wait_ops_futex.hpp
new file mode 100644
index 0000000..7e73cbe
--- /dev/null
+++ b/include/boost/atomic/detail/wait_ops_futex.hpp
@@ -0,0 +1,64 @@
+/*
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * http://www.boost.org/LICENSE_1_0.txt)
+ *
+ * Copyright (c) 2020 Andrey Semashev
+ */
+/*!
+ * \file   atomic/detail/wait_ops_futex.hpp
+ *
+ * This header contains implementation of the wait/notify atomic operations based on futexes.
+ */
+
+#ifndef BOOST_ATOMIC_DETAIL_WAIT_OPS_FUTEX_HPP_INCLUDED_
+#define BOOST_ATOMIC_DETAIL_WAIT_OPS_FUTEX_HPP_INCLUDED_
+
+#include <boost/memory_order.hpp>
+#include <boost/atomic/detail/config.hpp>
+#include <boost/atomic/detail/futex.hpp>
+#include <boost/atomic/detail/wait_operations_fwd.hpp>
+
+#ifdef BOOST_HAS_PRAGMA_ONCE
+#pragma once
+#endif
+
+namespace boost {
+namespace atomics {
+namespace detail {
+
+template< typename Base >
+struct wait_operations< Base, 4u, true > :
+    public Base
+{
+    typedef Base base_type;
+    typedef typename base_type::storage_type storage_type;
+
+    static BOOST_FORCEINLINE storage_type wait(storage_type const volatile& storage, storage_type old_val, memory_order order) BOOST_NOEXCEPT
+    {
+        storage_type new_val = base_type::load(storage, order);
+        while (new_val == old_val)
+        {
+            atomics::detail::futex_wait_private(const_cast< storage_type* >(&storage), old_val);
+            new_val = base_type::load(storage, order);
+        }
+
+        return new_val;
+    }
+
+    static BOOST_FORCEINLINE void notify_one(storage_type volatile& storage) BOOST_NOEXCEPT
+    {
+        atomics::detail::futex_signal_private(const_cast< storage_type* >(&storage));
+    }
+
+    static BOOST_FORCEINLINE void notify_all(storage_type volatile& storage) BOOST_NOEXCEPT
+    {
+        atomics::detail::futex_broadcast_private(const_cast< storage_type* >(&storage));
+    }
+};
+
+} // namespace detail
+} // namespace atomics
+} // namespace boost
+
+#endif // BOOST_ATOMIC_DETAIL_WAIT_OPS_FUTEX_HPP_INCLUDED_