ircd::util: Add a non-atomic compare_exchange().

include/ircd/util/compare_exchange.h

@@ -0,0 +1,37 @@
+// The Construct
+//
+// Copyright (C) The Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2020 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+#pragma once
+#define HAVE_IRCD_UTIL_COMPARE_EXCHANGE_H
+
+namespace ircd {
+inline namespace util
+{
+	template<class T>
+	bool compare_exchange(T &value, T &expect, T desire) noexcept;
+}}
+
+/// Non-atomic compare_exchange().
+template<class T>
+inline bool
+ircd::util::compare_exchange(T &value,
+                             T &expect,
+                             T desire)
+noexcept
+{
+	const bool ret
+	{
+		value == expect
+	};
+
+	value = ret? desire: value;
+	expect = ret? expect: value;
+	return ret;
+}

include/ircd/util/util.h

@@ -64,6 +64,7 @@ namespace ircd
 #include "boolean.h"
 #include "maybe.h"
 #include "all.h"
+#include "compare_exchange.h"
 
 // Unsorted section
 namespace ircd {