MXS-1632: Add C++ wrappers for __atomic builtins

Using the __atomic functions makes it easy to increment fundamental types
atomically without converting them to std::atomic<T>.

maxutils/maxbase/include/maxbase/atomic.hh

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2018 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+#pragma once
+
+#include <maxbase/ccdefs.hh>
+
+namespace maxbase
+{
+
+namespace atomic
+{
+
+constexpr int RELAXED = __ATOMIC_RELAXED;
+constexpr int CONSUME = __ATOMIC_CONSUME;
+constexpr int ACQUIRE = __ATOMIC_ACQUIRE;
+constexpr int RELEASE = __ATOMIC_RELEASE;
+constexpr int ACQ_REL = __ATOMIC_ACQ_REL;
+constexpr int SEQ_CST = __ATOMIC_SEQ_CST;
+
+/**
+ * Perform atomic fetch-and-add operation
+ *
+ * @param t    Variable to add to
+ * @param v    Value to add
+ * @param mode Memory ordering
+ *
+ * @return The old value
+ */
+template<class T, class R>
+T add(T* t, R v, int mode = SEQ_CST)
+{
+    return __atomic_fetch_add(t, v, mode);
+}
+
+/**
+ * Perform atomic load operation
+ *
+ * @param t    Variable to load
+ * @param mode Memory ordering
+ *
+ * @return The loaded value
+ */
+template<class T>
+T load(T* t, int mode = SEQ_CST)
+{
+    return __atomic_load_n(t, mode);
+}
+
+/**
+ * Perform atomic store operation
+ *
+ * @param t    Variable where the value is stored
+ * @param v    Value to store
+ * @param mode Memory ordering
+ *
+ * @return The loaded value
+ */
+template<class T, class R>
+void store(T* t, R v, int mode = SEQ_CST)
+{
+    __atomic_store_n(t, v, mode);
+}
+}
+}