--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdEpoch.cpp	Tue May 15 20:24:34 2018 +0200
@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2016, 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "jfr/recorder/checkpoint/types/traceid/jfrTraceIdEpoch.hpp"
+#include "runtime/safepoint.hpp"
+#include "runtime/orderAccess.inline.hpp"
+
+// Alternating epochs on each rotation allow for concurrent tagging.
+// The regular epoch shift happens only during a safepoint.
+// The fence is there only for the emergency dump case which happens outside of safepoint.
+bool JfrTraceIdEpoch::_epoch_state = false;
+void JfrTraceIdEpoch::shift_epoch() {
+  _epoch_state = !_epoch_state;
+  if (!SafepointSynchronize::is_at_safepoint()) {
+    OrderAccess::fence();
+  }
+}