--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jvmci/com.oracle.jvmci.code/src/com/oracle/jvmci/code/MemoryBarriers.java	Tue Jun 09 00:22:49 2015 +0200
@@ -0,0 +1,121 @@
+/*
+ * Copyright (c) 2011, 2011, 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.
+ */
+package com.oracle.jvmci.code;
+
+/**
+ * Constants and intrinsic definition for memory barriers.
+ *
+ * The documentation for each constant is taken from Doug Lea's <a
+ * href="http://gee.cs.oswego.edu/dl/jmm/cookbook.html">The JSR-133 Cookbook for Compiler
+ * Writers</a>.
+ * <p>
+ * The {@code JMM_*} constants capture the memory barriers necessary to implement the Java Memory
+ * Model with respect to volatile field accesses. Their values are explained by this comment from
+ * templateTable_i486.cpp in the HotSpot source code:
+ *
+ * <pre>
+ * Volatile variables demand their effects be made known to all CPU's in
+ * order.  Store buffers on most chips allow reads &amp; writes to reorder; the
+ * JMM's ReadAfterWrite.java test fails in -Xint mode without some kind of
+ * memory barrier (i.e., it's not sufficient that the interpreter does not
+ * reorder volatile references, the hardware also must not reorder them).
+ *
+ * According to the new Java Memory Model (JMM):
+ * (1) All volatiles are serialized wrt to each other.
+ * ALSO reads &amp; writes act as acquire &amp; release, so:
+ * (2) A read cannot let unrelated NON-volatile memory refs that happen after
+ * the read float up to before the read.  It's OK for non-volatile memory refs
+ * that happen before the volatile read to float down below it.
+ * (3) Similarly, a volatile write cannot let unrelated NON-volatile memory refs
+ * that happen BEFORE the write float down to after the write.  It's OK for
+ * non-volatile memory refs that happen after the volatile write to float up
+ * before it.
+ *
+ * We only put in barriers around volatile refs (they are expensive), not
+ * _between_ memory refs (which would require us to track the flavor of the
+ * previous memory refs).  Requirements (2) and (3) require some barriers
+ * before volatile stores and after volatile loads.  These nearly cover
+ * requirement (1) but miss the volatile-store-volatile-load case.  This final
+ * case is placed after volatile-stores although it could just as well go
+ * before volatile-loads.
+ * </pre>
+ */
+public class MemoryBarriers {
+
+    /**
+     * The sequence {@code Load1; LoadLoad; Load2} ensures that {@code Load1}'s data are loaded
+     * before data accessed by {@code Load2} and all subsequent load instructions are loaded. In
+     * general, explicit {@code LoadLoad} barriers are needed on processors that perform speculative
+     * loads and/or out-of-order processing in which waiting load instructions can bypass waiting
+     * stores. On processors that guarantee to always preserve load ordering, these barriers amount
+     * to no-ops.
+     */
+    public static final int LOAD_LOAD = 0x0001;
+
+    /**
+     * The sequence {@code Load1; LoadStore; Store2} ensures that {@code Load1}'s data are loaded
+     * before all data associated with {@code Store2} and subsequent store instructions are flushed.
+     * {@code LoadStore} barriers are needed only on those out-of-order processors in which waiting
+     * store instructions can bypass loads.
+     */
+    public static final int LOAD_STORE = 0x0002;
+
+    /**
+     * The sequence {@code Store1; StoreLoad; Load2} ensures that {@code Store1}'s data are made
+     * visible to other processors (i.e., flushed to main memory) before data accessed by
+     * {@code Load2} and all subsequent load instructions are loaded. {@code StoreLoad} barriers
+     * protect against a subsequent load incorrectly using {@code Store1}'s data value rather than
+     * that from a more recent store to the same location performed by a different processor.
+     * Because of this, on the processors discussed below, a {@code StoreLoad} is strictly necessary
+     * only for separating stores from subsequent loads of the same location(s) as were stored
+     * before the barrier. {@code StoreLoad} barriers are needed on nearly all recent
+     * multiprocessors, and are usually the most expensive kind. Part of the reason they are
+     * expensive is that they must disable mechanisms that ordinarily bypass cache to satisfy loads
+     * from write-buffers. This might be implemented by letting the buffer fully flush, among other
+     * possible stalls.
+     */
+    public static final int STORE_LOAD = 0x0004;
+
+    /**
+     * The sequence {@code Store1; StoreStore; Store2} ensures that {@code Store1}'s data are
+     * visible to other processors (i.e., flushed to memory) before the data associated with
+     * {@code Store2} and all subsequent store instructions. In general, {@code StoreStore} barriers
+     * are needed on processors that do not otherwise guarantee strict ordering of flushes from
+     * write buffers and/or caches to other processors or main memory.
+     */
+    public static final int STORE_STORE = 0x0008;
+
+    public static final int JMM_PRE_VOLATILE_WRITE = LOAD_STORE | STORE_STORE;
+    public static final int JMM_POST_VOLATILE_WRITE = STORE_LOAD | STORE_STORE;
+    public static final int JMM_PRE_VOLATILE_READ = 0;
+    public static final int JMM_POST_VOLATILE_READ = LOAD_LOAD | LOAD_STORE;
+
+    public static String barriersString(int barriers) {
+        StringBuilder sb = new StringBuilder();
+        sb.append((barriers & LOAD_LOAD) != 0 ? "LOAD_LOAD " : "");
+        sb.append((barriers & LOAD_STORE) != 0 ? "LOAD_STORE " : "");
+        sb.append((barriers & STORE_LOAD) != 0 ? "STORE_LOAD " : "");
+        sb.append((barriers & STORE_STORE) != 0 ? "STORE_STORE " : "");
+        return sb.toString().trim();
+    }
+}