1 /*
2 * Copyright (c) 2011, 2011, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
23 package jdk.vm.ci.code;
24
25 /**
26 * Constants and intrinsic definition for memory barriers.
27 *
28 * The documentation for each constant is taken from Doug Lea's <a
29 * href="http://gee.cs.oswego.edu/dl/jmm/cookbook.html">The JSR-133 Cookbook for Compiler
30 * Writers</a>.
31 * <p>
32 * The {@code JMM_*} constants capture the memory barriers necessary to implement the Java Memory
33 * Model with respect to volatile field accesses. Their values are explained by this comment from
34 * templateTable_i486.cpp in the HotSpot source code:
35 *
36 * <pre>
37 * Volatile variables demand their effects be made known to all CPU's in
38 * order. Store buffers on most chips allow reads & writes to reorder; the
39 * JMM's ReadAfterWrite.java test fails in -Xint mode without some kind of
40 * memory barrier (i.e., it's not sufficient that the interpreter does not
41 * reorder volatile references, the hardware also must not reorder them).
42 *
43 * According to the new Java Memory Model (JMM):
44 * (1) All volatiles are serialized wrt to each other.
45 * ALSO reads & writes act as acquire & release, so:
46 * (2) A read cannot let unrelated NON-volatile memory refs that happen after
47 * the read float up to before the read. It's OK for non-volatile memory refs
48 * that happen before the volatile read to float down below it.
49 * (3) Similarly, a volatile write cannot let unrelated NON-volatile memory refs
50 * that happen BEFORE the write float down to after the write. It's OK for
51 * non-volatile memory refs that happen after the volatile write to float up
52 * before it.
53 *
54 * We only put in barriers around volatile refs (they are expensive), not
55 * _between_ memory refs (which would require us to track the flavor of the
56 * previous memory refs). Requirements (2) and (3) require some barriers
57 * before volatile stores and after volatile loads. These nearly cover
58 * requirement (1) but miss the volatile-store-volatile-load case. This final
59 * case is placed after volatile-stores although it could just as well go
60 * before volatile-loads.
61 * </pre>
62 */
63 public class MemoryBarriers {
64
65 /**
66 * The sequence {@code Load1; LoadLoad; Load2} ensures that {@code Load1}'s data are loaded
67 * before data accessed by {@code Load2} and all subsequent load instructions are loaded. In
68 * general, explicit {@code LoadLoad} barriers are needed on processors that perform speculative
69 * loads and/or out-of-order processing in which waiting load instructions can bypass waiting
70 * stores. On processors that guarantee to always preserve load ordering, these barriers amount
71 * to no-ops.
72 */
73 public static final int LOAD_LOAD = 0x0001;
74
75 /**
76 * The sequence {@code Load1; LoadStore; Store2} ensures that {@code Load1}'s data are loaded
77 * before all data associated with {@code Store2} and subsequent store instructions are flushed.
78 * {@code LoadStore} barriers are needed only on those out-of-order processors in which waiting
79 * store instructions can bypass loads.
80 */
81 public static final int LOAD_STORE = 0x0002;
82
83 /**
84 * The sequence {@code Store1; StoreLoad; Load2} ensures that {@code Store1}'s data are made
85 * visible to other processors (i.e., flushed to main memory) before data accessed by
86 * {@code Load2} and all subsequent load instructions are loaded. {@code StoreLoad} barriers
87 * protect against a subsequent load incorrectly using {@code Store1}'s data value rather than
88 * that from a more recent store to the same location performed by a different processor.
89 * Because of this, on the processors discussed below, a {@code StoreLoad} is strictly necessary
90 * only for separating stores from subsequent loads of the same location(s) as were stored
91 * before the barrier. {@code StoreLoad} barriers are needed on nearly all recent
92 * multiprocessors, and are usually the most expensive kind. Part of the reason they are
93 * expensive is that they must disable mechanisms that ordinarily bypass cache to satisfy loads
94 * from write-buffers. This might be implemented by letting the buffer fully flush, among other
95 * possible stalls.
96 */
97 public static final int STORE_LOAD = 0x0004;
98
99 /**
100 * The sequence {@code Store1; StoreStore; Store2} ensures that {@code Store1}'s data are
101 * visible to other processors (i.e., flushed to memory) before the data associated with
102 * {@code Store2} and all subsequent store instructions. In general, {@code StoreStore} barriers
103 * are needed on processors that do not otherwise guarantee strict ordering of flushes from
104 * write buffers and/or caches to other processors or main memory.
105 */
106 public static final int STORE_STORE = 0x0008;
107
108 public static final int JMM_PRE_VOLATILE_WRITE = LOAD_STORE | STORE_STORE;
109 public static final int JMM_POST_VOLATILE_WRITE = STORE_LOAD | STORE_STORE;
110 public static final int JMM_PRE_VOLATILE_READ = 0;
111 public static final int JMM_POST_VOLATILE_READ = LOAD_LOAD | LOAD_STORE;
112
113 public static String barriersString(int barriers) {
114 StringBuilder sb = new StringBuilder();
115 sb.append((barriers & LOAD_LOAD) != 0 ? "LOAD_LOAD " : "");
116 sb.append((barriers & LOAD_STORE) != 0 ? "LOAD_STORE " : "");
117 sb.append((barriers & STORE_LOAD) != 0 ? "STORE_LOAD " : "");
118 sb.append((barriers & STORE_STORE) != 0 ? "STORE_STORE " : "");
119 return sb.toString().trim();
120 }
121 }