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annotate graal/com.oracle.graal.api.code/src/com/oracle/graal/api/code/MemoryBarriers.java @ 6498:6bc8aa568cb9

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moved MemoryBarriers to com.oracle.graal.api.code
author Doug Simon <doug.simon@oracle.com>
date Wed, 03 Oct 2012 18:14:17 +0200
parents graal/com.oracle.max.criutils/src/com/oracle/max/criutils/MemoryBarriers.java@956217932b8c
children 5e3d1a68664e
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1 /*
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2 * Copyright (c) 2011, 2011, Oracle and/or its affiliates. All rights reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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20 * or visit www.oracle.com if you need additional information or have any
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21 * questions.
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22 */
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23 package com.oracle.graal.api.code;
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24
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25 /**
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26 * Constants and intrinsic definition for memory barriers.
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27 *
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28 * The documentation for each constant is taken from Doug Lea's
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29 * <a href="http://gee.cs.oswego.edu/dl/jmm/cookbook.html">The JSR-133 Cookbook for Compiler Writers</a>.
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30 * <p>
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31 * The {@code JMM_*} constants capture the memory barriers necessary to implement the Java Memory Model
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32 * with respect to volatile field accesses. Their values are explained by this
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33 * comment from templateTable_i486.cpp in the HotSpot source code:
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34 * <pre>
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35 * Volatile variables demand their effects be made known to all CPU's in
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36 * order. Store buffers on most chips allow reads & writes to reorder; the
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37 * JMM's ReadAfterWrite.java test fails in -Xint mode without some kind of
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38 * memory barrier (i.e., it's not sufficient that the interpreter does not
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39 * reorder volatile references, the hardware also must not reorder them).
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40 *
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41 * According to the new Java Memory Model (JMM):
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42 * (1) All volatiles are serialized wrt to each other.
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43 * ALSO reads & writes act as acquire & release, so:
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44 * (2) A read cannot let unrelated NON-volatile memory refs that happen after
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45 * the read float up to before the read. It's OK for non-volatile memory refs
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46 * that happen before the volatile read to float down below it.
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47 * (3) Similarly, a volatile write cannot let unrelated NON-volatile memory refs
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48 * that happen BEFORE the write float down to after the write. It's OK for
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49 * non-volatile memory refs that happen after the volatile write to float up
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50 * before it.
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51 *
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52 * We only put in barriers around volatile refs (they are expensive), not
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53 * _between_ memory refs (which would require us to track the flavor of the
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54 * previous memory refs). Requirements (2) and (3) require some barriers
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55 * before volatile stores and after volatile loads. These nearly cover
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56 * requirement (1) but miss the volatile-store-volatile-load case. This final
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57 * case is placed after volatile-stores although it could just as well go
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58 * before volatile-loads.
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59 * </pre>
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60 */
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61 public class MemoryBarriers {
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62
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63 /**
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64 * The sequence {@code Load1; LoadLoad; Load2} ensures that {@code Load1}'s data are loaded before data accessed
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65 * by {@code Load2} and all subsequent load instructions are loaded. In general, explicit {@code LoadLoad}
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66 * barriers are needed on processors that perform speculative loads and/or out-of-order processing in which
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67 * waiting load instructions can bypass waiting stores. On processors that guarantee to always preserve load
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68 * ordering, these barriers amount to no-ops.
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69 */
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70 public static final int LOAD_LOAD = 0x0001;
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71
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72 /**
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73 * The sequence {@code Load1; LoadStore; Store2} ensures that {@code Load1}'s data are loaded before all data
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74 * associated with {@code Store2} and subsequent store instructions are flushed. {@code LoadStore} barriers are
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75 * needed only on those out-of-order processors in which waiting store instructions can bypass loads.
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76 */
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77 public static final int LOAD_STORE = 0x0002;
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78
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79 /**
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80 * The sequence {@code Store1; StoreLoad; Load2} ensures that {@code Store1}'s data are made visible to other
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81 * processors (i.e., flushed to main memory) before data accessed by {@code Load2} and all subsequent load
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82 * instructions are loaded. {@code StoreLoad} barriers protect against a subsequent load incorrectly using
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83 * {@code Store1}'s data value rather than that from a more recent store to the same location performed by a
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84 * different processor. Because of this, on the processors discussed below, a {@code StoreLoad} is strictly
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85 * necessary only for separating stores from subsequent loads of the same location(s) as were stored before the
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86 * barrier. {@code StoreLoad} barriers are needed on nearly all recent multiprocessors, and are usually the most
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87 * expensive kind. Part of the reason they are expensive is that they must disable mechanisms that ordinarily
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88 * bypass cache to satisfy loads from write-buffers. This might be implemented by letting the buffer fully
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89 * flush, among other possible stalls.
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90 */
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91 public static final int STORE_LOAD = 0x0004;
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92
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93 /**
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94 * The sequence {@code Store1; StoreStore; Store2} ensures that {@code Store1}'s data are visible to other
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95 * processors (i.e., flushed to memory) before the data associated with {@code Store2} and all subsequent store
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96 * instructions. In general, {@code StoreStore} barriers are needed on processors that do not otherwise
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97 * guarantee strict ordering of flushes from write buffers and/or caches to other processors or main memory.
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98 */
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99 public static final int STORE_STORE = 0x0008;
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100
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101 public static final int JMM_PRE_VOLATILE_WRITE = LOAD_STORE | STORE_STORE;
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102 public static final int JMM_POST_VOLATILE_WRITE = STORE_LOAD | STORE_STORE;
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103 public static final int JMM_PRE_VOLATILE_READ = 0;
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104 public static final int JMM_POST_VOLATILE_READ = LOAD_LOAD | LOAD_STORE;
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105
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106 public static String barriersString(int barriers) {
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107 StringBuilder sb = new StringBuilder();
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108 sb.append((barriers & LOAD_LOAD) != 0 ? "LOAD_LOAD " : "");
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109 sb.append((barriers & LOAD_STORE) != 0 ? "LOAD_STORE " : "");
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110 sb.append((barriers & STORE_LOAD) != 0 ? "STORE_LOAD " : "");
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111 sb.append((barriers & STORE_STORE) != 0 ? "STORE_STORE " : "");
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112 return sb.toString().trim();
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113 }
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114 }