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 29 * <a 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 barriersString(int barriers)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 } 122