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1 /*
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2 * Copyright 2001-2006 Sun Microsystems, Inc. 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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20 * CA 95054 USA or visit www.sun.com if you need additional information or
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21 * have any questions.
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22 *
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23 */
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24
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25 package sun.jvm.hotspot.utilities;
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26
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27 import java.io.*;
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28 import java.util.*;
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29 import sun.jvm.hotspot.debugger.*;
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30 import sun.jvm.hotspot.gc_interface.*;
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31 import sun.jvm.hotspot.memory.*;
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32 import sun.jvm.hotspot.oops.*;
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33 import sun.jvm.hotspot.runtime.*;
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34
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35 /** Finds all paths from roots to the specified set of objects. NOTE:
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36 currently only a subset of the roots known to the VM is exposed to
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37 the SA: objects on the stack, static fields in classes, and JNI
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38 handles. These should be most of the user-level roots keeping
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39 objects alive. */
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40
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41 public class LivenessAnalysis {
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42 // Used for debugging this code
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43 private static final boolean DEBUG = false;
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44
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45 private LivenessAnalysis() {}
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46
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47 public static LivenessPathList computeAllLivenessPaths(Oop target) {
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48 LivenessPathList list = computeAllLivenessPaths(target, true);
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49 if ((list == null) || (list.size() == 0)) {
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50 // Dead object
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51 return null;
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52 }
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53 return list;
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54 }
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55
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56 //---------------------------------------------------------------------------
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57 // Internals only below this point
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58 //
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59
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60 // Returns true if a new path was completed, otherwise false
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61 // indicating there were no more paths to complete.
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62 //
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63 // The trimPathsThroughPopularObjects flag alters the behavior of
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64 // the returned results. If true, then if multiple paths to
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65 // different roots all go through a particular popular object, those
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66 // paths will be truncated and only one (arbitrary one) will be be
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67 // returned. On the other hand, if the target object itself is
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68 // popular and there are multiple distinct paths to it (indicating
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69 // that there are multiple objects pointing directly to it) then all
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70 // of those paths will be reported.
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71 private static LivenessPathList computeAllLivenessPaths(Oop target, boolean trimPathsThroughPopularObjects) {
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72 ReversePtrs rev = VM.getVM().getRevPtrs();
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73 if (rev == null) {
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74 throw new RuntimeException("LivenessAnalysis requires ReversePtrs to have been computed");
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75 }
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76
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77 // Currently the reverse pointer analysis returns non-null results
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78 // only for live objects
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79 if (rev.get(target) == null) {
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80 // Object is dead
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81 return null;
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82 }
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83
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84 // HashSet of Oops acting as a bit mask indicating which ones have
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85 // already been traversed
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86 Set/*<Oop>*/ visitedOops = new HashSet/*<Oop>*/();
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87
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88 // IdentityHashMap of LivenessElements acting as a bit mask
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89 // indicating which roots have already been traversed
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90 Map/*<LivenessElement, LivenessElement>*/ visitedRoots =
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91 new IdentityHashMap/*<LivenessElement, LivenessElement>*/();
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92
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93 visitedOops.add(target);
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94
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95 // Construct the initial LivenessPath
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96 LivenessPathList list = new LivenessPathList();
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97 {
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98 LivenessPath path = new LivenessPath();
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99 path.push(new LivenessPathElement(target, null));
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100 list.add(path);
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101 }
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102
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103 // Iterate until done
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104 while (true) {
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105 // See if there are any incomplete paths left
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106 LivenessPath path = null;
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107
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108 for (int i = list.size() - 1; i >= 0; i--) {
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109 LivenessPath tmp = list.get(i);
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110 if (!tmp.isComplete()) {
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111 path = tmp;
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112 break;
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113 }
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114 }
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115
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116 // If no incomplete paths left, return
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117 if (path == null) {
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118 return list;
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119 }
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120
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121 // Try to complete this path
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122
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123 // Remove the path from the list of reported ones in
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124 // anticipation of completing it
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125 list.remove(path);
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126
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127 try {
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128 // Fetch next set of reverse pointers for the last object on
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129 // the list
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130 ArrayList/*<LivenessPathElement>*/ nextPtrs =
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131 rev.get(path.peek().getObj());
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132
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133 // Depending on exactly what the reverse pointers analysis
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134 // yields, these results may be null, although currently they
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135 // won't be
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136 if (nextPtrs != null) {
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137 // Iterate these
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138 for (Iterator iter = nextPtrs.iterator(); iter.hasNext(); ) {
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139 LivenessPathElement nextElement = (LivenessPathElement) iter.next();
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140 // See whether we've visited this element yet
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141 if ((nextElement.isRoot() && (visitedRoots.get(nextElement) == null)) ||
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142 (!nextElement.isRoot() && !visitedOops.contains(nextElement.getObj()))) {
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143 // Show we've visited this one
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144 if (nextElement.isRoot()) {
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145 visitedRoots.put(nextElement, nextElement);
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146 } else {
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147 visitedOops.add(nextElement.getObj());
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148 }
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149
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150 // Create a new LivenessPath for each one
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151 LivenessPath nextPath = path.copy();
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152 nextPath.push(nextElement);
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153
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154 // Regardless of whether we found a root, put the
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155 // original path back on the list because we're going to
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156 // do depth-first searching rather than breadth-first
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157 list.add(path);
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158 list.add(nextPath);
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159
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160 // See whether this path is complete (i.e., it
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161 // terminates in a root)
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162 if (trimPathsThroughPopularObjects && nextElement.isRoot()) {
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163 // Go back through the path list and remove all
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164 // incomplete paths ending in any of the intermediate
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165 // (non-root and non-terminal) nodes in this path.
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166 // This has the effect of removing multiple paths
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167 // going through popular objects.
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168 for (int i = 1; i < nextPath.size() - 1; i++) {
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169 LivenessPathElement el = nextPath.get(i);
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170 int j = 0;
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171 while (j < list.size()) {
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172 LivenessPath curPath = list.get(j);
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173 // We can use an object identity since these
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174 // intermediate nodes are canonicalized via the
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175 // ReversePtrsAnalysis
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176 if (curPath.peek() == el) {
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177 list.remove(curPath);
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178 } else {
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179 j++;
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180 }
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181 }
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182 }
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183 }
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184
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185 // Do depth-first searching, not breadth-first
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186 break;
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187 }
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188 }
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189 }
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190 } catch (Exception e) {
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191 System.err.println("LivenessAnalysis: WARNING: " + e +
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192 " during traversal");
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193 }
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194 }
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195 }
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196 }
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