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