1 /*
2 * Copyright (c) 2015, 2019, 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.hotspot;
24
25 import java.lang.ref.Reference;
26 import java.lang.ref.ReferenceQueue;
27 import java.lang.ref.WeakReference;
28 import java.util.Arrays;
29 import java.util.Iterator;
30
31 import jdk.vm.ci.meta.JavaKind;
32 import jdk.vm.ci.meta.ResolvedJavaType;
33
34 /**
35 * This class manages the set of metadata roots that must be scanned during garbage collection.
36 * Because of class redefinition Method* and ConstantPool* can be freed if they don't appear to be
37 * in use so they must be tracked when there are live references to them from Java.
38 *
39 * The general theory of operation is that all {@link MetaspaceWrapperObject}s are created by
40 * calling into the VM which calls back out to actually create the wrapper instance. During the call
41 * the VM keeps the metadata reference alive through the use of metadata handles. Once the call
42 * completes the wrapper object is registered here and will be scanned during metadata scanning. The
43 * weakness of the reference to the wrapper object allows them to be reclaimed when they are no
44 * longer used.
45 *
46 */
47 class HotSpotJVMCIMetaAccessContext {
48
49 /**
50 * The set of currently live contexts used for tracking of live metadata. Examined from the VM
51 * during garbage collection.
52 */
53 private static WeakReference<?>[] allContexts = new WeakReference<?>[0];
54
55 /**
56 * This is a chunked list of metadata roots. It can be read from VM native code so it's been
57 * marked volatile to ensure the order of updates are respected.
58 */
59 private volatile Object[] metadataRoots;
60
61 private ChunkedList<WeakReference<MetaspaceWrapperObject>> list = new ChunkedList<>();
62
63 /**
64 * The number of weak references freed since the last time the list was shrunk.
65 */
66 private int freed;
67
68 /**
69 * The {@link ReferenceQueue} tracking the weak references created by this context.
70 */
71 private final ReferenceQueue<MetaspaceWrapperObject> queue = new ReferenceQueue<>();
72
73 static synchronized void add(HotSpotJVMCIMetaAccessContext context) {
74 for (int i = 0; i < allContexts.length; i++) {
75 if (allContexts[i] == null || allContexts[i].get() == null) {
76 allContexts[i] = new WeakReference<>(context);
77 return;
78 }
79 }
80 int index = allContexts.length;
81 allContexts = Arrays.copyOf(allContexts, index + 2);
82 allContexts[index] = new WeakReference<>(context);
83 }
84
85 HotSpotJVMCIMetaAccessContext() {
86 add(this);
87 }
88
89 /**
90 * Periodically trim the list of tracked metadata. A new list is created to replace the old to
91 * avoid concurrent scanning issues.
92 */
93 private void clean() {
94 Reference<?> ref = queue.poll();
95 if (ref == null) {
96 return;
97 }
98 while (ref != null) {
99 freed++;
100 ref = queue.poll();
101 }
102 if (freed > list.size() / 2) {
103 ChunkedList<WeakReference<MetaspaceWrapperObject>> newList = new ChunkedList<>();
104 for (WeakReference<MetaspaceWrapperObject> element : list) {
105 /*
106 * The referent could become null anywhere in here but it doesn't matter. It will
107 * get cleaned up next time.
108 */
109 if (element != null && element.get() != null) {
110 newList.add(element);
111 }
112 }
113 list = newList;
114 metadataRoots = list.getHead();
115 freed = 0;
116 }
117 }
118
119 /**
120 * Add a {@link MetaspaceWrapperObject} to tracked by the GC. It's assumed that the caller is
121 * responsible for keeping the reference alive for the duration of the call. Once registration
122 * is complete then the VM will ensure it's kept alive.
123 *
124 * @param metaspaceObject
125 */
126
127 public synchronized void add(MetaspaceWrapperObject metaspaceObject) {
128 clean();
129 list.add(new WeakReference<>(metaspaceObject, queue));
130 if (list.getHead() != metadataRoots) {
131 /*
132 * The list enlarged so update the head.
133 */
134 metadataRoots = list.getHead();
135 }
136 assert isRegistered(metaspaceObject);
137 }
138
139 protected ResolvedJavaType createClass(Class<?> javaClass) {
140 if (javaClass.isPrimitive()) {
141 JavaKind kind = JavaKind.fromJavaClass(javaClass);
142 return new HotSpotResolvedPrimitiveType(kind);
143 } else {
144 return new HotSpotResolvedObjectTypeImpl(javaClass, this);
145 }
146 }
147
148 private static class WeakTypeRef {
149 private WeakReference<ResolvedJavaType> ref;
150 WeakTypeRef(ResolvedJavaType value) {
151 set(value);
152 }
153 void set(ResolvedJavaType value) {
154 ref = new WeakReference<ResolvedJavaType>(value);
155 }
156 ResolvedJavaType get() {
157 return ref.get();
158 }
159 };
160
161 private final ClassValue<WeakTypeRef> resolvedJavaType = new ClassValue<>() {
162 @Override
163 protected WeakTypeRef computeValue(Class<?> type) {
164 return new WeakTypeRef(createClass(type));
165 }
166 };
167
168 /**
169 * Gets the JVMCI mirror for a {@link Class} object.
170 *
171 * @return the {@link ResolvedJavaType} corresponding to {@code javaClass}
172 */
173 public ResolvedJavaType fromClass(Class<?> javaClass) {
174 WeakTypeRef ref = resolvedJavaType.get(javaClass);
175 ResolvedJavaType javaType = ref.get();
176 if (javaType == null) {
177 /*
178 * If the referent has become null, create a new value and
179 * update cached weak reference.
180 */
181 javaType = createClass(javaClass);
182 ref.set(javaType);
183 }
184 return javaType;
185 }
186
187 /**
188 * A very simple append only chunked list implementation.
189 */
190 static class ChunkedList<T> implements Iterable<T> {
191 private static final int CHUNK_SIZE = 32;
192
193 private static final int NEXT_CHUNK_INDEX = CHUNK_SIZE - 1;
194
195 private Object[] head;
196 private int index;
197 private int size;
198
199 ChunkedList() {
200 head = new Object[CHUNK_SIZE];
201 index = 0;
202 }
203
204 void add(T element) {
205 if (index == NEXT_CHUNK_INDEX) {
206 Object[] newHead = new Object[CHUNK_SIZE];
207 newHead[index] = head;
208 head = newHead;
209 index = 0;
210 }
211 head[index++] = element;
212 size++;
213 }
214
215 Object[] getHead() {
216 return head;
217 }
218
219 @Override
220 public Iterator<T> iterator() {
221 return new ChunkIterator<>();
222 }
223
224 int size() {
225 return size;
226 }
227
228 class ChunkIterator<V> implements Iterator<V> {
229
230 ChunkIterator() {
231 currentChunk = head;
232 currentIndex = -1;
233 next = findNext();
234 }
235
236 Object[] currentChunk;
237 int currentIndex;
238 V next;
239
240 @SuppressWarnings("unchecked")
241 V findNext() {
242 V result;
243 do {
244 currentIndex++;
245 if (currentIndex == NEXT_CHUNK_INDEX) {
246 currentChunk = (Object[]) currentChunk[currentIndex];
247 currentIndex = 0;
248 if (currentChunk == null) {
249 return null;
250 }
251 }
252 result = (V) currentChunk[currentIndex];
253 } while (result == null);
254 return result;
255 }
256
257 @Override
258 public boolean hasNext() {
259 return next != null;
260 }
261
262 @Override
263 public V next() {
264 V result = next;
265 next = findNext();
266 return result;
267 }
268
269 }
270
271 }
272
273 synchronized boolean isRegistered(MetaspaceWrapperObject wrapper) {
274 for (WeakReference<MetaspaceWrapperObject> m : list) {
275 if (m != null && m.get() == wrapper) {
276 return true;
277 }
278 }
279 return false;
280 }
281 }