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MostlyThreadLocal.java
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MostlyThreadLocal.java
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/*
* Copyright 2022 Carl Mastrangelo
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package io.perfmark.impl;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicReferenceArray;
/**
* This class tries to access the mark holder in a thread local. If the class is unable to store the created
* MarkHolder in the thread local, a concurrent map of Thread to MarkHolders is used for lookup. In Java 19
* Thread Locals can be disabled due to use of Virtual threads.
*
* <p>
* Note that this class avoids using {@link #initialValue()} because it may be called multiple times, in the
* case that ThreadLocals are unsettable.
*/
final class MostlyThreadLocal extends ThreadLocal<MarkHolder> {
private static final int BITS = 11;
private static final int SIZE = 1 << BITS;
private static final int MASK = SIZE - 1;
private static final AtomicReferenceArray<CopyOnWriteArrayList<Storage.MarkHolderHandle>> threadToHandles =
new AtomicReferenceArray<CopyOnWriteArrayList<Storage.MarkHolderHandle>>(SIZE);
MostlyThreadLocal() {}
@Override
public MarkHolder get() {
MarkHolder markHolder = super.get();
if (markHolder != null) {
return markHolder;
}
return getAndSetSlow();
}
private MarkHolder getAndSetSlow() {
assert super.get() == null;
Storage.MarkHolderHandle markHolderHandle;
MarkHolder markHolder;
Storage.MarkHolderAndHandle markHolderAndHandle;
Thread thread = Thread.currentThread();
CopyOnWriteArrayList<Storage.MarkHolderHandle> handles = getHandles(thread);
if (handles == null) {
markHolderAndHandle = Storage.allocateMarkHolder();
markHolder = markHolderAndHandle.markHolder();
assert markHolder != null;
markHolderHandle = markHolderAndHandle.handle();
try {
set(markHolder);
return markHolder;
} catch (UnsupportedOperationException e) {
// ignore.
}
handles = getOrCreateHandles(thread);
} else {
if ((markHolderHandle = getConcurrent(handles)) != null) {
if ((markHolder = markHolderHandle.markHolder()) != null) {
return markHolder;
}
}
markHolderAndHandle = Storage.allocateMarkHolder();
markHolder = markHolderAndHandle.markHolder();
assert markHolder != null;
markHolderHandle = markHolderAndHandle.handle();
}
handles.add(markHolderHandle);
return markHolder;
}
@Override
public void remove() {
Thread thread = Thread.currentThread();
CopyOnWriteArrayList<Storage.MarkHolderHandle> handles = getHandles(thread);
if (handles == null) {
super.remove();
} else {
assert super.get() == null;
for (Storage.MarkHolderHandle handle : handles) {
Thread t = handle.threadRef().get();
if (t == null || t == thread) {
handles.remove(handle);
}
}
}
}
/**
* Returns the handles for the given thread index bucket, or {@code null}.
*/
private static CopyOnWriteArrayList<Storage.MarkHolderHandle> getHandles(Thread thread) {
int hashCode = System.identityHashCode(thread);
int index = hashCode & MASK;
return threadToHandles.get(index);
}
private static CopyOnWriteArrayList<Storage.MarkHolderHandle> getOrCreateHandles(Thread thread) {
int hashCode = System.identityHashCode(thread);
int index = hashCode & MASK;
CopyOnWriteArrayList<Storage.MarkHolderHandle> handles;
do {
if ((handles = threadToHandles.get(index)) != null) {
break;
}
} while (
!threadToHandles.compareAndSet(index, null, (handles = new CopyOnWriteArrayList<Storage.MarkHolderHandle>())));
return handles;
}
/**
* May return {@code null} if not found.
*/
private static Storage.MarkHolderHandle getConcurrent(CopyOnWriteArrayList<Storage.MarkHolderHandle> handles) {
if (!handles.isEmpty()) {
Storage.MarkHolderHandle handle;
try {
handle = handles.get(0);
} catch (IndexOutOfBoundsException e) {
return null;
}
if (handle.threadRef().get() == Thread.currentThread()) {
return handle;
}
return slowGetConcurrent(handles);
}
return null;
}
/**
* May return {@code null} if not found.
*/
private static Storage.MarkHolderHandle slowGetConcurrent(CopyOnWriteArrayList<Storage.MarkHolderHandle> handles) {
for (Storage.MarkHolderHandle handle : handles) {
Thread thread = handle.threadRef().get();
if (thread == null) {
handles.remove(handle);
} else if (thread == Thread.currentThread()) {
return handle;
}
}
return null;
}
}