001/*
002 *  This file is part of the Jikes RVM project (http://jikesrvm.org).
003 *
004 *  This file is licensed to You under the Eclipse Public License (EPL);
005 *  You may not use this file except in compliance with the License. You
006 *  may obtain a copy of the License at
007 *
008 *      http://www.opensource.org/licenses/eclipse-1.0.php
009 *
010 *  See the COPYRIGHT.txt file distributed with this work for information
011 *  regarding copyright ownership.
012 */
013package org.vmmagic.pragma;
014
015import java.lang.annotation.Retention;
016import java.lang.annotation.Target;
017import java.lang.annotation.RetentionPolicy;
018import java.lang.annotation.ElementType;
019import org.vmmagic.Pragma;
020
021/**
022 * By default all Java code is interruptible, that is scheduling or garbage
023 * collection may occur at points within the code. Code can be marked as
024 * {@link Unpreemptible} or {@link Uninterruptible}, that instructs the JVM to
025 * avoid garbage collection and thread scheduling. The {@link Uninterruptible}
026 * annotation disallows any operation that may cause garbage collection or
027 * thread scheduling, for example memory allocation. The {@link Unpreemptible}
028 * annotation doesn't disallow operations that can cause garbage collection or
029 * scheduling, but instructs the JVM to avoid inserting such operations during a
030 * block of code.<p>
031 *
032 * In the internals of a VM most code wants to be {@link Uninterruptible}.
033 * However, code involved in scheduling and locking will cause context switches,
034 * and creating exception objects may trigger garbage collection, this code is
035 * therefore {@link Unpreemptible}.<p>
036 *
037 * Any method that is marked as unpreemptible is treated specially by the
038 * machine code compiler:
039 * <ol>
040 *  <li>the normal thread switch test that would be emitted in the method
041 * prologue is omitted.</li>
042 *  <li>the stack overflow test that would be emitted in the method prologue is
043 * omitted.</li>
044 *  <li>calls to preemptible code causes warnings.</li>
045 * </ol>
046 * <p>
047 * This is the inverse of {@link Preemptible}.
048 */
049@Retention(RetentionPolicy.RUNTIME)
050@Target({ElementType.TYPE, ElementType.METHOD})
051@Pragma
052public @interface Unpreemptible {
053  /**
054   * @return Explanation of why code needs to be unpreemptible
055   */
056  String value() default "";
057}