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     */
013    package org.vmmagic.pragma;
014    
015    import java.lang.annotation.Retention;
016    import java.lang.annotation.Target;
017    import java.lang.annotation.RetentionPolicy;
018    import java.lang.annotation.ElementType;
019    import 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:<p>
039     *
040     * <ol>
041     *  <li>the normal thread switch test that would be emitted in the method
042     * prologue is omitted.</li>
043     *  <li>the stack overflow test that would be emitted in the method prologue is
044     * omitted.</li>
045     *  <li>calls to preemptible code causes warnings.</li>
046     * <ol>
047     * <p>
048     *
049     * This is the inverse of {@link Preemptible}.
050     */
051    @Retention(RetentionPolicy.RUNTIME)
052    @Target({ElementType.TYPE, ElementType.METHOD})
053    @Pragma
054    public @interface Unpreemptible {
055      /**
056       * @return Explanation of why code needs to be unpreemptible
057       */
058      String value() default "";
059    }