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 uninterruptible 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     *   <li>bytecodes that can cause interruption cause warnings.</li>
046     *   <li>uninterruptible code will be generated assuming no RuntimeExceptions are
047     * raised and without any GC maps (since by definition there can be noGC if
048     * control is not lost).</li>
049     * </ol>
050     * <p>
051     *
052     * This is the inverse of {@link Interruptible}.
053     */
054    @Retention(RetentionPolicy.RUNTIME)
055    @Target({ElementType.TYPE, ElementType.METHOD})
056    @Pragma
057    public @interface Uninterruptible {
058      /**
059       * @return Explanation of why code needs to be uninterruptible
060       */
061      String value() default "";
062    }