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.
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}.
036     *
037     * Any method that is marked as uninterruptible is treated specially by the
038     * machine code compiler:
039     *
040     * (1) the normal thread switch test that would be emitted in the method
041     * prologue is omitted.
042     *
043     * (2) the stack overflow test that would be emitted in the method prologue is
044     * omitted.
045     *
046     * (3) calls to preemptible code causes warnings.
047     *
048     * (4) bytecodes that can cause interruption cause warnings.
049     *
050     * (5) uninterruptible code will be generated assuming no RuntimeExceptions are
051     * raised and without any GC maps (since by definition there can be noGC if
052     * control is not lost).
053     *
054     * This is the inverse of {@link Interruptible}.
055     */
056    @Retention(RetentionPolicy.RUNTIME)
057    @Target({ElementType.TYPE, ElementType.METHOD})
058    @Pragma
059    public @interface Uninterruptible {
060      /**
061       * @return Explanation of why code needs to be uninterruptible
062       */
063      String value() default "";
064    }