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.mmtk.plan.markcompact;
014    
015    import org.mmtk.plan.TraceLocal;
016    import org.mmtk.plan.Trace;
017    import org.mmtk.policy.Space;
018    import org.mmtk.vm.VM;
019    
020    import org.vmmagic.pragma.*;
021    import org.vmmagic.unboxed.*;
022    
023    /**
024     * This class implments the thread-local functionality for a transitive
025     * closure over a mark-compact space during the forwarding phase.
026     */
027    @Uninterruptible
028    public final class MCForwardTraceLocal extends TraceLocal {
029      /**
030       * Constructor
031       */
032      public MCForwardTraceLocal(Trace trace) {
033        super(MC.SCAN_FORWARD, trace);
034      }
035    
036      /****************************************************************************
037       *
038       * Externally visible Object processing and tracing
039       */
040    
041      /**
042       * Is the specified object live?
043       *
044       * @param object The object.
045       * @return True if the object is live.
046       */
047      public boolean isLive(ObjectReference object) {
048        if (object.isNull()) return false;
049        if (Space.isInSpace(MC.MARK_COMPACT, object)) {
050          return MC.mcSpace.isLive(object);
051        }
052        return super.isLive(object);
053      }
054    
055      /**
056       * This method is the core method during the trace of the object graph.
057       * The role of this method is to:
058       *
059       * 1. Ensure the traced object is not collected.
060       * 2. If this is the first visit to the object enqueue it to be scanned.
061       * 3. Return the forwarded reference to the object.
062       *
063       * In this instance, we refer objects in the mark-sweep space to the
064       * msSpace for tracing, and defer to the superclass for all others.
065       *
066       * @param object The object to be traced.
067       * @return The new reference to the same object instance.
068       */
069      @Inline
070      public ObjectReference traceObject(ObjectReference object) {
071        if (object.isNull()) return object;
072        if (Space.isInSpace(MC.MARK_COMPACT, object))
073          return MC.mcSpace.traceForwardObject(this, object);
074        return super.traceObject(object);
075      }
076    
077      /**
078       * Will this object move from this point on, during the current trace ?
079       *
080       * @param object The object to query.
081       * @return True if the object will not move.
082       */
083      public boolean willNotMoveInCurrentCollection(ObjectReference object) {
084        return !Space.isInSpace(MC.MARK_COMPACT, object);
085      }
086    
087      /**
088       * Ensure that this object will not move for the rest of the GC.
089       *
090       * @param object The object that must not move
091       * @return The new object, guaranteed stable for the rest of the GC.
092       */
093      @Inline
094      public ObjectReference precopyObject(ObjectReference object) {
095        if (VM.VERIFY_ASSERTIONS) {
096          // All precopying must occur during the initial trace.
097          VM.assertions._assert(!Space.isInSpace(MC.MARK_COMPACT, object));
098        }
099        return super.precopyObject(object);
100      }
101    
102    }