1 /*
   2  * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 package sun.jvm.hotspot.utilities;
  26 
  27 import sun.jvm.hotspot.debugger.*;
  28 import sun.jvm.hotspot.gc.shared.*;
  29 import sun.jvm.hotspot.memory.*;
  30 import sun.jvm.hotspot.oops.*;
  31 import sun.jvm.hotspot.runtime.*;
  32 
  33 /** Helper class which covers the reserved area of the heap with an
  34     (object-external) set of mark bits, used for GC-like scans through
  35     the heap like liveness analysis. */
  36 
  37 public class MarkBits {
  38   public MarkBits(CollectedHeap heap) {
  39     MemRegion reserved = heap.reservedRegion();
  40     // Must cover "reserved" with one bit for each OopHandle
  41     start = reserved.start();
  42     end   = reserved.end();
  43     long numOopHandles = end.minus(start) / VM.getVM().getOopSize();
  44     bits = heap.createBitMap(numOopHandles);
  45   }
  46 
  47   public void clear() {
  48     bits.clear();
  49   }
  50 
  51   /** Returns true if a mark was newly placed for the given Oop, or
  52       false if the Oop was already marked. If the Oop happens to lie
  53       outside the heap (should not happen), prints a warning and
  54       returns false. */
  55   public boolean mark(Oop obj) {
  56     if (obj == null) {
  57       System.err.println("MarkBits: WARNING: null object, ignoring");
  58       return false;
  59     }
  60 
  61     OopHandle handle = obj.getHandle();
  62     long idx = handle.minus(start) / VM.getVM().getOopSize();
  63     if (bits.at(idx)) {
  64       return false; // already marked
  65     }
  66     bits.atPut(idx, true);
  67     return true;
  68   }
  69 
  70   /** Forces clearing of a given mark bit. */
  71   public void clear(Oop obj) {
  72     OopHandle handle = obj.getHandle();
  73     long idx = handle.minus(start) / VM.getVM().getOopSize();
  74     bits.atPut(idx, false);
  75   }
  76 
  77   private BitMapInterface bits;
  78   private Address start;
  79   private Address end;
  80 }