1 /*
   2  * Copyright (c) 2018, 2019, 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 package jdk.vm.ci.hotspot;
  24 
  25 import static jdk.vm.ci.hotspot.UnsafeAccess.UNSAFE;
  26 
  27 /**
  28  * This class manages a set of {@code jobject} and {@code jmetadata} handles whose lifetimes are
  29  * dependent on associated {@link IndirectHotSpotObjectConstantImpl} and
  30  * {@link MetaspaceHandleObject} wrapper objects respectively.
  31  *
  32  * The general theory of operation is that all wrappers are created by calling into the VM which
  33  * calls back out to actually create the wrapper instance. During the call the VM keeps the object
  34  * or metadata reference alive through the use of handles. Once the call completes the wrapper
  35  * object is registered here and will be scanned during metadata scanning. The weakness of the
  36  * reference to the wrapper object allows the handles to be reclaimed when they are no longer used.
  37  */
  38 final class HandleCleaner extends Cleaner {
  39 
  40     /**
  41      * A {@code jmetadata} or {@code jobject} handle.
  42      */
  43     private final long handle;
  44 
  45     /**
  46      * Specifies if {@link #handle} is a {@code jobject} or {@code jmetadata}.
  47      */
  48     private final boolean isJObject;
  49 
  50     private HandleCleaner(Object wrapper, long handle, boolean isJObject) {
  51         super(wrapper);
  52         this.handle = handle;
  53         this.isJObject = isJObject;
  54     }
  55 
  56     /**
  57      * Releases the resource associated with {@code this.handle}.
  58      */
  59     @Override
  60     void doCleanup() {
  61         if (isJObject) {
  62             // The sentinel value used to denote a free handle is
  63             // an object on the HotSpot heap so we call into the
  64             // VM to set the target of an object handle to this value.
  65             CompilerToVM.compilerToVM().deleteGlobalHandle(handle);
  66         } else {
  67             // Setting the target of a jmetadata handle to 0 enables
  68             // the handle to be reused. See MetadataHandleBlock in
  69             // jvmciRuntime.cpp for more info.
  70             long value = UNSAFE.getLong(null, handle);
  71             UNSAFE.compareAndSetLong(null, handle, value, 0);
  72         }
  73     }
  74 
  75     /**
  76      * Registers a cleaner for {@code handle}. The cleaner will release the handle some time after
  77      * {@code wrapper} is detected as unreachable by the garbage collector.
  78      */
  79     @SuppressWarnings("unused")
  80     static void create(Object wrapper, long handle) {
  81         assert wrapper instanceof IndirectHotSpotObjectConstantImpl || wrapper instanceof MetaspaceHandleObject;
  82         new HandleCleaner(wrapper, handle, wrapper instanceof IndirectHotSpotObjectConstantImpl);
  83     }
  84 }