/*

 * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package jdk.vm.ci.hotspot;

import static jdk.vm.ci.hotspot.UnsafeAccess.UNSAFE;

/**

 * This class manages a set of {@code jobject} and {@code jmetadata} handles whose lifetimes are

 * dependent on associated {@link IndirectHotSpotObjectConstantImpl} and

 * {@link MetaspaceHandleObject} wrapper objects respectively.

 *

 * The general theory of operation is that all wrappers are created by calling into the VM which

 * calls back out to actually create the wrapper instance. During the call the VM keeps the object

 * or metadata reference alive through the use of handles. Once the call completes the wrapper

 * object is registered here and will be scanned during metadata scanning. The weakness of the

 * reference to the wrapper object allows the handles to be reclaimed when they are no longer used.

 */

final class HandleCleaner extends Cleaner {

    /**

     * A {@code jmetadata} or {@code jobject} handle.

     */

    private final long handle;

    /**

     * Specifies if {@link #handle} is a {@code jobject} or {@code jmetadata}.

     */

    private final boolean isJObject;

    private HandleCleaner(Object wrapper, long handle, boolean isJObject) {

        super(wrapper);

        this.handle = handle;

        this.isJObject = isJObject;

    }

    /**

     * Releases the resource associated with {@code this.handle}.

     */

    @Override

    void doCleanup() {

        if (isJObject) {

            // The sentinel value used to denote a free handle is

            // an object on the HotSpot heap so we call into the

            // VM to set the target of an object handle to this value.

            CompilerToVM.compilerToVM().deleteGlobalHandle(handle);

        } else {

            // Setting the target of a jmetadata handle to 0 enables

            // the handle to be reused. See MetadataHandleBlock in

            // jvmciRuntime.cpp for more info.

            long value = UNSAFE.getLong(null, handle);

            UNSAFE.compareAndSetLong(null, handle, value, 0);

        }

    }

    /**

     * Registers a cleaner for {@code handle}. The cleaner will release the handle some time after

     * {@code wrapper} is detected as unreachable by the garbage collector.

     */

    @SuppressWarnings("unused")

    static void create(Object wrapper, long handle) {

        assert wrapper instanceof IndirectHotSpotObjectConstantImpl || wrapper instanceof MetaspaceHandleObject;

        new HandleCleaner(wrapper, handle, wrapper instanceof IndirectHotSpotObjectConstantImpl);

    }

}