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 * Copyright (c) 2014, 2017, 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.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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
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package com.oracle.svm.core.threadlocal;

import org.graalvm.nativeimage.Platform;
import org.graalvm.nativeimage.Platforms;
import org.graalvm.word.LocationIdentity;

Base class for fast thread local variables.
/**
 * Base class for fast thread local variables.
 */
public abstract class FastThreadLocal {

    static class FastThreadLocalLocationIdentity extends LocationIdentity {
        @Override
        public boolean isImmutable() {
            return false;
        }
    }

    private final LocationIdentity locationIdentity;

    @Platforms(Platform.HOSTED_ONLY.class)
    FastThreadLocal() {
        this.locationIdentity = new FastThreadLocalLocationIdentity();
    }

    
Returns the LocationIdentity used for memory accesses performed by the get and set methods of the subclasses. /**
     * Returns the {@link LocationIdentity} used for memory accesses performed by the {@code get}
     * and {@code set} methods of the subclasses.
     */
    public LocationIdentity getLocationIdentity() {
        return locationIdentity;
    }

    @Platforms(Platform.HOSTED_ONLY.class) //
    private int maxOffset = Integer.MAX_VALUE;

    
Useful value for setMaxOffset: The thread local variable is in the first cache line of the memory block. This allows grouping of the most frequently accessed variables. We are not using a real cache line size, but instead assume that 64 bytes is the common minimum size on all platforms. /**
     * Useful value for {@link #setMaxOffset}: The thread local variable is in the first cache line
     * of the memory block. This allows grouping of the most frequently accessed variables.
     * 
     * We are not using a real cache line size, but instead assume that 64 bytes is the common
     * minimum size on all platforms.
     */
    public static final int FIRST_CACHE_LINE = 63;

    
Useful value for setMaxOffset: The thread local variable has an offset that can be expressed as a signed 8-bit value. Some architectures, e.g. AMD64, need fewer bytes to encode such offsets. /**
     * Useful value for {@link #setMaxOffset}: The thread local variable has an offset that can be
     * expressed as a signed 8-bit value. Some architectures, e.g. AMD64, need fewer bytes to encode
     * such offsets.
     */
    public static final int BYTE_OFFSET = 127;

    
Sets the maximum offset of this thread local variable in the memory block reserved for each
thread. This can be used for performance and footprint optimization, to group frequently
accessed values closely together with low offsets that can be encoded more efficiently in
machine code.
/**
     * Sets the maximum offset of this thread local variable in the memory block reserved for each
     * thread. This can be used for performance and footprint optimization, to group frequently
     * accessed values closely together with low offsets that can be encoded more efficiently in
     * machine code.
     */
    @SuppressWarnings("unchecked")
    @Platforms(Platform.HOSTED_ONLY.class)
    public <T extends FastThreadLocal> T setMaxOffset(int maxOffset) {
        this.maxOffset = maxOffset;
        return (T) this;
    }

    @Platforms(Platform.HOSTED_ONLY.class)
    public int getMaxOffset() {
        return maxOffset;
    }
}