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 * Copyright (c) 2013, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * The Universal Permissive License (UPL), Version 1.0
 *
 * Subject to the condition set forth below, permission is hereby granted to any
 * person obtaining a copy of this software, associated documentation and/or
 * data (collectively the "Software"), free of charge and under any and all
 * copyright rights in the Software, and any and all patent rights owned or
 * freely licensable by each licensor hereunder covering either (i) the
 * unmodified Software as contributed to or provided by such licensor, or (ii)
 * the Larger Works (as defined below), to deal in both
 *
 * (a) the Software, and
 *
 * (b) any piece of software and/or hardware listed in the lrgrwrks.txt file if
 * one is included with the Software each a "Larger Work" to which the Software
 * is contributed by such licensors),
 *
 * without restriction, including without limitation the rights to copy, create
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 * Software and the Larger Work(s), and to sublicense the foregoing rights on
 * either these or other terms.
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 * This license is subject to the following condition:
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 * The above copyright notice and either this complete permission notice or at a
 * minimum a reference to the UPL must be included in all copies or substantial
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package com.oracle.truffle.api.utilities;

import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

import com.oracle.truffle.api.Assumption;
import com.oracle.truffle.api.CompilerDirectives;
import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
import com.oracle.truffle.api.Truffle;
import com.oracle.truffle.api.nodes.InvalidAssumptionException;

/**
 * A value that the compiler can assume is constant, but can be changed by invalidation.
 * 
 * Compiled code that uses the value will be invalidated each time the value changes, so you should
 * take care to only change values infrequently, or to monitor the number of times the value has
 * changed and at some point to replace the value with something more generic so that it does not
 * have to be changed and code does not have to keep being recompiled.
 *
 * @since 0.8 or earlier
 */
public class AssumedValue {

    private final String name;

    // value behaves as volatile by piggybacking on Assumption semantics
    @CompilationFinal private T value;
    @CompilationFinal private volatile Assumption assumption;

    @SuppressWarnings("rawtypes") private static final AtomicReferenceFieldUpdater ASSUMPTION_UPDATER = //
                    AtomicReferenceFieldUpdater.newUpdater(AssumedValue.class, Assumption.class, "assumption");

    /** @since 0.8 or earlier */
    public AssumedValue(T initialValue) {
        this(null, initialValue);
    }

    /** @since 0.8 or earlier */
    public AssumedValue(String name, T initialValue) {
        this.name = name;
        value = initialValue;
        assumption = Truffle.getRuntime().createAssumption(name);
    }

    /**
     * Get the current value, updating it if it has been {@link #set}. The compiler may be able to
     * make this method return a constant value, but still accommodate mutation.
     *
     * @since 0.8 or earlier
     */
    public T get() {
        try {
            assumption.check();
        } catch (InvalidAssumptionException e) {
            // No need to rewrite anything - just pick up the new values
        }

        return value;
    }

    /**
     * Set a new value, which will be picked up the next time {@link #get} is called.
     *
     * @since 0.8 or earlier
     */
    public void set(T newValue) {
        CompilerDirectives.transferToInterpreter();
        value = newValue;

        Assumption newAssumption = Truffle.getRuntime().createAssumption(name);
        Assumption oldAssumption = ASSUMPTION_UPDATER.getAndSet(this, newAssumption);
        oldAssumption.invalidate();
    }

}