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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.CompilerAsserts;
import com.oracle.truffle.api.CompilerDirectives;
import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
import com.oracle.truffle.api.Truffle;
/**
* 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 */
@TruffleBoundary
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() {
if (CompilerDirectives.isPartialEvaluationConstant(assumption)) {
/*
* Only check the assumption if we actually constant-fold the value. Otherwise, we might
* deoptimize for no reason.
*
* Note that it's important that the first if condition in this method does a volatile
* read on the assumption variable. That acts as a memory barrier, preventing the value
* read from floating above in the case where the value is not constant folded and in
* the interpreter code. That makes the value field effectively volatile, making this
* class thread-safe.
*/
CompilerAsserts.partialEvaluationConstant(value);
if (!assumption.isValid()) {
CompilerDirectives.transferToInterpreterAndInvalidate();
}
}
return value;
}
/**
* Set a new value, which will be picked up the next time {@link #get} is called.
*
* @since 0.8 or earlier
*/
@TruffleBoundary
public void set(T newValue) {
value = newValue;
Assumption newAssumption = Truffle.getRuntime().createAssumption(name);
Assumption oldAssumption = ASSUMPTION_UPDATER.getAndSet(this, newAssumption);
oldAssumption.invalidate();
}
}