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package com.oracle.truffle.api.instrument;

import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
import com.oracle.truffle.api.*;
import com.oracle.truffle.api.frame.*;
import com.oracle.truffle.api.instrument.impl.*;
import com.oracle.truffle.api.nodes.*;

// TODO (mlvdv) migrate some of this to external documentation.
/**
 * A dynamically added/removed binding between a {@link Probe}, which provides notification of
 * {@linkplain TruffleEventListener execution events} taking place at a {@link Node} in a Guest
 * Language (GL) Truffle AST, and a {@linkplain TruffleEventListener listener}, which consumes
 * notifications on behalf of an external tool.
 * <p>
 * <h4>Summary: How to "instrument" an AST location:</h4>
 * <ol>
 * <li>Create an implementation of {@link TruffleEventListener} that responds to events on behalf of
 * a tool.</li>
 * <li>Create an Instrument via factory method {@link Instrument#create(TruffleEventListener)}.</li>
 * <li>"Attach" the Instrument to a Probe via {@link Probe#attach(Instrument)}, at which point event
 * notifications begin to arrive at the listener.</li>
 * <li>When no longer needed, "detach" the Instrument via {@link Instrument#dispose()}, at which
 * point event notifications to the listener cease, and the Instrument becomes unusable.</li>
 * </ol>
 * <p>
 * <h4>Options for creating listeners:</h4>
 * <p>
 * <ol>
 * <li>Implement the interface {@link TruffleEventListener}. The event handling methods account for
 * both the entry into an AST node (about to call) and several possible kinds of exit from an AST
 * node (just returned).</li>
 * <li>Extend {@link DefaultEventListener}, which provides a no-op implementation of every
 * {@link TruffleEventListener} method; override the methods of interest.</li>
 * <li>Extend {@link SimpleEventListener}, where return values are ignored so only two methods (for
 * "enter" and "return") will notify all events.</li>
 * </ol>
 * <p>
 * <h4>General guidelines for listener implementation:</h4>
 * <p>
 * When an Instrument is attached to a Probe, the listener effectively becomes part of the executing
 * GL program; performance can be affected by the listener's implementation.
 * <ul>
 * <li>Do not store {@link Frame} or {@link Node} references in fields.</li>
 * <li>Prefer {@code final} fields and (where performance is important) short methods.</li>
 * <li>If needed, pass along the {@link VirtualFrame} reference from an event notification as far as
 * possible through code that is expected to be inlined, since this incurs no runtime overhead. When
 * access to frame data is needed, substitute a more expensive {@linkplain Frame#materialize()
 * materialized} representation of the frame.</li>
 * <li>If a listener calls back to its tool during event handling, and if performance is an issue,
 * then this should be through a final "callback" field in the instrument, and the called methods
 * should be minimal.</li>
 * <li>On the other hand, implementations should prevent Truffle from inlining beyond a reasonable
 * point with the method annotation {@link TruffleBoundary}.</li>
 * <li>The implicit "outer" pointer in a non-static inner class is a useful way to implement
 * callbacks to owner tools.</li>
 * <li>Primitive-valued return events are boxed for notification, but Truffle will eliminate the
 * boxing if they are cast back to their primitive values quickly (in particular before crossing any
 * {@link TruffleBoundary} annotations).
 * </ul>
 * <p>
 * <h4>Allowing for AST cloning:</h4>
 * <p>
 * Truffle routinely <em>clones</em> ASTs, which has consequences for listener implementation.
 * <ul>
 * <li>Even though a {@link Probe} is uniquely associated with a particular location in the
 * executing Guest Language program, execution events at that location will in general be
 * implemented by different {@link Node} instances, i.e. <em>clones</em> of the originally probed
 * node.</li>
 * <li>Because of <em>cloning</em> the {@link Node} supplied with notifications to a particular
 * listener will vary, but because they all represent the same GL program location the events should
 * be treated as equivalent for most purposes.</li>
 * </ul>
 * <p>
 * <h4>Access to execution state:</h4>
 * <p>
 * <ul>
 * <li>Event notification arguments provide primary access to the GL program's execution states:
 * <ul>
 * <li>{@link Node}: the concrete node (in one of the AST's clones) from which the event originated.
 * </li>
 * <li>{@link VirtualFrame}: the current execution frame.
 * </ul>
 * <li>Some global information is available, for example the execution
 * {@linkplain TruffleRuntime#iterateFrames(FrameInstanceVisitor) stack}.</li>
 * <li>Additional information needed by a listener could be stored when created, preferably
 * {@code final} of course. For example, a reference to the {@link Probe} to which the listener's
 * Instrument has been attached would give access to its corresponding
 * {@linkplain Probe#getProbedSourceSection() source location} or to the collection of
 * {@linkplain SyntaxTag tags} currently applied to the Probe.</li>
 * </ul>
 * <p>
 * <h4>Activating and deactivating Instruments:</h4>
 * <p>
 * Instruments are <em>single-use</em>:
 * <ul>
 * <li>An instrument becomes active only when <em>attached</em> to a Probe via
 * {@link Probe#attach(Instrument)}, and it may only be attached to a single Probe. It is a runtime
 * error to attempt attaching a previously attached instrument.</li>
 * <li>Attaching an instrument modifies every existing clone of the AST to which it is being
 * attached, which can trigger deoptimization.</li>
 * <li>The method {@link Instrument#dispose()} makes an instrument inactive by removing it from the
 * Probe to which it was attached and rendering it permanently inert.</li>
 * <li>Disposal removes the implementation of an instrument from all ASTs to which it was attached,
 * which can trigger deoptimization.</li>
 * </ul>
 * <p>
 * <h4>Sharing listeners:</h4>
 * <p>
 * Although an Instrument may only be attached to a single Probe, a listener can be shared among
 * multiple Instruments. This can be useful for observing events that might happen at different
 * locations in a single AST, for example all assignments to a particular variable. In this case a
 * new Instrument would be created and attached at each assignment node, but all the Instruments
 * would be created with the same listener.
 * <p>
 * <strong>Disclaimer:</strong> experimental; under development.
 *
 * @see Probe
 * @see TruffleEventListener
 */
public final class Instrument {

    /**
     * Creates an instrument that will route execution events to a listener.
     *
     * @param listener a listener for event generated by the instrument
     * @param instrumentInfo optional description of the instrument's role
     * @return a new instrument, ready for attachment at a probe
     */
    public static Instrument create(TruffleEventListener listener, String instrumentInfo) {
        return new Instrument(listener, instrumentInfo);
    }

    /**
     * Creates an instrument that will route execution events to a listener.
     */
    public static Instrument create(TruffleEventListener listener) {
        return new Instrument(listener, null);
    }

    /**
     * Tool-supplied listener for events.
     */
    private final TruffleEventListener toolEventListener;

    /**
     * Optional documentation, mainly for debugging.
     */
    private final String instrumentInfo;

    /**
     * Has this instrument been disposed? stays true once set.
     */
    private boolean isDisposed = false;

    private Probe probe = null;

    private Instrument(TruffleEventListener listener, String instrumentInfo) {
        this.toolEventListener = listener;
        this.instrumentInfo = instrumentInfo;
    }

    /**
     * Removes this instrument (and any clones) from the probe to which it attached and renders the
     * instrument inert.
     *
     * @throws IllegalStateException if this instrument has already been disposed
     */
    public void dispose() throws IllegalStateException {
        if (isDisposed) {
            throw new IllegalStateException("Attempt to dispose an already disposed Instrumennt");
        }
        if (probe != null) {
            // It's attached
            probe.disposeInstrument(this);
        }
        this.isDisposed = true;
    }

    Probe getProbe() {
        return probe;
    }

    void setAttachedTo(Probe probe) {
        this.probe = probe;
    }

    /**
     * Has this instrument been disposed and rendered unusable?
     */
    boolean isDisposed() {
        return isDisposed;
    }

    InstrumentNode addToChain(InstrumentNode nextNode) {
        return new InstrumentNode(nextNode);
    }

    /**
     * Removes this instrument from an instrument chain.
     */
    InstrumentNode removeFromChain(InstrumentNode instrumentNode) {
        boolean found = false;
        if (instrumentNode != null) {
            if (instrumentNode.getInstrument() == this) {
                // Found the match at the head of the chain
                return instrumentNode.nextInstrument;
            }
            // Match not at the head of the chain; remove it.
            found = instrumentNode.removeFromChain(Instrument.this);
        }
        if (!found) {
            throw new IllegalStateException("Couldn't find instrument node to remove: " + this);
        }
        return instrumentNode;
    }

    @NodeInfo(cost = NodeCost.NONE)
    final class InstrumentNode extends Node implements TruffleEventListener, InstrumentationNode {

        @Child private InstrumentNode nextInstrument;

        private InstrumentNode(InstrumentNode nextNode) {
            this.nextInstrument = nextNode;
        }

        @Override
        public boolean isInstrumentable() {
            return false;
        }

        /**
         * Gets the instrument that created this node.
         */
        private Instrument getInstrument() {
            return Instrument.this;
        }

        /**
         * Removes the node from this chain that was added by a particular instrument, assuming that
         * the head of the chain is not the one to be replaced. This is awkward, but is required
         * because {@link Node#replace(Node)} won't take a {@code null} argument. This doesn't work
         * for the tail of the list, which would be replacing itself with null. So the replacement
         * must be directed the parent of the node being removed.
         */
        private boolean removeFromChain(Instrument instrument) {
            assert getInstrument() != instrument;
            if (nextInstrument == null) {
                return false;
            }
            if (nextInstrument.getInstrument() == instrument) {
                // Next is the one to remove
                if (nextInstrument.nextInstrument == null) {
                    // Next is at the tail; just forget
                    nextInstrument = null;
                } else {
                    // Replace next with its successor
                    nextInstrument.replace(nextInstrument.nextInstrument);
                }
                return true;
            }
            return nextInstrument.removeFromChain(instrument);
        }

        public void enter(Node node, VirtualFrame vFrame) {
            Instrument.this.toolEventListener.enter(node, vFrame);
            if (nextInstrument != null) {
                nextInstrument.enter(node, vFrame);
            }
        }

        public void returnVoid(Node node, VirtualFrame vFrame) {
            Instrument.this.toolEventListener.returnVoid(node, vFrame);
            if (nextInstrument != null) {
                nextInstrument.returnVoid(node, vFrame);
            }
        }

        public void returnValue(Node node, VirtualFrame vFrame, Object result) {
            Instrument.this.toolEventListener.returnValue(node, vFrame, result);
            if (nextInstrument != null) {
                nextInstrument.returnValue(node, vFrame, result);
            }
        }

        public void returnExceptional(Node node, VirtualFrame vFrame, Exception exception) {
            Instrument.this.toolEventListener.returnExceptional(node, vFrame, exception);
            if (nextInstrument != null) {
                nextInstrument.returnExceptional(node, vFrame, exception);
            }
        }

        public String instrumentationInfo() {
            if (Instrument.this.instrumentInfo != null) {
                return Instrument.this.instrumentInfo;
            }
            return toolEventListener.getClass().getSimpleName();
        }
    }

}