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Truffle SL is an example language implemented using the Truffle API.
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package com.oracle.truffle.sl.nodes.local;

import com.oracle.truffle.api.CompilerDirectives;
import com.oracle.truffle.api.dsl.NodeField;
import com.oracle.truffle.api.dsl.Specialization;
import com.oracle.truffle.api.frame.FrameSlot;
import com.oracle.truffle.api.frame.FrameUtil;
import com.oracle.truffle.api.frame.VirtualFrame;
import com.oracle.truffle.api.instrumentation.StandardTags.ReadVariableTag;
import com.oracle.truffle.api.instrumentation.Tag;
import com.oracle.truffle.sl.nodes.SLExpressionNode;
import com.oracle.truffle.sl.nodes.interop.NodeObjectDescriptor;

/**
 * Node to read a local variable from a function's {@link VirtualFrame frame}. The Truffle frame API
 * allows to store primitive values of all Java primitive types, and Object values. This means that
 * all SL types that are objects are handled by the {@link #readObject} method.
 * 
 * We use the primitive type only when the same primitive type is uses for all writes. If the local
 * variable is type-polymorphic, then the value is always stored as an Object, i.e., primitive
 * values are boxed. Even a mixture of {@code long} and {@code boolean} writes leads to both being
 * stored boxed.
 */
@NodeField(name = "slot", type = FrameSlot.class)
public abstract class SLReadLocalVariableNode extends SLExpressionNode {

    /**
     * Returns the descriptor of the accessed local variable. The implementation of this method is
     * created by the Truffle DSL based on the {@link NodeField} annotation on the class.
     */
    protected abstract FrameSlot getSlot();

    @Specialization(guards = "frame.isLong(getSlot())")
    protected long readLong(VirtualFrame frame) {
        /*
         * When the FrameSlotKind is Long, we know that only primitive long values have ever been
         * written to the local variable. So we do not need to check that the frame really contains
         * a primitive long value.
         */
        return FrameUtil.getLongSafe(frame, getSlot());
    }

    @Specialization(guards = "frame.isBoolean(getSlot())")
    protected boolean readBoolean(VirtualFrame frame) {
        return FrameUtil.getBooleanSafe(frame, getSlot());
    }

    @Specialization(replaces = {"readLong", "readBoolean"})
    protected Object readObject(VirtualFrame frame) {
        if (!frame.isObject(getSlot())) {
            /*
             * The FrameSlotKind has been set to Object, so from now on all writes to the local
             * variable will be Object writes. However, now we are in a frame that still has an old
             * non-Object value. This is a slow-path operation: we read the non-Object value, and
             * write it immediately as an Object value so that we do not hit this path again
             * multiple times for the same variable of the same frame.
             */
            CompilerDirectives.transferToInterpreter();
            Object result = frame.getValue(getSlot());
            frame.setObject(getSlot(), result);
            return result;
        }

        return FrameUtil.getObjectSafe(frame, getSlot());
    }

    @Override
    public boolean hasTag(Class tag) {
        return tag == ReadVariableTag.class || super.hasTag(tag);
    }

    @Override
    public Object getNodeObject() {
        return NodeObjectDescriptor.readVariable(getSlot().getIdentifier().toString());
    }
}