SLAddNode.java

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package com.oracle.truffle.sl.nodes.expression;

import java.math.BigInteger;

import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
import com.oracle.truffle.api.ExactMath;
import com.oracle.truffle.api.dsl.ImplicitCast;
import com.oracle.truffle.api.dsl.Specialization;
import com.oracle.truffle.api.nodes.NodeInfo;
import com.oracle.truffle.sl.nodes.SLBinaryNode;
import com.oracle.truffle.sl.nodes.SLTypes;

/**
 * SL node that performs the "+" operation, which performs addition on arbitrary precision numbers,
 * as well as String concatenation if one of the operands is a String.
 * <p>
 * Type specialization on the input values is essential for the performance. This is achieved via
 * node rewriting: specialized subclasses handle just a single type, so that the generic node that
 * can handle all types is used only in cases where different types were encountered. The subclasses
 * are automatically generated by the Truffle DSL. In addition, a {@link SLAddNodeGen factory class}
 * is generated that provides, e.g., {@link SLAddNodeGen#create node creation}.
 */
@NodeInfo(shortName = "+")
public abstract class SLAddNode extends SLBinaryNode {

    /**
     * Specialization for primitive {@code long} values. This is the fast path of the
     * arbitrary-precision arithmetic. We need to check for overflows of the addition, and switch to
     * the {@link #add(BigInteger, BigInteger) slow path}. Therefore, we use an
     * {@link ExactMath#addExact(long, long) addition method that throws an exception on overflow}.
     * The {@code rewriteOn} attribute on the {@link Specialization} annotation automatically
     * triggers the node rewriting on the exception.
     * <p>
     * In compiled code, {@link ExactMath#addExact(long, long) addExact} is compiled to efficient
     * machine code that uses the processor's overflow flag. Therefore, this method is compiled to
     * only two machine code instructions on the fast path.
     * <p>
     * This specialization is automatically selected by the Truffle DSL if both the left and right
     * operand are {@code long} values.
     */
    @Specialization(rewriteOn = ArithmeticException.class)
    protected long add(long left, long right) {
        return ExactMath.addExact(left, right);
    }

    /**
     * This is the slow path of the arbitrary-precision arithmetic. The {@link BigInteger} type of
     * Java is doing everything we need.
     * <p>
     * This specialization is automatically selected by the Truffle DSL if both the left and right
     * operand are {@link BigInteger} values. Because the type system defines an {@link ImplicitCast
     * implicit conversion} from {@code long} to {@link BigInteger} in
     * {@link SLTypes#castBigInteger(long)}, this specialization is also taken if the left or the
     * right operand is a {@code long} value. Because the {@link #add(long, long) long}
     * specialization} has the {@code rewriteOn} attribute, this specialization is also taken if
     * both input values are {@code long} values but the primitive addition overflows.
     */
    @Specialization
    @TruffleBoundary
    protected BigInteger add(BigInteger left, BigInteger right) {
        return left.add(right);
    }

    /**
     * Specialization for String concatenation. The SL specification says that String concatenation
     * works if either the left or the right operand is a String. The non-string operand is
     * converted then automatically converted to a String.
     * <p>
     * To implement these semantics, we tell the Truffle DSL to use a custom guard. The guard
     * function is defined in {@link #isString this class}, but could also be in any superclass.
     */
    @Specialization(guards = "isString(left, right)")
    @TruffleBoundary
    protected String add(Object left, Object right) {
        return left.toString() + right.toString();
    }

    /**
     * Guard for String concatenation: returns true if either the left or the right operand is a
     * {@link String}.
     */
    protected boolean isString(Object a, Object b) {
        return a instanceof String || b instanceof String;
    }
}