PeepholeFoldConstants.java

/*
 * Copyright 2004 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.javascript.jscomp;

import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.NodeUtil.ValueType;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import com.google.javascript.rhino.jstype.TernaryValue;

/**
 * Peephole optimization to fold constants (e.g. x + 1 + 7 --> x + 8).
 *
 */
class PeepholeFoldConstants extends AbstractPeepholeOptimization {

  // TODO(johnlenz): optimizations should not be emiting errors. Move these to
  // a check pass.
  static final DiagnosticType INVALID_GETELEM_INDEX_ERROR =
      DiagnosticType.warning(
          "JSC_INVALID_GETELEM_INDEX_ERROR",
          "Array index not integer: {0}");

  static final DiagnosticType INDEX_OUT_OF_BOUNDS_ERROR =
      DiagnosticType.warning(
          "JSC_INDEX_OUT_OF_BOUNDS_ERROR",
          "Array index out of bounds: {0}");

  static final DiagnosticType NEGATING_A_NON_NUMBER_ERROR =
      DiagnosticType.warning(
          "JSC_NEGATING_A_NON_NUMBER_ERROR",
          "Can''t negate non-numeric value: {0}");

  static final DiagnosticType FRACTIONAL_BITWISE_OPERAND =
      DiagnosticType.warning(
          "JSC_FRACTIONAL_BITWISE_OPERAND",
          "Fractional bitwise operand: {0}");

  private static final double MAX_FOLD_NUMBER = Math.pow(2, 53);

  private final boolean late;

  private final boolean shouldUseTypes;

  /**
   * @param late When late is false, this mean we are currently running before
   * most of the other optimizations. In this case we would avoid optimizations
   * that would make the code harder to analyze. When this is true, we would
   * do anything to minimize for size.
   */
  PeepholeFoldConstants(boolean late, boolean shouldUseTypes) {
    this.late = late;
    this.shouldUseTypes = shouldUseTypes;
  }

  @Override
  Node optimizeSubtree(Node subtree) {
    switch (subtree.getToken()) {
      case CALL:
        return tryFoldCall(subtree);
      case NEW:
        return tryFoldCtorCall(subtree);

      case TYPEOF:
        return tryFoldTypeof(subtree);

      case NOT:
      case POS:
      case NEG:
      case BITNOT:
        tryReduceOperandsForOp(subtree);
        return tryFoldUnaryOperator(subtree);

      case VOID:
        return tryReduceVoid(subtree);

      default:
        tryReduceOperandsForOp(subtree);
        return tryFoldBinaryOperator(subtree);
    }
  }

  private Node tryFoldBinaryOperator(Node subtree) {
    Node left = subtree.getFirstChild();

    if (left == null) {
      return subtree;
    }

    Node right = left.getNext();

    if (right == null) {
      return subtree;
    }

    // If we've reached here, node is truly a binary operator.
    switch (subtree.getToken()) {
      case GETPROP:
        return tryFoldGetProp(subtree, left, right);

      case GETELEM:
        return tryFoldGetElem(subtree, left, right);

      case INSTANCEOF:
        return tryFoldInstanceof(subtree, left, right);

      case AND:
      case OR:
        return tryFoldAndOr(subtree, left, right);

      case LSH:
      case RSH:
      case URSH:
        return tryFoldShift(subtree, left, right);

      case ASSIGN:
        return tryFoldAssign(subtree, left, right);

      case ASSIGN_BITOR:
      case ASSIGN_BITXOR:
      case ASSIGN_BITAND:
      case ASSIGN_LSH:
      case ASSIGN_RSH:
      case ASSIGN_URSH:
      case ASSIGN_ADD:
      case ASSIGN_SUB:
      case ASSIGN_MUL:
      case ASSIGN_DIV:
      case ASSIGN_MOD:
        return tryUnfoldAssignOp(subtree, left, right);

      case ADD:
        return tryFoldAdd(subtree, left, right);

      case SUB:
      case DIV:
      case MOD:
        return tryFoldArithmeticOp(subtree, left, right);

      case MUL:
      case BITAND:
      case BITOR:
      case BITXOR:
        Node result = tryFoldArithmeticOp(subtree, left, right);
        if (result != subtree) {
          return result;
        }
        return tryFoldLeftChildOp(subtree, left, right);

      case LT:
      case GT:
      case LE:
      case GE:
      case EQ:
      case NE:
      case SHEQ:
      case SHNE:
        return tryFoldComparison(subtree, left, right);

      default:
        return subtree;
    }
  }

  private Node tryReduceVoid(Node n) {
    Node child = n.getFirstChild();
    if ((!child.isNumber() || child.getDouble() != 0.0) && !mayHaveSideEffects(n)) {
      n.replaceChild(child, IR.number(0));
      reportCodeChange();
    }
    return n;
  }

  private void tryReduceOperandsForOp(Node n) {
    switch (n.getToken()) {
      case ADD:
        Node left = n.getFirstChild();
        Node right = n.getLastChild();
        if (!NodeUtil.mayBeString(left, shouldUseTypes)
            && !NodeUtil.mayBeString(right, shouldUseTypes)) {
          tryConvertOperandsToNumber(n);
        }
        break;
      case ASSIGN_BITOR:
      case ASSIGN_BITXOR:
      case ASSIGN_BITAND:
        // TODO(johnlenz): convert these to integers.
      case ASSIGN_LSH:
      case ASSIGN_RSH:
      case ASSIGN_URSH:
      case ASSIGN_SUB:
      case ASSIGN_MUL:
      case ASSIGN_MOD:
      case ASSIGN_DIV:
        tryConvertToNumber(n.getLastChild());
        break;
      case BITNOT:
      case BITOR:
      case BITXOR:
      case BITAND:
      case LSH:
      case RSH:
      case URSH:
      case SUB:
      case MUL:
      case MOD:
      case DIV:
      case POS:
      case NEG:
        tryConvertOperandsToNumber(n);
        break;
      default:
        break;
    }
  }

  private void tryConvertOperandsToNumber(Node n) {
    Node next;
    for (Node c = n.getFirstChild(); c != null; c = next) {
      next = c.getNext();
      tryConvertToNumber(c);
    }
  }

  private void tryConvertToNumber(Node n) {
    switch (n.getToken()) {
      case NUMBER:
        // Nothing to do
        return;
      case AND:
      case OR:
      case COMMA:
        tryConvertToNumber(n.getLastChild());
        return;
      case HOOK:
        tryConvertToNumber(n.getSecondChild());
        tryConvertToNumber(n.getLastChild());
        return;
      case NAME:
        if (!NodeUtil.isUndefined(n)) {
          return;
        }
        break;
      default:
        break;
    }

    Double result = NodeUtil.getNumberValue(n, shouldUseTypes);
    if (result == null) {
      return;
    }

    double value = result;

    Node replacement = NodeUtil.numberNode(value, n);
    if (replacement.isEquivalentTo(n)) {
      return;
    }

    n.replaceWith(replacement);
    reportCodeChange();
  }

  /**
   * Folds 'typeof(foo)' if foo is a literal, e.g.
   * typeof("bar") --> "string"
   * typeof(6) --> "number"
   */
  private Node tryFoldTypeof(Node originalTypeofNode) {
    Preconditions.checkArgument(originalTypeofNode.isTypeOf());

    Node argumentNode = originalTypeofNode.getFirstChild();
    if (argumentNode == null || !NodeUtil.isLiteralValue(argumentNode, true)) {
      return originalTypeofNode;
    }

    String typeNameString = null;

    switch (argumentNode.getToken()) {
      case FUNCTION:
        typeNameString = "function";
        break;
      case STRING:
        typeNameString = "string";
        break;
      case NUMBER:
        typeNameString = "number";
        break;
      case TRUE:
      case FALSE:
        typeNameString = "boolean";
        break;
      case NULL:
      case OBJECTLIT:
      case ARRAYLIT:
        typeNameString = "object";
        break;
      case VOID:
        typeNameString = "undefined";
        break;
      case NAME:
        // We assume here that programs don't change the value of the
        // keyword undefined to something other than the value undefined.
        if ("undefined".equals(argumentNode.getString())) {
          typeNameString = "undefined";
        }
        break;
      default:
        break;
    }

    if (typeNameString != null) {
      Node newNode = IR.string(typeNameString);
      originalTypeofNode.replaceWith(newNode);
      reportCodeChange();

      return newNode;
    }

    return originalTypeofNode;
  }

  private Node tryFoldUnaryOperator(Node n) {
    Preconditions.checkState(n.hasOneChild(), n);

    Node left = n.getFirstChild();
    Node parent = n.getParent();

    if (left == null) {
      return n;
    }

    TernaryValue leftVal = NodeUtil.getPureBooleanValue(left);
    if (leftVal == TernaryValue.UNKNOWN) {
      return n;
    }

    switch (n.getToken()) {
      case NOT:
        // Don't fold !0 and !1 back to false.
        if (late && left.isNumber()) {
          double numValue = left.getDouble();
          if (numValue == 0 || numValue == 1) {
            return n;
          }
        }
        Node replacementNode = NodeUtil.booleanNode(!leftVal.toBoolean(true));
        parent.replaceChild(n, replacementNode);
        reportCodeChange();
        return replacementNode;
      case POS:
        if (NodeUtil.isNumericResult(left)) {
          // POS does nothing to numeric values.
          parent.replaceChild(n, left.detach());
          reportCodeChange();
          return left;
        }
        return n;
      case NEG:
        if (left.isName()) {
          if (left.getString().equals("Infinity")) {
            // "-Infinity" is valid and a literal, don't modify it.
            return n;
          } else if (left.getString().equals("NaN")) {
            // "-NaN" is "NaN".
            n.removeChild(left);
            parent.replaceChild(n, left);
            reportCodeChange();
            return left;
          }
        }

        if (left.isNumber()) {
          double negNum = -left.getDouble();

          Node negNumNode = IR.number(negNum);
          parent.replaceChild(n, negNumNode);
          reportCodeChange();
          return negNumNode;
        } else {
          // left is not a number node, so do not replace, but warn the
          // user because they can't be doing anything good
          report(NEGATING_A_NON_NUMBER_ERROR, left);
          return n;
        }
      case BITNOT:
        try {
          double val = left.getDouble();
          if (Math.floor(val) == val) {
            int intVal = jsConvertDoubleToBits(val);
            Node notIntValNode = IR.number(~intVal);
            parent.replaceChild(n, notIntValNode);
            reportCodeChange();
            return notIntValNode;
          } else {
            report(FRACTIONAL_BITWISE_OPERAND, left);
            return n;
          }
        } catch (UnsupportedOperationException ex) {
          // left is not a number node, so do not replace, but warn the
          // user because they can't be doing anything good
          report(NEGATING_A_NON_NUMBER_ERROR, left);
          return n;
        }
        default:
          return n;
    }
  }

  /**
   * Uses a method for treating a double as 32bits that is equivalent to
   * how JavaScript would convert a number before applying a bit operation.
   */
  private int jsConvertDoubleToBits(double d) {
    return (int) (((long) Math.floor(d)) & 0xffffffff);
  }

  /**
   * Try to fold {@code left instanceof right} into {@code true}
   * or {@code false}.
   */
  private Node tryFoldInstanceof(Node n, Node left, Node right) {
    Preconditions.checkArgument(n.isInstanceOf());

    // TODO(johnlenz) Use type information if available to fold
    // instanceof.
    if (NodeUtil.isLiteralValue(left, true)
        && !mayHaveSideEffects(right)) {

      Node replacementNode = null;

      if (NodeUtil.isImmutableValue(left)) {
        // Non-object types are never instances.
        replacementNode = IR.falseNode();
      } else if (right.isName()
          && "Object".equals(right.getString())) {
        replacementNode = IR.trueNode();
      }

      if (replacementNode != null) {
        n.replaceWith(replacementNode);
        reportCodeChange();
        return replacementNode;
      }
    }

    return n;
  }

  private Node tryFoldAssign(Node n, Node left, Node right) {
    Preconditions.checkArgument(n.isAssign());

    if (!late) {
      return n;
    }

    // Tries to convert x = x + y -> x += y;
    if (!right.hasChildren() ||
        right.getSecondChild() != right.getLastChild()) {
      // RHS must have two children.
      return n;
    }

    if (mayHaveSideEffects(left)) {
      return n;
    }

    Node newRight;
    if (areNodesEqualForInlining(left, right.getFirstChild())) {
      newRight = right.getLastChild();
    } else if (NodeUtil.isCommutative(right.getToken())
        && areNodesEqualForInlining(left, right.getLastChild())) {
      newRight = right.getFirstChild();
    } else {
      return n;
    }

    Token newType = null;
    switch (right.getToken()) {
      case ADD:
        newType = Token.ASSIGN_ADD;
        break;
      case BITAND:
        newType = Token.ASSIGN_BITAND;
        break;
      case BITOR:
        newType = Token.ASSIGN_BITOR;
        break;
      case BITXOR:
        newType = Token.ASSIGN_BITXOR;
        break;
      case DIV:
        newType = Token.ASSIGN_DIV;
        break;
      case LSH:
        newType = Token.ASSIGN_LSH;
        break;
      case MOD:
        newType = Token.ASSIGN_MOD;
        break;
      case MUL:
        newType = Token.ASSIGN_MUL;
        break;
      case RSH:
        newType = Token.ASSIGN_RSH;
        break;
      case SUB:
        newType = Token.ASSIGN_SUB;
        break;
      case URSH:
        newType = Token.ASSIGN_URSH;
        break;
      default:
        return n;
    }

    Node newNode = new Node(newType,
        left.detach(), newRight.detach());
    n.replaceWith(newNode);

    reportCodeChange();

    return newNode;
  }

  private Node tryUnfoldAssignOp(Node n, Node left, Node right) {
    if (late) {
      return n;
    }

    if (!n.hasChildren() ||
        n.getSecondChild() != n.getLastChild()) {
      return n;
    }

    if (mayHaveSideEffects(left)) {
      return n;
    }

    // Tries to convert x += y -> x = x + y;
    Token op = NodeUtil.getOpFromAssignmentOp(n);
    Node replacement = IR.assign(left.detach(),
        new Node(op, left.cloneTree(), right.detach())
            .srcref(n));
    n.replaceWith(replacement);
    reportCodeChange();

    return replacement;
  }

  /**
   * Try to fold a AND/OR node.
   */
  private Node tryFoldAndOr(Node n, Node left, Node right) {
    Node parent = n.getParent();

    Node result = null;

    Token type = n.getToken();

    TernaryValue leftVal = NodeUtil.getImpureBooleanValue(left);

    if (leftVal != TernaryValue.UNKNOWN) {
      boolean lval = leftVal.toBoolean(true);

      // (TRUE || x) => TRUE (also, (3 || x) => 3)
      // (FALSE && x) => FALSE
      if ((lval && type == Token.OR) || (!lval && type == Token.AND)) {
        result = left;

      } else if (!mayHaveSideEffects(left)) {
        // (FALSE || x) => x
        // (TRUE && x) => x
        result = right;
      } else {
        // Left side may have side effects, but we know its boolean value.
        // e.g. true_with_sideeffects || foo() => true_with_sideeffects, foo()
        // or: false_with_sideeffects && foo() => false_with_sideeffects, foo()
        // This, combined with PeepholeRemoveDeadCode, helps reduce expressions
        // like "x() || false || z()".
        n.detachChildren();
        result = IR.comma(left, right);
      }
    }

    // Note: Right hand side folding is handled by
    // PeepholeMinimizeConditions#tryMinimizeCondition

    if (result != null) {
      // Fold it!
      n.detachChildren();
      parent.replaceChild(n, result);
      reportCodeChange();

      return result;
    } else {
      return n;
    }
  }

  /**
   * Expressions such as [foo() + 'a' + 'b'] generate parse trees
   * where no node has two const children ((foo() + 'a') + 'b'), so
   * tryFoldAdd() won't fold it -- tryFoldLeftChildAdd() will (for Strings).
   * Specifically, it folds Add expressions where:
   *  - The left child is also and add expression
   *  - The right child is a constant value
   *  - The left child's right child is a STRING constant.
   */
  private Node tryFoldChildAddString(Node n, Node left, Node right) {

    if (NodeUtil.isLiteralValue(right, false) &&
        left.isAdd()) {

      Node ll = left.getFirstChild();
      Node lr = ll.getNext();

      // Left's right child MUST be a string. We would not want to fold
      // foo() + 2 + 'a' because we don't know what foo() will return, and
      // therefore we don't know if left is a string concat, or a numeric add.
      if (lr.isString()) {
        String leftString = NodeUtil.getStringValue(lr);
        String rightString = NodeUtil.getStringValue(right);
        if (leftString != null && rightString != null) {
          left.removeChild(ll);
          String result = leftString + rightString;
          n.replaceChild(left, ll);
          n.replaceChild(right, IR.string(result));
          reportCodeChange();
          return n;
        }
      }
    }

    if (NodeUtil.isLiteralValue(left, false) &&
        right.isAdd()) {

      Node rl = right.getFirstChild();
      Node rr = right.getLastChild();

      // Left's right child MUST be a string. We would not want to fold
      // foo() + 2 + 'a' because we don't know what foo() will return, and
      // therefore we don't know if left is a string concat, or a numeric add.
      if (rl.isString()) {
        String leftString = NodeUtil.getStringValue(left);
        String rightString = NodeUtil.getStringValue(rl);
        if (leftString != null && rightString != null) {
          right.removeChild(rr);
          String result = leftString + rightString;
          n.replaceChild(right, rr);
          n.replaceChild(left, IR.string(result));
          reportCodeChange();
          return n;
        }
      }
    }

    return n;
  }

  /**
   * Try to fold an ADD node with constant operands
   */
  private Node tryFoldAddConstantString(Node n, Node left, Node right) {
    if (left.isString() || right.isString()
        || left.isArrayLit() || right.isArrayLit()) {
      // Add strings.
      String leftString = NodeUtil.getStringValue(left);
      String rightString = NodeUtil.getStringValue(right);
      if (leftString != null && rightString != null) {
        Node newStringNode = IR.string(leftString + rightString);
        n.replaceWith(newStringNode);
        reportCodeChange();
        return newStringNode;
      }
    }

    return n;
  }

  /**
   * Try to fold arithmetic binary operators
   */
  private Node tryFoldArithmeticOp(Node n, Node left, Node right) {
    Node result = performArithmeticOp(n.getToken(), left, right);
    if (result != null) {
      result.useSourceInfoIfMissingFromForTree(n);
      n.replaceWith(result);
      reportCodeChange();
      return result;
    }
    return n;
  }

  /**
   * Try to fold arithmetic binary operators
   */
  private Node performArithmeticOp(Token opType, Node left, Node right) {
    // Unlike other operations, ADD operands are not always converted
    // to Number.
    if (opType == Token.ADD
        && (NodeUtil.mayBeString(left, shouldUseTypes)
            || NodeUtil.mayBeString(right, shouldUseTypes))) {
      return null;
    }

    double result;

    // TODO(johnlenz): Handle NaN with unknown value. BIT ops convert NaN
    // to zero so this is a little awkward here.

    Double lValObj = NodeUtil.getNumberValue(left, shouldUseTypes);
    if (lValObj == null) {
      return null;
    }
    Double rValObj = NodeUtil.getNumberValue(right, shouldUseTypes);
    if (rValObj == null) {
      return null;
    }

    double lval = lValObj;
    double rval = rValObj;

    switch (opType) {
      case BITAND:
        result = NodeUtil.toInt32(lval) & NodeUtil.toInt32(rval);
        break;
      case BITOR:
        result = NodeUtil.toInt32(lval) | NodeUtil.toInt32(rval);
        break;
      case BITXOR:
        result = NodeUtil.toInt32(lval) ^ NodeUtil.toInt32(rval);
        break;
      case ADD:
        result = lval + rval;
        break;
      case SUB:
        result = lval - rval;
        break;
      case MUL:
        result = lval * rval;
        break;
      case MOD:
        if (rval == 0) {
          return null;
        }
        result = lval % rval;
        break;
      case DIV:
        if (rval == 0) {
          return null;
        }
        result = lval / rval;
        break;
      default:
        throw new Error("Unexpected arithmetic operator");
    }

    // TODO(johnlenz): consider removing the result length check.
    // length of the left and right value plus 1 byte for the operator.
    if ((String.valueOf(result).length() <=
        String.valueOf(lval).length() + String.valueOf(rval).length() + 1

        // Do not try to fold arithmetic for numbers > 2^53. After that
        // point, fixed-point math starts to break down and become inaccurate.
        && Math.abs(result) <= MAX_FOLD_NUMBER)
        || Double.isNaN(result)
        || result == Double.POSITIVE_INFINITY
        || result == Double.NEGATIVE_INFINITY) {
      return NodeUtil.numberNode(result, null);
    }
    return null;
  }

  /**
   * Expressions such as [foo() * 10 * 20] generate parse trees
   * where no node has two const children ((foo() * 10) * 20), so
   * performArithmeticOp() won't fold it -- tryFoldLeftChildOp() will.
   * Specifically, it folds associative expressions where:
   *  - The left child is also an associative expression of the same time.
   *  - The right child is a constant NUMBER constant.
   *  - The left child's right child is a NUMBER constant.
   */
  private Node tryFoldLeftChildOp(Node n, Node left, Node right) {
    Token opType = n.getToken();
    Preconditions.checkState(
        (NodeUtil.isAssociative(opType) && NodeUtil.isCommutative(opType))
        || n.isAdd());

    Preconditions.checkState(!n.isAdd() || !NodeUtil.mayBeString(n, shouldUseTypes));

    // Use getNumberValue to handle constants like "NaN" and "Infinity"
    // other values are converted to numbers elsewhere.
    Double rightValObj = NodeUtil.getNumberValue(right, shouldUseTypes);
    if (rightValObj != null && left.getToken() == opType) {
      Preconditions.checkState(left.hasTwoChildren());

      Node ll = left.getFirstChild();
      Node lr = ll.getNext();

      Node valueToCombine = ll;
      Node replacement = performArithmeticOp(opType, valueToCombine, right);
      if (replacement == null) {
        valueToCombine = lr;
        replacement = performArithmeticOp(opType, valueToCombine, right);
      }
      if (replacement != null) {
        // Remove the child that has been combined
        left.removeChild(valueToCombine);
        // Replace the left op with the remaining child.
        n.replaceChild(left, left.removeFirstChild());
        // New "-Infinity" node need location info explicitly
        // added.
        replacement.useSourceInfoIfMissingFromForTree(right);
        n.replaceChild(right, replacement);
        reportCodeChange();
      }
    }

    return n;
  }

  private Node tryFoldAdd(Node node, Node left, Node right) {
    Preconditions.checkArgument(node.isAdd());

    if (NodeUtil.mayBeString(node, shouldUseTypes)) {
      if (NodeUtil.isLiteralValue(left, false) &&
          NodeUtil.isLiteralValue(right, false)) {
        // '6' + 7
        return tryFoldAddConstantString(node, left, right);
      } else {
        // a + 7 or 6 + a
        return tryFoldChildAddString(node, left, right);
      }
    } else {
      // Try arithmetic add
      Node result = tryFoldArithmeticOp(node, left, right);
      if (result != node) {
        return result;
      }
      return tryFoldLeftChildOp(node, left, right);
    }
  }

  /**
   * Try to fold shift operations
   */
  private Node tryFoldShift(Node n, Node left, Node right) {
    if (left.isNumber() &&
        right.isNumber()) {

      double result;
      double lval = left.getDouble();
      double rval = right.getDouble();

      // only the lower 5 bits are used when shifting, so don't do anything
      // if the shift amount is outside [0,32)
      if (!(rval >= 0 && rval < 32)) {
        return n;
      }

      int rvalInt = (int) rval;
      if (rvalInt != rval) {
        report(FRACTIONAL_BITWISE_OPERAND, right);
        return n;
      }

      if (Math.floor(lval) != lval) {
        report(FRACTIONAL_BITWISE_OPERAND, left);
        return n;
      }

      int bits = jsConvertDoubleToBits(lval);

      switch (n.getToken()) {
        case LSH:
          result = bits << rvalInt;
          break;
        case RSH:
          result = bits >> rvalInt;
          break;
        case URSH:
          // JavaScript always treats the result of >>> as unsigned.
          // We must force Java to do the same here.
          result = 0xffffffffL & (bits >>> rvalInt);
          break;
        default:
          throw new AssertionError("Unknown shift operator: " + n.getToken());
      }

      Node newNumber = IR.number(result);
      n.replaceWith(newNumber);
      reportCodeChange();

      return newNumber;
    }

    return n;
  }

  /**
   * Try to fold comparison nodes, e.g ==
   */
  private Node tryFoldComparison(Node n, Node left, Node right) {
    TernaryValue result = evaluateComparison(n.getToken(), left, right, shouldUseTypes);
    if (result == TernaryValue.UNKNOWN) {
      return n;
    }

    Node newNode = NodeUtil.booleanNode(result.toBoolean(true));
    n.replaceWith(newNode);
    reportCodeChange();

    return newNode;
  }

  /** http://www.ecma-international.org/ecma-262/6.0/#sec-abstract-relational-comparison */
  private static TernaryValue tryAbstractRelationalComparison(Node left, Node right,
      boolean useTypes, boolean willNegate) {
    // First, try to evaluate based on the general type.
    ValueType leftValueType = NodeUtil.getKnownValueType(left);
    ValueType rightValueType = NodeUtil.getKnownValueType(right);
    if (leftValueType != ValueType.UNDETERMINED && rightValueType != ValueType.UNDETERMINED) {
      if (leftValueType == ValueType.STRING && rightValueType == ValueType.STRING) {
        String lv = NodeUtil.getStringValue(left);
        String rv = NodeUtil.getStringValue(right);
        if (lv != null && rv != null) {
          // In JS, browsers parse \v differently. So do not compare strings if one contains \v.
          if (lv.indexOf('\u000B') != -1 || rv.indexOf('\u000B') != -1) {
            return TernaryValue.UNKNOWN;
          } else {
            return TernaryValue.forBoolean(lv.compareTo(rv) < 0);
          }
        } else if (left.isTypeOf() && right.isTypeOf()
            && left.getFirstChild().isName() && right.getFirstChild().isName()
            && left.getFirstChild().getString().equals(right.getFirstChild().getString())) {
          // Special case: `typeof a < typeof a` is always false.
          return TernaryValue.FALSE;
        }
      }
    }
    // Then, try to evaluate based on the value of the node. Try comparing as numbers.
    Double lv = NodeUtil.getNumberValue(left, useTypes);
    Double rv = NodeUtil.getNumberValue(right, useTypes);
    if (lv == null || rv == null) {
      // Special case: `x < x` is always false.
      //
      // TODO(moz): If we knew the named value wouldn't be NaN, it would be nice to handle
      // LE and GE. We should use type information if available here.
      if (!willNegate && left.isName() && right.isName()) {
        if (left.getString().equals(right.getString())) {
          return TernaryValue.FALSE;
        }
      }
      return TernaryValue.UNKNOWN;
    }
    if (Double.isNaN(lv) || Double.isNaN(rv)) {
      return TernaryValue.forBoolean(willNegate);
    } else {
      return TernaryValue.forBoolean(lv.doubleValue() < rv.doubleValue());
    }
  }

  /** http://www.ecma-international.org/ecma-262/6.0/#sec-abstract-equality-comparison */
  private static TernaryValue tryAbstractEqualityComparison(Node left, Node right,
      boolean useTypes) {
    // Evaluate based on the general type.
    ValueType leftValueType = NodeUtil.getKnownValueType(left);
    ValueType rightValueType = NodeUtil.getKnownValueType(right);
    if (leftValueType != ValueType.UNDETERMINED && rightValueType != ValueType.UNDETERMINED) {
      // Delegate to strict equality comparison for values of the same type.
      if (leftValueType == rightValueType) {
        return tryStrictEqualityComparison(left, right, useTypes);
      }
      if ((leftValueType == ValueType.NULL && rightValueType == ValueType.VOID)
          || (leftValueType == ValueType.VOID && rightValueType == ValueType.NULL)) {
        return TernaryValue.TRUE;
      }
      if ((leftValueType == ValueType.NUMBER && rightValueType == ValueType.STRING)
          || rightValueType == ValueType.BOOLEAN) {
        Double rv = NodeUtil.getNumberValue(right, useTypes);
        return rv == null
            ? TernaryValue.UNKNOWN
            : tryAbstractEqualityComparison(left, IR.number(rv), useTypes);
      }
      if ((leftValueType == ValueType.STRING && rightValueType == ValueType.NUMBER)
          || leftValueType == ValueType.BOOLEAN) {
        Double lv = NodeUtil.getNumberValue(left, useTypes);
        return lv == null
            ? TernaryValue.UNKNOWN
            : tryAbstractEqualityComparison(IR.number(lv), right, useTypes);
      }
      if ((leftValueType == ValueType.STRING || leftValueType == ValueType.NUMBER)
          && rightValueType == ValueType.OBJECT) {
        return TernaryValue.UNKNOWN;
      }
      if (leftValueType == ValueType.OBJECT
          && (rightValueType == ValueType.STRING || rightValueType == ValueType.NUMBER)) {
        return TernaryValue.UNKNOWN;
      }
      return TernaryValue.FALSE;
    }
    // In general, the rest of the cases cannot be folded.
    return TernaryValue.UNKNOWN;
  }

  /** http://www.ecma-international.org/ecma-262/6.0/#sec-strict-equality-comparison */
  private static TernaryValue tryStrictEqualityComparison(Node left, Node right, boolean useTypes) {
    // First, try to evaluate based on the general type.
    ValueType leftValueType = NodeUtil.getKnownValueType(left);
    ValueType rightValueType = NodeUtil.getKnownValueType(right);
    if (leftValueType != ValueType.UNDETERMINED && rightValueType != ValueType.UNDETERMINED) {
      // Strict equality can only be true for values of the same type.
      if (leftValueType != rightValueType) {
        return TernaryValue.FALSE;
      }
      switch (leftValueType) {
        case VOID:
        case NULL:
          return TernaryValue.TRUE;
        case NUMBER: {
          if (NodeUtil.isNaN(left)) {
            return TernaryValue.FALSE;
          }
          if (NodeUtil.isNaN(right)) {
            return TernaryValue.FALSE;
          }
          Double lv = NodeUtil.getNumberValue(left, useTypes);
          Double rv = NodeUtil.getNumberValue(right, useTypes);
          if (lv != null && rv != null) {
            return TernaryValue.forBoolean(lv.doubleValue() == rv.doubleValue());
          }
          break;
        }
        case STRING: {
          String lv = NodeUtil.getStringValue(left);
          String rv = NodeUtil.getStringValue(right);
          if (lv != null && rv != null) {
            // In JS, browsers parse \v differently. So do not consider strings
            // equal if one contains \v.
            if (lv.indexOf('\u000B') != -1 || rv.indexOf('\u000B') != -1) {
              return TernaryValue.UNKNOWN;
            } else {
              return lv.equals(rv) ? TernaryValue.TRUE : TernaryValue.FALSE;
            }
          } else if (left.isTypeOf() && right.isTypeOf()
              && left.getFirstChild().isName() && right.getFirstChild().isName()
              && left.getFirstChild().getString().equals(right.getFirstChild().getString())) {
            // Special case, typeof a == typeof a is always true.
            return TernaryValue.TRUE;
          }
          break;
        }
        case BOOLEAN: {
          TernaryValue lv = NodeUtil.getPureBooleanValue(left);
          TernaryValue rv = NodeUtil.getPureBooleanValue(right);
          return lv.and(rv).or(lv.not().and(rv.not()));
        }
        default: // Symbol and Object cannot be folded in the general case.
          return TernaryValue.UNKNOWN;
      }
    }

    // Then, try to evaluate based on the value of the node. There's only one special case:
    // Any strict equality comparison against NaN returns false.
    if (NodeUtil.isNaN(left) || NodeUtil.isNaN(right)) {
      return TernaryValue.FALSE;
    }
    return TernaryValue.UNKNOWN;
  }

  static TernaryValue evaluateComparison(Token op, Node left, Node right, boolean useTypes) {
    // Don't try to minimize side-effects here.
    if (NodeUtil.mayHaveSideEffects(left) || NodeUtil.mayHaveSideEffects(right)) {
      return TernaryValue.UNKNOWN;
    }

    switch (op) {
      case EQ:
        return tryAbstractEqualityComparison(left, right, useTypes);
      case NE:
        return tryAbstractEqualityComparison(left, right, useTypes).not();
      case SHEQ:
        return tryStrictEqualityComparison(left, right, useTypes);
      case SHNE:
        return tryStrictEqualityComparison(left, right, useTypes).not();
      case LT:
        return tryAbstractRelationalComparison(left, right, useTypes, false);
      case GT:
        return tryAbstractRelationalComparison(right, left, useTypes, false);
      case LE:
        return tryAbstractRelationalComparison(right, left, useTypes, true).not();
      case GE:
        return tryAbstractRelationalComparison(left, right, useTypes, true).not();
      default:
        break;
    }
    throw new IllegalStateException("Unexpected operator for comparison");
  }

  /**
   * Try to fold away unnecessary object instantiation.
   * e.g. this[new String('eval')] -> this.eval
   */
  private Node tryFoldCtorCall(Node n) {
    Preconditions.checkArgument(n.isNew());

    // we can remove this for GETELEM calls (anywhere else?)
    if (inForcedStringContext(n)) {
      return tryFoldInForcedStringContext(n);
    }
    return n;
  }

  /**
   * Remove useless calls:
   *   Object.defineProperties(o, {})  ->  o
   */
  private Node tryFoldCall(Node n) {
    Preconditions.checkArgument(n.isCall());

    if (NodeUtil.isObjectDefinePropertiesDefinition(n)) {
      Node srcObj = n.getLastChild();
      if (srcObj.isObjectLit() && !srcObj.hasChildren()) {
        Node parent = n.getParent();
        Node destObj = n.getSecondChild().detach();
        parent.replaceChild(n, destObj);
        reportCodeChange();
      }
    }
    return n;
  }

  /** Returns whether this node must be coerced to a string. */
  private static boolean inForcedStringContext(Node n) {
    if (n.getParent().isGetElem()
        && n.getParent().getLastChild() == n) {
      return true;
    }

    // we can fold in the case "" + new String("")
    return n.getParent().isAdd();
  }

  private Node tryFoldInForcedStringContext(Node n) {
    // For now, we only know how to fold ctors.
    Preconditions.checkArgument(n.isNew());

    Node objectType = n.getFirstChild();
    if (!objectType.isName()) {
      return n;
    }

    if (objectType.getString().equals("String")) {
      Node value = objectType.getNext();
      String stringValue = null;
      if (value == null) {
        stringValue = "";
      } else {
        if (!NodeUtil.isImmutableValue(value)) {
          return n;
        }

        stringValue = NodeUtil.getStringValue(value);
      }

      if (stringValue == null) {
        return n;
      }

      Node parent = n.getParent();
      Node newString = IR.string(stringValue);

      parent.replaceChild(n, newString);
      newString.useSourceInfoIfMissingFrom(parent);
      reportCodeChange();

      return newString;
    }
    return n;
  }

  /**
   * Try to fold array-element. e.g [1, 2, 3][10];
   */
  private Node tryFoldGetElem(Node n, Node left, Node right) {
    Preconditions.checkArgument(n.isGetElem());

    if (left.isObjectLit()) {
      return tryFoldObjectPropAccess(n, left, right);
    }

    if (left.isArrayLit()) {
      return tryFoldArrayAccess(n, left, right);
    }

    if (left.isString()) {
      return tryFoldStringArrayAccess(n, left, right);
    }
    return n;
  }

  /**
   * Try to fold array-length. e.g [1, 2, 3].length ==> 3, [x, y].length ==> 2
   */
  private Node tryFoldGetProp(Node n, Node left, Node right) {
    Preconditions.checkArgument(n.isGetProp());

    if (left.isObjectLit()) {
      return tryFoldObjectPropAccess(n, left, right);
    }

    if (right.isString() &&
        right.getString().equals("length")) {
      int knownLength = -1;
      switch (left.getToken()) {
        case ARRAYLIT:
          if (mayHaveSideEffects(left)) {
            // Nope, can't fold this, without handling the side-effects.
            return n;
          }
          knownLength = left.getChildCount();
          break;
        case STRING:
          knownLength = left.getString().length();
          break;
        default:
          // Not a foldable case, forget it.
          return n;
      }

      Preconditions.checkState(knownLength != -1);
      Node lengthNode = IR.number(knownLength);
      n.replaceWith(lengthNode);
      reportCodeChange();

      return lengthNode;
    }

    return n;
  }

  private Node tryFoldArrayAccess(Node n, Node left, Node right) {
    // If GETPROP/GETELEM is used as assignment target the array literal is
    // acting as a temporary we can't fold it here:
    //    "[][0] += 1"
    if (NodeUtil.isAssignmentTarget(n)) {
      return n;
    }

    if (!right.isNumber()) {
      // Sometimes people like to use complex expressions to index into
      // arrays, or strings to index into array methods.
      return n;
    }

    double index = right.getDouble();
    int intIndex = (int) index;
    if (intIndex != index) {
      report(INVALID_GETELEM_INDEX_ERROR, right);
      return n;
    }

    if (intIndex < 0) {
      report(INDEX_OUT_OF_BOUNDS_ERROR, right);
      return n;
    }

    Node current = left.getFirstChild();
    Node elem = null;
    for (int i = 0; current != null; i++) {
      if (i != intIndex) {
        if (mayHaveSideEffects(current)) {
          return n;
        }
      } else {
        elem = current;
      }

      current = current.getNext();
    }

    if (elem == null) {
      report(INDEX_OUT_OF_BOUNDS_ERROR, right);
      return n;
    }

    if (elem.isEmpty()) {
      elem = NodeUtil.newUndefinedNode(elem);
    } else {
      left.removeChild(elem);
    }

    // Replace the entire GETELEM with the value
    n.replaceWith(elem);
    reportCodeChange();
    return elem;
  }

  private Node tryFoldStringArrayAccess(Node n, Node left, Node right) {
    // If GETPROP/GETELEM is used as assignment target the array literal is
    // acting as a temporary we can't fold it here:
    //    "[][0] += 1"
    if (NodeUtil.isAssignmentTarget(n)) {
      return n;
    }

    if (!right.isNumber()) {
      // Sometimes people like to use complex expressions to index into
      // arrays, or strings to index into array methods.
      return n;
    }

    double index = right.getDouble();
    int intIndex = (int) index;
    if (intIndex != index) {
      report(INVALID_GETELEM_INDEX_ERROR, right);
      return n;
    }

    if (intIndex < 0) {
      report(INDEX_OUT_OF_BOUNDS_ERROR, right);
      return n;
    }

    Preconditions.checkState(left.isString());
    String value = left.getString();
    if (intIndex >= value.length()) {
      report(INDEX_OUT_OF_BOUNDS_ERROR, right);
      return n;
    }

    char c = 0;
    // Note: For now skip the strings with unicode
    // characters as I don't understand the differences
    // between Java and JavaScript.
    for (int i = 0; i <= intIndex; i++) {
      c = value.charAt(i);
      if (c < 32 || c > 127) {
        return n;
      }
    }
    Node elem = IR.string(Character.toString(c));

    // Replace the entire GETELEM with the value
    n.replaceWith(elem);
    reportCodeChange();
    return elem;
  }

  private Node tryFoldObjectPropAccess(Node n, Node left, Node right) {
    Preconditions.checkArgument(NodeUtil.isGet(n));

    if (!left.isObjectLit() || !right.isString()) {
      return n;
    }

    if (NodeUtil.isAssignmentTarget(n)) {
      // If GETPROP/GETELEM is used as assignment target the object literal is
      // acting as a temporary we can't fold it here:
      //    "{a:x}.a += 1" is not "x += 1"
      return n;
    }

    // find the last definition in the object literal
    Node key = null;
    Node value = null;
    for (Node c = left.getFirstChild(); c != null; c = c.getNext()) {
      if (c.getString().equals(right.getString())) {
        switch (c.getToken()) {
          case SETTER_DEF:
            continue;
          case GETTER_DEF:
          case STRING_KEY:
            if (value != null && mayHaveSideEffects(value)) {
              // The previously found value had side-effects
              return n;
            }
            key = c;
            value = key.getFirstChild();
            break;
          default:
            throw new IllegalStateException();
        }
      } else if (mayHaveSideEffects(c.getFirstChild())) {
        // We don't handle the side-effects here as they might need a temporary
        // or need to be reordered.
        return n;
      }
    }

    // Didn't find a definition of the name in the object literal, it might
    // be coming from the Object prototype
    if (value == null) {
      return n;
    }

    if (value.isFunction() && NodeUtil.referencesThis(value)) {
      // 'this' may refer to the object we are trying to remove
      return n;
    }

    Node replacement = value.detach();
    if (key.isGetterDef()){
      replacement = IR.call(replacement);
      replacement.putBooleanProp(Node.FREE_CALL, true);
    }

    n.replaceWith(replacement);
    reportCodeChange();
    return n;
  }
}