Review

13

com/google/javascript/jscomp/PeepholeRemoveDeadCode.java

trySimpilifyUnusedResult(Node, boolean)

class PeepholeRemoveDeadCode {
  /**
   * General cascading unused operation node removal.
   * @param n The root of the expression to simplify.
   * @param removeUnused If true, the node is removed from the AST if
   *     it is not useful, otherwise it replaced with an EMPTY node.
   * @return The replacement node, or null if the node was is not useful.
   */
  private Node trySimpilifyUnusedResult(Node n, boolean removeUnused) {
    Node result = n;

    // Simplify the results of conditional expressions
    switch (n.getType()) {
      case Token.HOOK:
        Node trueNode = trySimpilifyUnusedResult(n.getFirstChild().getNext());
        Node falseNode = trySimpilifyUnusedResult(n.getLastChild());
        // If one or more of the conditional children were removed,
        // transform the HOOK to an equivalent operation:
        //    x() ? foo() : 1 --> x() && foo()
        //    x() ? 1 : foo() --> x() || foo()
        //    x() ? 1 : 1 --> x()
        //    x ? 1 : 1 --> null
        if (trueNode == null && falseNode != null) {
          n.setType(Token.OR);
          Preconditions.checkState(n.getChildCount() == 2);
        } else if (trueNode != null && falseNode == null) {
          n.setType(Token.AND);
          Preconditions.checkState(n.getChildCount() == 2);
        } else if (trueNode == null && falseNode == null) {
          result = trySimpilifyUnusedResult(n.getFirstChild());
        } else {
          // The structure didn't change.
          result = n;
        }
        break;
      case Token.AND:
      case Token.OR:
        // Try to remove the second operand from a AND or OR operations:
        //    x() || f --> x()
        //    x() && f --> x()
        Node conditionalResultNode = trySimpilifyUnusedResult(
            n.getLastChild());
        if (conditionalResultNode == null) {
          Preconditions.checkState(n.hasOneChild());
          // The conditionally executed code was removed, so
          // replace the AND/OR with its LHS or remove it if it isn't useful.
          result = trySimpilifyUnusedResult(n.getFirstChild());
        }
        break;
      case Token.FUNCTION:
        // A function expression isn't useful if it isn't used, remove it and
        // don't bother to look at its children.
        result = null;
        break;
      case Token.COMMA:
        // We rewrite other operations as COMMA expressions (which will later
        // get split into individual EXPR_RESULT statement, if possible), so
        // we special case COMMA (we don't want to rewrite COMMAs as new COMMAs
        // nodes.
        Node left = trySimpilifyUnusedResult(n.getFirstChild());
        Node right = trySimpilifyUnusedResult(n.getLastChild());
        if (left == null && right == null) {
          result = null;
        } else if (left == null) {
          result = right;
        } else if (right == null){
          result = left;
        } else {
          // The structure didn't change.
          result = n;
        }
        break;
      default:
        if (!NodeUtil.nodeTypeMayHaveSideEffects(n)) {
          // This is the meat of this function. The node itself doesn't generate
          // any side-effects but preserve any side-effects in the children.
          Node resultList = null;
          for (Node next, c = n.getFirstChild(); c != null; c = next) {
            next = c.getNext();
            c = trySimpilifyUnusedResult(c);
            if (c != null) {
              c.detachFromParent();
              if (resultList == null)  {
                // The first side-effect can be used stand-alone.
                resultList = c;
              } else {
                // Leave the side-effects in-place, simplifying it to a COMMA
                // expression.
                resultList = new Node(Token.COMMA, resultList, c)
                    .copyInformationFrom(c);
              }
            }
          }
          result = resultList;
        }
    }

    // Fix up the AST, replace or remove the an unused node (if requested).
    if (n != result) {
      Node parent = n.getParent();
      if (result == null) {
        if (removeUnused) {
          parent.removeChild(n);
        } else {
          result = new Node(Token.EMPTY).copyInformationFrom(n);
          parent.replaceChild(n, result);
        }
      } else {
        // A new COMMA expression may not have an existing parent.
        if (result.getParent() != null) {
          result.detachFromParent();
        }
        n.getParent().replaceChild(n, result);
      }
      reportCodeChange();
    }

    return result;
  }

}