PeepholeCollectPropertyAssignments.java
/*
* Copyright 2011 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 static com.google.common.base.Preconditions.checkState;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
/**
* A pass that looks for assignments to properties of an object or array
* immediately following its creation using the abbreviated syntax.
* <p>
* E.g. {@code var a = [];a[0] = 0} is optimized to {@code var a = [0]} and
* similarly for the object constructor.
*
* @author msamuel@google.com (Mike Samuel)
*/
final class PeepholeCollectPropertyAssignments extends AbstractPeepholeOptimization {
@Override
Node optimizeSubtree(Node subtree) {
if (!subtree.isScript() && !subtree.isNormalBlock()) {
return subtree;
}
boolean codeChanged = false;
// Look for variable declarations or simple assignments
// and start processing there.
for (Node child = subtree.getFirstChild();
child != null; child = child.getNext()) {
if (!child.isVar() && !NodeUtil.isExprAssign(child)) {
continue;
}
if (!isPropertyAssignmentToName(child.getNext())) {
// Quick check to see if there's anything to collapse.
continue;
}
checkState(child.hasOneChild());
Node name = getName(child);
if (!name.isName()) {
// The assignment target is not a simple name.
continue;
}
Node value = getValue(child);
if (value == null || !isInterestingValue(value)) {
// No initializer or not an Object or Array literal.
continue;
}
Node propertyCandidate;
while ((propertyCandidate = child.getNext()) != null) {
// This does not infinitely loop because collectProperty always
// removes propertyCandidate from its parent when it returns true.
if (!collectProperty(propertyCandidate, name.getString(), value)) {
break;
}
codeChanged = true;
}
}
if (codeChanged) {
compiler.reportChangeToEnclosingScope(subtree);
}
return subtree;
}
private static Node getName(Node n) {
if (n.isVar()) {
return n.getFirstChild();
} else if (NodeUtil.isExprAssign(n)) {
return n.getFirstFirstChild();
}
throw new IllegalStateException();
}
private static Node getValue(Node n) {
if (n.isVar()) {
return n.getFirstFirstChild();
} else if (NodeUtil.isExprAssign(n)) {
return n.getFirstChild().getLastChild();
}
throw new IllegalStateException();
}
static boolean isInterestingValue(Node n) {
return n.isObjectLit() || n.isArrayLit();
}
private static boolean isPropertyAssignmentToName(Node propertyCandidate) {
if (propertyCandidate == null) { return false; }
// Must be an assignment...
if (!NodeUtil.isExprAssign(propertyCandidate)) {
return false;
}
Node expr = propertyCandidate.getFirstChild();
// to a property...
Node lhs = expr.getFirstChild();
if (!NodeUtil.isGet(lhs)) {
return false;
}
// of a variable.
Node obj = lhs.getFirstChild();
return obj.isName();
}
private boolean collectProperty(
Node propertyCandidate, String name, Node value) {
if (!isPropertyAssignmentToName(propertyCandidate)) {
return false;
}
Node lhs = propertyCandidate.getFirstFirstChild();
// Must be an assignment to the recent variable...
if (!name.equals(lhs.getFirstChild().getString())) {
return false;
}
Node rhs = lhs.getNext();
// with a value that cannot change the values of the variables,
if (mayHaveSideEffects(rhs)
|| NodeUtil.canBeSideEffected(rhs)) {
return false;
}
// and does not have a reference to a variable initialized after it.
if (!NodeUtil.isLiteralValue(rhs, true)
&& mightContainForwardReference(rhs, name)) {
return false;
}
switch (value.getToken()) {
case ARRAYLIT:
if (!collectArrayProperty(value, propertyCandidate)) {
return false;
}
break;
case OBJECTLIT:
if (!collectObjectProperty(value, propertyCandidate)) {
return false;
}
break;
default:
throw new IllegalStateException();
}
return true;
}
private static boolean collectArrayProperty(
Node arrayLiteral, Node propertyCandidate) {
Node assignment = propertyCandidate.getFirstChild();
final int sizeOfArrayAtStart = arrayLiteral.getChildCount();
int maxIndexAssigned = sizeOfArrayAtStart - 1;
Node lhs = assignment.getFirstChild();
Node rhs = lhs.getNext();
if (!lhs.isGetElem()) {
return false;
}
Node obj = lhs.getFirstChild();
Node property = obj.getNext();
// The left hand side must have a numeric index
if (!property.isNumber()) {
return false;
}
// that is a valid array index
double dindex = property.getDouble();
if (!(dindex >= 0) // Handles NaN and negatives.
|| Double.isInfinite(dindex) || dindex > 0x7fffffffL) {
return false;
}
int index = (int) dindex;
if (dindex != index) {
return false;
}
// that would not make the array so sparse that they take more space
// when rendered than x[9]=1.
if (maxIndexAssigned + 4 < index) {
return false;
}
if (index > maxIndexAssigned) {
while (maxIndexAssigned < index - 1) {
// Pad the array if it is sparse.
// So if array is [0] and integer 3 is assigned at index is 2, then
// we want to produce [0,,2].
Node emptyNode = IR.empty().srcref(arrayLiteral);
arrayLiteral.addChildToBack(emptyNode);
++maxIndexAssigned;
}
arrayLiteral.addChildToBack(rhs.detach());
} else {
// An out of order assignment. Allow it if it's a hole.
Node currentValue = arrayLiteral.getChildAtIndex(index);
if (!currentValue.isEmpty()) {
// We've already collected a value for this index.
return false;
}
arrayLiteral.replaceChild(currentValue, rhs.detach());
}
propertyCandidate.detach();
return true;
}
private static boolean collectObjectProperty(
Node objectLiteral, Node propertyCandidate) {
Node assignment = propertyCandidate.getFirstChild();
Node lhs = assignment.getFirstChild();
Node rhs = lhs.getNext();
Node obj = lhs.getFirstChild();
Node property = obj.getNext();
// The property must be statically known.
if (lhs.isGetElem()
&& (!property.isString()
&& !property.isNumber())) {
return false;
}
String propertyName;
if (property.isNumber()) {
propertyName = NodeUtil.getStringValue(property);
} else {
propertyName = property.getString();
}
// Check if the new property already exists in the object literal
// Note: Duplicate keys are invalid in strict mode
Node existingProperty = null;
for (Node currentProperty : objectLiteral.children()) {
// Get the name of the current property
String currentPropertyName = currentProperty.getString();
// Get the value of the property
Node currentValue = currentProperty.getFirstChild();
// Compare the current property name with the new property name
if (currentPropertyName.equals(propertyName)) {
existingProperty = currentProperty;
// Check if the current value and the new value are side-effect
boolean isCurrentValueSideEffect = NodeUtil.canBeSideEffected(currentValue);
boolean isNewValueSideEffect = NodeUtil.canBeSideEffected(rhs);
// If they are side-effect free then replace the current value with the new one
if (isCurrentValueSideEffect || isNewValueSideEffect) {
return false;
}
// Break the loop if the property exists
break;
}
}
Node newProperty = IR.stringKey(propertyName)
.useSourceInfoIfMissingFrom(property);
// Preserve the quotedness of a property reference
if (lhs.isGetElem()) {
newProperty.setQuotedString();
}
Node newValue = rhs.detach();
newProperty.addChildToBack(newValue);
if (existingProperty != null) {
objectLiteral.removeChild(existingProperty);
}
// If the property does not already exist we can safely add it
objectLiteral.addChildToBack(newProperty);
propertyCandidate.detach();
return true;
}
private static boolean mightContainForwardReference(
Node node, String varName) {
if (node.isName()) {
return varName.equals(node.getString());
}
for (Node child = node.getFirstChild(); child != null;
child = child.getNext()) {
if (mightContainForwardReference(child, varName)) {
return true;
}
}
return false;
}
}