/*
* Copyright 2009 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.collect.HashMultimap;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Multimap;
import com.google.javascript.jscomp.NodeTraversal.Callback;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.Collection;
import java.util.List;
/**
* Reduces the size of common function expressions.
*
* This pass will rewrite:
*
* C.prototype.getA = function() { return this.a_ };
* C.prototype.setA = function(newValue) { this.a_ = newValue };
*
* as:
*
* C.prototype.getA = JSCompiler_get("a");
* C.prototype.setA = JSCompiler_set("a");
*
* if by doing so we will save bytes, after the helper functions are
* added and renaming is done.
*
* NOTE: JSCompiler_get and JSCompiler_set turn dotted accesses to
* computed accesses, which causes JS engines to use dictionary lookups.
* Because of this perf regression, this pass is off by default in advanced
* mode even though it improves code size.
*
*/
class FunctionRewriter implements CompilerPass {
private final AbstractCompiler compiler;
// Safety margin used to avoid growing simple programs by a few bytes.
// Selected arbitrarily.
private static final int SAVINGS_THRESHOLD = 16;
FunctionRewriter(AbstractCompiler compiler) {
this.compiler = compiler;
}
@Override
public void process(Node externs, Node root) {
List<Reducer> reducers = ImmutableList.of(new ReturnConstantReducer(),
new GetterReducer(),
new SetterReducer(),
new EmptyFunctionReducer(),
new IdentityReducer());
Multimap<Reducer, Reduction> reductionMap = HashMultimap.create();
// Accumulate possible reductions in the reduction multi-map. They
// will be applied in the loop below.
NodeTraversal.traverseEs6(compiler, root,
new ReductionGatherer(reducers, reductionMap));
// Apply reductions iff they will provide some savings.
for (Reducer reducer : reducers) {
Collection<Reduction> reductions = reductionMap.get(reducer);
if (reductions.isEmpty()) {
continue;
}
Node helperCode = parseHelperCode(reducer);
if (helperCode == null) {
continue;
}
int helperCodeCost = InlineCostEstimator.getCost(helperCode);
// Estimate savings
int savings = 0;
for (Reduction reduction : reductions) {
savings += reduction.estimateSavings();
}
// Compare estimated savings against the helper cost. Apply
// reductions if doing so will result in some savings.
if (savings > (helperCodeCost + SAVINGS_THRESHOLD)) {
for (Reduction reduction : reductions) {
reduction.apply();
}
Node addingRoot = compiler.getNodeForCodeInsertion(null);
NodeUtil.markNewScopesChanged(helperCode, compiler);
addingRoot.addChildToFront(helperCode);
compiler.reportChangeToEnclosingScope(addingRoot);
}
}
}
/**
* Parse helper code needed by a reducer.
*
* @return Helper code root. If parse fails, return null.
*/
public Node parseHelperCode(Reducer reducer) {
Node root =
compiler.parseSyntheticCode(reducer.getClass() + ":helper", reducer.getHelperSource());
return (root != null) ? root.removeFirstChild() : null;
}
private static boolean isReduceableFunctionExpression(Node n) {
return NodeUtil.isFunctionExpression(n)
&& !NodeUtil.isGetOrSetKey(n.getParent());
}
/**
* Information needed to apply a reduction.
*/
private class Reduction {
private final Node parent;
private final Node oldChild;
private final Node newChild;
Reduction(Node parent, Node oldChild, Node newChild) {
this.parent = parent;
this.oldChild = oldChild;
this.newChild = newChild;
}
/**
* Apply the reduction by replacing the old child with the new child.
*/
void apply() {
parent.replaceChild(oldChild, newChild);
compiler.reportChangeToEnclosingScope(newChild);
}
/**
* Estimate number of bytes saved by applying this reduction.
*/
int estimateSavings() {
return InlineCostEstimator.getCost(oldChild) -
InlineCostEstimator.getCost(newChild);
}
}
/**
* Gathers a list of reductions to apply later by doing an in-order
* AST traversal. If a suitable reduction is found, stop traversal
* in that branch.
*/
private class ReductionGatherer implements Callback {
private final List<Reducer> reducers;
private final Multimap<Reducer, Reduction> reductions;
/**
* @param reducers List of reducers to apply during traversal.
* @param reductions Reducer -> Reduction multimap,
* populated during traversal.
*/
ReductionGatherer(List<Reducer> reducers,
Multimap<Reducer, Reduction> reductions) {
this.reducers = reducers;
this.reductions = reductions;
}
@Override
public boolean shouldTraverse(NodeTraversal raversal,
Node node,
Node parent) {
for (Reducer reducer : reducers) {
Node replacement = reducer.reduce(node);
if (replacement != node) {
reductions.put(reducer, new Reduction(parent, node, replacement));
return false;
}
}
return true;
}
@Override
public void visit(NodeTraversal traversal, Node node, Node parent) {
}
}
/**
* Interface implemented by the strength-reduction optimizers below.
*/
abstract static class Reducer {
/**
* @return JS source for helper methods used by this reduction.
*/
abstract String getHelperSource();
/**
* @return root of the reduced subtree if a reduction was applied;
* otherwise returns the node argument.
*/
abstract Node reduce(Node node);
/**
* Builds a method call based on the the given method name,
* argument and history.
*
* @param methodName Method to call.
* @param argumentNode Method argument.
*/
protected final Node buildCallNode(String methodName, Node argumentNode,
Node srcref) {
Node call = IR.call(IR.name(methodName)).srcref(srcref);
call.putBooleanProp(Node.FREE_CALL, true);
if (argumentNode != null) {
call.addChildToBack(argumentNode.cloneTree());
}
return call;
}
}
/**
* Reduces return immutable constant literal methods declarations
* with calls to a constant return method factory.
*
* Example:
* a.prototype.b = function() {}
* is reduced to:
* a.prototype.b = emptyFn();
*/
private static class EmptyFunctionReducer extends Reducer {
static final String FACTORY_METHOD_NAME = "JSCompiler_emptyFn";
static final String HELPER_SOURCE =
"function " + FACTORY_METHOD_NAME + "() {" +
" return function() {}" +
"}";
@Override
public String getHelperSource() {
return HELPER_SOURCE;
}
@Override
public Node reduce(Node node) {
if (NodeUtil.isEmptyFunctionExpression(node)) {
return buildCallNode(FACTORY_METHOD_NAME, null, node);
} else {
return node;
}
}
}
/**
* Base class for reducers that match functions that contain a
* single return statement.
*/
abstract static class SingleReturnStatementReducer extends Reducer {
/**
* @return function return value node if function body contains a
* single return statement. Otherwise, null.
*/
protected final Node maybeGetSingleReturnRValue(Node functionNode) {
Node body = functionNode.getLastChild();
if (!body.hasOneChild()) {
return null;
}
Node statement = body.getFirstChild();
if (statement.isReturn()) {
return statement.getFirstChild();
}
return null;
}
}
/**
* Reduces property getter method declarations with calls to a
* getter method factory.
*
* Example:
* a.prototype.b = function(a) {return a}
* is reduced to:
* a.prototype.b = getter(a);
*/
private static class IdentityReducer extends SingleReturnStatementReducer {
static final String FACTORY_METHOD_NAME = "JSCompiler_identityFn";
static final String HELPER_SOURCE =
"function " + FACTORY_METHOD_NAME + "() {" +
" return function(" + FACTORY_METHOD_NAME + "_value) {" +
"return " + FACTORY_METHOD_NAME + "_value}" +
"}";
@Override
public String getHelperSource() {
return HELPER_SOURCE;
}
@Override
public Node reduce(Node node) {
if (!isReduceableFunctionExpression(node)) {
return node;
}
if (isIdentityFunction(node)) {
return buildCallNode(FACTORY_METHOD_NAME, null, node);
} else {
return node;
}
}
/**
* Checks if the function matches the pattern:
* function(<value>, <rest>) {return <value>}
*
* @return Whether the function matches the pattern.
*/
private boolean isIdentityFunction(Node functionNode) {
Node argList = functionNode.getSecondChild();
Node paramNode = argList.getFirstChild();
if (paramNode == null) {
return false;
}
Node value = maybeGetSingleReturnRValue(functionNode);
return value != null && value.isName() && value.getString().equals(paramNode.getString());
}
}
/**
* Reduces return immutable constant literal methods declarations
* with calls to a constant return method factory.
*
* Example:
* a.prototype.b = function() {return 10}
* is reduced to:
* a.prototype.b = returnconst(10);
*/
private static class ReturnConstantReducer
extends SingleReturnStatementReducer {
static final String FACTORY_METHOD_NAME = "JSCompiler_returnArg";
static final String HELPER_SOURCE =
"function " + FACTORY_METHOD_NAME +
"(" + FACTORY_METHOD_NAME + "_value) {" +
" return function() {return " + FACTORY_METHOD_NAME + "_value}" +
"}";
@Override
public String getHelperSource() {
return HELPER_SOURCE;
}
@Override
public Node reduce(Node node) {
if (!isReduceableFunctionExpression(node)) {
return node;
}
Node valueNode = getValueNode(node);
if (valueNode != null) {
return buildCallNode(FACTORY_METHOD_NAME, valueNode, node);
} else {
return node;
}
}
/**
* Checks if the function matches the pattern:
* function(<args>) {return <immutable value>}
* and returns <immutable value> if a match is found.
*
* @return the immutable value node; or null.
*/
private Node getValueNode(Node functionNode) {
Node value = maybeGetSingleReturnRValue(functionNode);
if (value != null &&
NodeUtil.isImmutableValue(value)) {
return value;
}
return null;
}
}
/**
* Reduces property getter method declarations with calls to a
* getter method factory.
*
* Example:
* a.prototype.b = function() {return this.b_}
* is reduced to:
* a.prototype.b = getter("b_");
*/
private static class GetterReducer extends SingleReturnStatementReducer {
static final String FACTORY_METHOD_NAME = "JSCompiler_get";
static final String HELPER_SOURCE =
"function " + FACTORY_METHOD_NAME + "(" +
FACTORY_METHOD_NAME + "_name) {" +
" return function() {return this[" + FACTORY_METHOD_NAME + "_name]}" +
"}";
@Override
public String getHelperSource() {
return HELPER_SOURCE;
}
@Override
public Node reduce(Node node) {
if (!isReduceableFunctionExpression(node)) {
return node;
}
Node propName = getGetPropertyName(node);
if (propName != null) {
if (!propName.isString()) {
throw new IllegalStateException("Expected STRING, got " + propName.getToken());
}
return buildCallNode(FACTORY_METHOD_NAME, propName, node);
} else {
return node;
}
}
/**
* Checks if the function matches the pattern:
* function(<args>) {return this.<name>}
* and returns <name> if a match is found.
*
* @return STRING node that is the RHS of a this property get; or null.
*/
private Node getGetPropertyName(Node functionNode) {
Node value = maybeGetSingleReturnRValue(functionNode);
if (value != null &&
value.isGetProp() &&
value.getFirstChild().isThis()) {
return value.getLastChild();
}
return null;
}
}
/**
* Reduces property setter method declarations with calls to a
* setter method factory.
*
* Example:
* a.prototype.setB = function(value) {this.b_ = value}
* reduces to:
* a.prototype.setB = getter("b_");
*/
private static class SetterReducer extends Reducer {
static final String FACTORY_METHOD_NAME = "JSCompiler_set";
static final String HELPER_SOURCE =
"function " + FACTORY_METHOD_NAME + "(" +
FACTORY_METHOD_NAME + "_name) {" +
" return function(" + FACTORY_METHOD_NAME + "_value) {" +
"this[" + FACTORY_METHOD_NAME + "_name] = " +
FACTORY_METHOD_NAME + "_value}" +
"}";
@Override
public String getHelperSource() {
return HELPER_SOURCE;
}
@Override
public Node reduce(Node node) {
if (!isReduceableFunctionExpression(node)) {
return node;
}
Node propName = getSetPropertyName(node);
if (propName != null) {
if (!propName.isString()) {
throw new IllegalStateException("Expected STRING, got " + propName.getToken());
}
return buildCallNode(FACTORY_METHOD_NAME, propName, node);
} else {
return node;
}
}
/**
* Checks if the function matches the pattern:
* function(<value>, <rest>) {this.<name> = <value>}
* and returns <name> if a match is found.
*
* @return STRING node that is the RHS of a this property get; or null.
*/
private Node getSetPropertyName(Node functionNode) {
Node body = functionNode.getLastChild();
if (!body.hasOneChild()) {
return null;
}
Node argList = functionNode.getSecondChild();
Node paramNode = argList.getFirstChild();
if (paramNode == null) {
return null;
}
Node statement = body.getFirstChild();
if (!NodeUtil.isExprAssign(statement)) {
return null;
}
Node assign = statement.getFirstChild();
Node lhs = assign.getFirstChild();
if (lhs.isGetProp() && lhs.getFirstChild().isThis()) {
Node rhs = assign.getLastChild();
if (rhs.isName() &&
rhs.getString().equals(paramNode.getString())) {
Node propertyName = lhs.getLastChild();
return propertyName;
}
}
return null;
}
}
}