com.google.javascript.jscomp.DeadPropertyAssignmentElimination Maven / Gradle / Ivy
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/*
* Copyright 2016 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.Iterators;
import com.google.common.collect.PeekingIterator;
import com.google.common.collect.Sets;
import com.google.javascript.jscomp.NodeTraversal.Callback;
import com.google.javascript.jscomp.NodeTraversal.ChangeScopeRootCallback;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Objects;
import java.util.Queue;
import java.util.Set;
import javax.annotation.Nullable;
/**
* An optimization pass that finds and removes dead property assignments within functions and
* classes.
*
* This pass does not currently use the control-flow graph. It makes the following assumptions:
*
* - Functions with inner functions are not processed.
* - All properties are read whenever entering a block node. Dead assignments within a block
* are processed.
* - Hook nodes are not processed (it's assumed they read everything)
* - Switch blocks are not processed (it's assumed they read everything)
* - Any reference to a property getter/setter is treated like a call that escapes all props.
* - If there's an Object.definePropert{y,ies} call where the object or property name is aliased
* then the optimization does not run at all.
* - Properties names defined in externs will not be pruned.
*
*/
public class DeadPropertyAssignmentElimination implements CompilerPass {
private final AbstractCompiler compiler;
// TODO(kevinoconnor): Try to give special treatment to constructor, else remove this field
// and cleanup dead code.
@VisibleForTesting
static final boolean ASSUME_CONSTRUCTORS_HAVENT_ESCAPED = false;
DeadPropertyAssignmentElimination(AbstractCompiler compiler) {
this.compiler = compiler;
}
@Override
public void process(Node externs, Node root) {
// GatherExternProperties must be enabled for this pass to safely know what property writes are
// eligible for removal.
if (compiler.getExternProperties() == null || compiler.getAccessorSummary() == null) {
return;
}
Set blacklistedPropNames =
Sets.union(
compiler.getAccessorSummary().getAccessors().keySet(), compiler.getExternProperties());
NodeTraversal.traverseChangedFunctions(compiler, new FunctionVisitor(blacklistedPropNames));
}
private static class FunctionVisitor implements ChangeScopeRootCallback {
/**
* A set of properties names that are potentially unsafe to remove duplicate writes to.
*/
private final Set blacklistedPropNames;
FunctionVisitor(Set blacklistedPropNames) {
this.blacklistedPropNames = blacklistedPropNames;
}
@Override
public void enterChangeScopeRoot(AbstractCompiler compiler, Node root) {
if (!root.isFunction()) {
return;
}
Node body = NodeUtil.getFunctionBody(root);
if (!body.hasChildren() || NodeUtil.containsFunction(body)) {
return;
}
FindCandidateAssignmentTraversal traversal =
new FindCandidateAssignmentTraversal(blacklistedPropNames, NodeUtil.isConstructor(root));
NodeTraversal.traverse(compiler, body, traversal);
// Any candidate property assignment can have a write removed if that write is never read
// and it's written to at least one more time.
for (Property property : traversal.propertyMap.values()) {
if (property.writes.size() <= 1) {
continue;
}
PeekingIterator iter = Iterators.peekingIterator(property.writes.iterator());
while (iter.hasNext()) {
PropertyWrite propertyWrite = iter.next();
if (iter.hasNext() && propertyWrite.isSafeToRemove(iter.peek())) {
Node lhs = propertyWrite.assignedAt;
Node rhs = lhs.getNext();
Node assignNode = lhs.getParent();
if (assignNode.isAssign()) {
// replace "a.b.c = " with ""
rhs.detach();
assignNode.replaceWith(rhs);
compiler.reportChangeToEnclosingScope(rhs);
} else {
checkState(NodeUtil.isAssignmentOp(assignNode));
// replace "a.b.c += " with "a.b.c + expr"
Token opType = NodeUtil.getOpFromAssignmentOp(assignNode);
assignNode.setToken(opType);
compiler.reportChangeToEnclosingScope(assignNode);
}
}
}
}
}
}
private static class Property {
private final String name;
// This pass doesn't use a control-flow graph; this field contains a rough approximation
// of the control flow. For writes in the same block, they appear in this list in
// program-execution order.
// All writes in a list are to the same property name, but the full qualified names may
// differ, eg, a.b.c and e.d.c can be in the list. Consecutive writes to the same qname
// may mean that the first write can be removed (see isSafeToRemove).
private final Deque writes = new ArrayDeque<>();
private final Set children = new HashSet<>();
Property(String name) {
this.name = name;
}
void markLastWriteRead() {
if (!writes.isEmpty()) {
writes.getLast().markRead();
}
}
/**
* Marks all children of this property as read.
*/
void markChildrenRead() {
// If a property is in propertiesSet, it has been added to the queue and processed,
// it will not be added to the queue again.
Set propertiesSet = new HashSet<>(children);
Queue propertyQueue = new ArrayDeque<>(propertiesSet);
// Ensure we don't process ourselves.
propertiesSet.add(this);
while (!propertyQueue.isEmpty()) {
Property childProperty = propertyQueue.remove();
childProperty.markLastWriteRead();
for (Property grandchildProperty : childProperty.children) {
if (propertiesSet.add(grandchildProperty)) {
propertyQueue.add(grandchildProperty);
}
}
}
}
void addWrite(Node lhs) {
checkArgument(lhs.isQualifiedName());
writes.addLast(new PropertyWrite(lhs));
}
@Override
public String toString() {
return "Property " + name;
}
}
private static class PropertyWrite {
private final Node assignedAt;
private boolean isRead = false;
private final String qualifiedName;
PropertyWrite(Node assignedAt) {
checkArgument(assignedAt.isQualifiedName());
this.assignedAt = assignedAt;
this.qualifiedName = assignedAt.getQualifiedName();
}
boolean isSafeToRemove(@Nullable PropertyWrite nextWrite) {
return !isRead && nextWrite != null && Objects.equals(qualifiedName, nextWrite.qualifiedName);
}
void markRead() {
isRead = true;
}
boolean isChildPropOf(String lesserPropertyQName) {
return qualifiedName != null && qualifiedName.startsWith(lesserPropertyQName + ".");
}
}
/**
* A NodeTraversal that operates within a function block and collects candidate properties
* assignments.
*/
private static class FindCandidateAssignmentTraversal implements Callback {
/**
* A map of property names to their nodes.
*
* Note: the references {@code a.b} and {@code c.b} will assume that it's the same b,
* because a and c may be aliased, and we don't track aliasing.
*/
Map propertyMap = new HashMap<>();
/**
* A set of properties names that are potentially unsafe to remove duplicate writes to.
*/
private final Set blacklistedPropNames;
/**
* Whether or not the function being analyzed is a constructor.
*/
private final boolean isConstructor;
FindCandidateAssignmentTraversal(Set blacklistedPropNames, boolean isConstructor) {
this.blacklistedPropNames = blacklistedPropNames;
this.isConstructor = isConstructor;
}
/**
* Gets a {@link Property} given the node that references it; the {@link Property} is created
* if it does not already exist.
*
* @return A {@link Property}, or null if the provided node is not a qualified name.
*/
private Property getOrCreateProperty(Node propNode) {
if (!propNode.isQualifiedName()) {
return null;
}
String propName =
propNode.isGetProp() ? propNode.getLastChild().getString() : propNode.getQualifiedName();
Property property = propertyMap.computeIfAbsent(propName, name -> new Property(name));
/* Using the GETPROP chain, build out the tree of children properties.
For example, from a.b.c and a.c we can build:
a
/ \
b c
/
c
Note: c is the same Property in this tree.
*/
if (propNode.isGetProp()) {
Property parentProperty = getOrCreateProperty(propNode.getFirstChild());
if (parentProperty != null) {
parentProperty.children.add(property);
}
}
return property;
}
@Override
public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
return visitNode(n, parent);
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
// Visit the LHS of an assignment in post-order
if (NodeUtil.isAssignmentOp(n)) {
visitAssignmentLhs(n.getFirstChild());
}
// Assume that all properties may be read when control flow leaves the function
if (NodeUtil.isInvocation(n) || n.isYield() || n.isAwait()) {
if (ASSUME_CONSTRUCTORS_HAVENT_ESCAPED
&& isConstructor
&& !NodeUtil.referencesThis(n)
&& NodeUtil.getEnclosingType(n, Token.TRY) == null) {
// this.x properties are okay.
markAllPropsReadExceptThisProps();
} else {
markAllPropsRead();
}
}
// Mark all properties as read when leaving a block since we haven't proven that the block
// will execute.
if (n.isBlock()) {
visitBlock(n);
}
}
private void visitBlock(Node blockNode) {
checkArgument(blockNode.isBlock());
// We don't do flow analysis yet so we're going to assume everything written up to this
// block is read.
if (blockNode.hasChildren()) {
markAllPropsRead();
}
}
private static boolean isConditionalExpression(Node n) {
switch (n.getToken()) {
case AND:
case OR:
case HOOK:
return true;
default:
return false;
}
}
private void visitAssignmentLhs(Node lhs) {
Property property = getOrCreateProperty(lhs);
if (property == null) {
return;
}
if (!lhs.isGetProp()) {
property.markLastWriteRead();
property.markChildrenRead();
return;
}
Node assignNode = lhs.getParent();
// If it's mutating assignment (+=, *=, etc.) then mark the last assignment read first.
if (!assignNode.isAssign()) {
property.markLastWriteRead();
}
// Reassignment of a qualified name prefix might change what child properties are referenced
// later on, so consider children properties as read.
// Ex. a.b.c = 10; a.b = other; a.b.c = 20;
property.markChildrenRead();
property.addWrite(lhs);
// Now we need to go up the prop chain and mark those as read.
Node child = lhs.getFirstChild();
while (child != null) {
Property childProperty = getOrCreateProperty(child);
if (childProperty == null) {
break;
}
childProperty.markLastWriteRead();
child = child.getFirstChild();
}
}
private boolean visitNode(Node n, Node parent) {
switch (n.getToken()) {
case GETPROP:
// Handle potential getters/setters.
if (n.isGetProp()
&& blacklistedPropNames.contains(n.getLastChild().getString())) {
// We treat getters/setters as if they were a call, thus we mark all properties as read.
markAllPropsRead();
return true;
}
if (NodeUtil.isAssignmentOp(parent) && parent.getFirstChild() == n) {
// We always visit the LHS assignment in post-order
return false;
}
Property property = getOrCreateProperty(n);
if (property != null) {
// Mark all children properties as read.
property.markLastWriteRead();
// Only mark children properties as read if we're at at the end of the referenced
// property chain.
// Ex. A read of "a.b.c" should mark a, a.b, a.b.c, and a.b.c.* as read, but not a.d
if (!parent.isGetProp()) {
property.markChildrenRead();
}
}
return true;
case THIS:
case NAME:
Property nameProp = checkNotNull(getOrCreateProperty(n));
nameProp.markLastWriteRead();
if (!parent.isGetProp()) {
nameProp.markChildrenRead();
}
return true;
case THROW:
case FOR:
case FOR_IN:
case SWITCH:
// TODO(kevinoconnor): Switch/for statements need special consideration since they may
// execute out of order.
markAllPropsRead();
return false;
case BLOCK:
visitBlock(n);
return true;
default:
if (isConditionalExpression(n)) {
markAllPropsRead();
return false;
}
return true;
}
}
private void markAllPropsRead() {
markAllPropsReadHelper(false /* excludeThisProps*/);
}
private void markAllPropsReadExceptThisProps() {
markAllPropsReadHelper(true /* excludeThisProps */);
}
private void markAllPropsReadHelper(boolean excludeThisProps) {
for (Property property : propertyMap.values()) {
if (property.writes.isEmpty()) {
continue;
}
if (excludeThisProps && property.writes.getLast().isChildPropOf("this")) {
continue;
}
property.markLastWriteRead();
}
}
}
}