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Closure Compiler is a JavaScript optimizing compiler. It parses your
JavaScript, analyzes it, removes dead code and rewrites and minimizes
what's left. It also checks syntax, variable references, and types, and
warns about common JavaScript pitfalls. It is used in many of Google's
JavaScript apps, including Gmail, Google Web Search, Google Maps, and
Google Docs.
The newest version!
/*
* 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.checkArgument;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
/**
* Tries to fuse all the statements in a block into a one statement by using
* COMMAs.
*
* Because COMMAs has the lowest precedence, we never need to insert
* extra () around. Once we have only one statement in a block, we can then
* eliminate a pair of {}'s. Further more, we can also fold a single
* statement IF into && or create further opportunities for all the other
* goodies in {@link PeepholeMinimizeConditions}.
*
*/
class StatementFusion extends AbstractPeepholeOptimization {
// TODO(user): We probably need to test this more. The current compiler
// assumes that there are more ;'s than ,'s in a real program. However,
// this assumption may be incorrect. We can probably do a quick traverse
// to check this assumption if that's necessary.
public static final boolean SHOULD_FAVOR_COMMA_OVER_SEMI_COLON = false;
private final boolean favorsCommaOverSemiColon;
public StatementFusion() {
this(SHOULD_FAVOR_COMMA_OVER_SEMI_COLON);
}
public StatementFusion(boolean favorsCommaOverSemiColon) {
this.favorsCommaOverSemiColon = favorsCommaOverSemiColon;
}
@Override
Node optimizeSubtree(Node n) {
// TODO(user): It is much cleaner to have two algorithms depending
// on favorsCommaOverSemiColon. If we decided the less aggressive one is
// no longer useful, delete it.
if (favorsCommaOverSemiColon) {
return tryFuseStatementsAggressively(n);
} else {
return tryFuseStatements(n);
}
}
Node tryFuseStatements(Node n) {
if (!n.getParent().isFunction() && canFuseIntoOneStatement(n)) {
Node start = n.getFirstChild();
Node end = n.getLastChild();
Node result = fuseIntoOneStatement(n, start, end);
fuseExpressionIntoControlFlowStatement(result, n.getLastChild());
reportChangeToEnclosingScope(n);
}
return n;
}
Node tryFuseStatementsAggressively(Node n) {
if (!NodeUtil.isStatementBlock(n)) {
return n;
}
Node cur = n.getFirstChild();
while (cur != null) {
if (!cur.isExprResult()) {
cur = cur.getNext();
continue;
}
Node next = cur.getNext();
while (next != null && next.isExprResult()) {
next = next.getNext();
}
if (cur.getNext() != next) {
cur = fuseIntoOneStatement(n, cur, next);
reportChangeToEnclosingScope(cur);
}
if (cur.isExprResult() &&
next != null && isFusableControlStatement(next)) {
fuseExpressionIntoControlFlowStatement(cur, next);
reportChangeToEnclosingScope(next);
next = next.getNext();
}
cur = next;
}
return n;
}
private boolean canFuseIntoOneStatement(Node block) {
// If we are favoring semi-colon, we shouldn't fuse script blocks.
if (!favorsCommaOverSemiColon && !block.isBlock()) {
return false;
}
// Nothing to do here.
if (!block.hasChildren() || block.hasOneChild()) {
return false;
}
Node last = block.getLastChild();
for (Node c = block.getFirstChild(); c != null; c = c.getNext()) {
if (!c.isExprResult() && c != last) {
return false;
}
}
return isFusableControlStatement(last);
}
private boolean isFusableControlStatement(Node n) {
switch (n.getToken()) {
case IF:
case THROW:
case SWITCH:
case EXPR_RESULT:
return true;
case RETURN:
// We don't want to add a new return value.
return n.hasChildren();
case FOR:
// Avoid cases where we have for(var x;_;_) { ....
return !NodeUtil.isNameDeclaration(n.getFirstChild());
case FOR_IN:
// Avoid cases where we have for(var x = foo() in a) { ....
return !mayHaveSideEffects(n.getFirstChild());
case LABEL:
return isFusableControlStatement(n.getLastChild());
case BLOCK:
return (isASTNormalized() || NodeUtil.canMergeBlock(n))
&& !n.isSyntheticBlock()
&& isFusableControlStatement(n.getFirstChild());
default:
break;
}
return false;
}
/**
* Given a block, fuse a list of statements with comma's.
*
* @param parent The parent that contains the statements.
* @param first The first statement to fuse (inclusive)
* @param last The last statement to fuse (exclusive)
* @return A single statement that contains all the fused statement as one.
*/
private static Node fuseIntoOneStatement(Node parent, Node first, Node last) {
// Nothing to fuse if there is only one statement.
if (first.getNext() == last) {
return first;
}
// Step one: Create a comma tree that contains all the statements.
Node commaTree = first.removeFirstChild();
Node next = null;
for (Node cur = first.getNext(); cur != last; cur = next) {
commaTree = fuseExpressionIntoExpression(
commaTree, cur.removeFirstChild());
next = cur.getNext();
parent.removeChild(cur);
}
// Step two: The last EXPR_RESULT will now hold the comma tree with all
// the fused statements.
first.addChildToBack(commaTree);
return first;
}
private static void fuseExpressionIntoControlFlowStatement(
Node before, Node control) {
checkArgument(before.isExprResult(), "before must be expression result");
// Now we are just left with two statements. The comma tree of the first
// n - 1 statements (which can be used in an expression) and the last
// statement. We perform specific fusion based on the last statement's type.
switch (control.getToken()) {
case IF:
case RETURN:
case THROW:
case SWITCH:
case EXPR_RESULT:
case FOR:
before.getParent().removeChild(before);
fuseExpressionIntoFirstChild(before.removeFirstChild(), control);
return;
case FOR_IN:
before.getParent().removeChild(before);
fuseExpressionIntoSecondChild(before.removeFirstChild(), control);
return;
case LABEL:
fuseExpressionIntoControlFlowStatement(before, control.getLastChild());
return;
case BLOCK:
fuseExpressionIntoControlFlowStatement(before, control.getFirstChild());
return;
default:
throw new IllegalStateException("Statement fusion missing.");
}
}
// exp1, exp1
static Node fuseExpressionIntoExpression(Node exp1, Node exp2) {
if (exp2.isEmpty()) {
return exp1;
}
Node comma = new Node(Token.COMMA, exp1);
comma.useSourceInfoIfMissingFrom(exp2);
// We can just join the new comma expression with another comma but
// lets keep all the comma's in a straight line. That way we can use
// tree comparison.
if (exp2.isComma()) {
Node leftMostChild = exp2;
while (leftMostChild.isComma()) {
leftMostChild = leftMostChild.getFirstChild();
}
Node parent = leftMostChild.getParent();
comma.addChildToBack(leftMostChild.detach());
parent.addChildToFront(comma);
return exp2;
} else {
comma.addChildToBack(exp2);
return comma;
}
}
protected static void fuseExpressionIntoFirstChild(Node exp, Node stmt) {
Node val = stmt.removeFirstChild();
Node comma = fuseExpressionIntoExpression(exp, val);
stmt.addChildToFront(comma);
}
protected static void fuseExpressionIntoSecondChild(Node exp, Node stmt) {
Node val = stmt.getSecondChild().detach();
Node comma = fuseExpressionIntoExpression(exp, val);
stmt.addChildAfter(comma, stmt.getFirstChild());
}
}