/*
* Copyright 2008 Google Inc.
*
* 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.gwt.dev.jjs.impl;
import com.google.gwt.dev.jjs.ast.Context;
import com.google.gwt.dev.jjs.ast.JBinaryOperation;
import com.google.gwt.dev.jjs.ast.JBinaryOperator;
import com.google.gwt.dev.jjs.ast.JClassType;
import com.google.gwt.dev.jjs.ast.JExpression;
import com.google.gwt.dev.jjs.ast.JMethod;
import com.google.gwt.dev.jjs.ast.JMethodCall;
import com.google.gwt.dev.jjs.ast.JModVisitor;
import com.google.gwt.dev.jjs.ast.JProgram;
import com.google.gwt.dev.jjs.ast.JReferenceType;
import com.google.gwt.dev.jjs.ast.JType;
/**
* <p>
* Rewrite Java <code>==</code> so that it will execute correctly in JavaScript.
* After this pass, Java's <code>==</code> is considered equivalent to
* JavaScript's <code>===</code>.
* </p>
* <p>
* Whenever possible, a Java <code>==</code> is replaced by a JavaScript
* <code>==</code>. This is shorter than <code>===</code>, and it avoids any
* complication due to GWT treating both <code>null</code> and
* <code>undefined</code> as a valid translation of a Java <code>null</code>.
* </p>
* <p>
* However, whenever something that may be a String is compared to something
* that may not be a <code>String</code>, use <code>===</code>. A Java object
* compared to a string should always yield false, but that's not true when
* comparing in JavaScript using <code>==</code>. The cases where
* <code>===</code> must be used are:
* </p>
* <ul>
* <li>One or both sides have unknown <code>String</code> status.</li>
* <li>One side is definitely <code>String</code> and one side is definitely !
* <code>String</code>. <br/>
* TODO: This case could be optimized as
* <code>(a == null) & (b == null)</code>.</li>
* </ul>
* <p>
* Since <code>null !== undefined</code>, it is also necessary to normalize
* <code>null</code> vs. <code>undefined</code> if it's possible for one side to
* be <code>null</code> and the other to be <code>undefined</code>.
* </p>
*/
public class EqualityNormalizer {
/**
* Breaks apart certain complex assignments.
*/
private class BreakupAssignOpsVisitor extends JModVisitor {
@Override
public void endVisit(JBinaryOperation x, Context ctx) {
JBinaryOperator op = x.getOp();
if (op != JBinaryOperator.EQ && op != JBinaryOperator.NEQ) {
return;
}
JExpression lhs = x.getLhs();
JExpression rhs = x.getRhs();
JType lhsType = lhs.getType();
JType rhsType = rhs.getType();
if (!(lhsType instanceof JReferenceType)) {
assert !(rhsType instanceof JReferenceType);
return;
}
StringStatus lhsStatus = getStringStatus((JReferenceType) lhsType);
StringStatus rhsStatus = getStringStatus((JReferenceType) rhsType);
int strat = COMPARISON_STRAT[lhsStatus.getIndex()][rhsStatus.getIndex()];
switch (strat) {
case STRAT_TRIPLE: {
if (canBeNull(lhs) && canBeNull(rhs)) {
/*
* If it's possible for one side to be null and the other side
* undefined, then mask both sides.
*/
lhs = maskUndefined(lhs);
rhs = maskUndefined(rhs);
}
JBinaryOperation binOp =
new JBinaryOperation(x.getSourceInfo(), x.getType(), x.getOp(), lhs, rhs);
ctx.replaceMe(binOp);
break;
}
case STRAT_DOUBLE: {
boolean lhsNullLit = lhs == program.getLiteralNull();
boolean rhsNullLit = rhs == program.getLiteralNull();
if ((lhsNullLit && rhsStatus == StringStatus.NOTSTRING)
|| (rhsNullLit && lhsStatus == StringStatus.NOTSTRING)) {
/*
* If either side is a null literal and the other is non-String,
* replace with a null-check.
*/
String methodName;
if (op == JBinaryOperator.EQ) {
methodName = "Cast.isNull";
} else {
methodName = "Cast.isNotNull";
}
JMethod isNullMethod = program.getIndexedMethod(methodName);
JMethodCall call = new JMethodCall(x.getSourceInfo(), null, isNullMethod);
call.addArg(lhsNullLit ? rhs : lhs);
ctx.replaceMe(call);
} else {
// Replace with a call to Cast.jsEquals, which does a == internally.
String methodName;
if (op == JBinaryOperator.EQ) {
methodName = "Cast.jsEquals";
} else {
methodName = "Cast.jsNotEquals";
}
JMethod eqMethod = program.getIndexedMethod(methodName);
JMethodCall call = new JMethodCall(x.getSourceInfo(), null, eqMethod);
call.addArgs(lhs, rhs);
ctx.replaceMe(call);
}
break;
}
}
}
private StringStatus getStringStatus(JReferenceType type) {
JClassType stringType = program.getTypeJavaLangString();
if (type == program.getTypeNull()) {
return StringStatus.NULL;
} else if (program.typeOracle.canTriviallyCast(type, stringType)) {
return StringStatus.STRING;
} else if (program.typeOracle.canTheoreticallyCast(type, stringType)) {
return StringStatus.UNKNOWN;
} else {
return StringStatus.NOTSTRING;
}
}
private JExpression maskUndefined(JExpression lhs) {
assert ((JReferenceType) lhs.getType()).canBeNull();
JMethod maskMethod = program.getIndexedMethod("Cast.maskUndefined");
JMethodCall lhsCall = new JMethodCall(lhs.getSourceInfo(), null, maskMethod, lhs.getType());
lhsCall.addArg(lhs);
return lhsCall;
}
}
/**
* Represents what we know about an operand type in terms of its type and
* <code>null</code> status.
*/
private enum StringStatus {
NOTSTRING(2), NULL(3), STRING(1), UNKNOWN(0);
private int index;
StringStatus(int index) {
this.index = index;
}
public int getIndex() {
return index;
}
}
/**
* A map of the combinations where each comparison strategy should be used.
*/
private static int[][] COMPARISON_STRAT = {
// ..U..S.!S..N
{1, 1, 1, 0,}, // UNKNOWN
{1, 0, 1, 0,}, // STRING
{1, 1, 0, 0,}, // NOTSTRING
{0, 0, 0, 0,}, // NULL
};
/**
* The comparison strategy of using ==.
*/
private static final int STRAT_DOUBLE = 0;
/**
* The comparison strategy of using ===.
*/
private static final int STRAT_TRIPLE = 1;
public static void exec(JProgram program) {
new EqualityNormalizer(program).execImpl();
}
private static boolean canBeNull(JExpression x) {
return ((JReferenceType) x.getType()).canBeNull();
}
private final JProgram program;
private EqualityNormalizer(JProgram program) {
this.program = program;
}
private void execImpl() {
BreakupAssignOpsVisitor breaker = new BreakupAssignOpsVisitor();
breaker.accept(program);
}
}