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/*
* Copyright 2006 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;
/**
* Tries to chain assignments together.
*
*/
class ExploitAssigns extends AbstractPeepholeOptimization {
@Override
Node optimizeSubtree(Node subtree) {
for (Node child = subtree.getFirstChild(); child != null;) {
Node next = child.getNext();
if (NodeUtil.isExprAssign(child)) {
collapseAssign(child.getFirstChild(), child, subtree);
}
child = next;
}
return subtree;
}
/** Try to collapse the given assign into subsequent expressions. */
private void collapseAssign(Node assign, Node expr, Node exprParent) {
Node leftValue = assign.getFirstChild();
Node rightValue = leftValue.getNext();
if (leftValue.isDestructuringPattern()) {
// We don't collapse expressions containing these because they can have side effects:
// - Reassigning RHS names. (e.g. `() => { ({c: a} = a); return a; }`).
// - Calling a getter defined on the RHS.
// - Evaluating a default value.
// TODO(b/123102446): Check for these issues and optimize when they aren't present.
return;
} else if (isCollapsibleValue(leftValue, true)
&& collapseAssignEqualTo(expr, exprParent, leftValue)) {
// Condition has side-effects.
} else if (isCollapsibleValue(rightValue, false)
&& collapseAssignEqualTo(expr, exprParent, rightValue)) {
// Condition has side-effects.
} else if (rightValue.isAssign()) {
// Recursively deal with nested assigns.
collapseAssign(rightValue, expr, exprParent);
}
}
/**
* Determines whether we know enough about the given value to be able to collapse it into
* subsequent expressions.
*
* For example, we can collapse booleans and variable names:
*
*
*
* x = 3; y = x; // y = x = 3;
* a = true; b = true; // b = a = true;
*
*
*
* But we won't try to collapse complex expressions.
*
* @param value The value node.
* @param isLValue Whether it's on the left-hand side of an expr.
*/
private static boolean isCollapsibleValue(Node value, boolean isLValue) {
switch (value.getToken()) {
case GETPROP:
case OPTCHAIN_GETPROP:
// Do not collapse GETPROPs on arbitrary objects, because
// they may be implemented setter functions, and oftentimes
// setter functions fail on native objects. This is OK for "THIS"
// objects, because we assume that they are non-native.
return !isLValue || value.getFirstChild().isThis();
case NAME:
return true;
default:
return NodeUtil.isImmutableValue(value);
}
}
/**
* Collapse the given assign expression into the expression directly following it, if possible.
*
* @param expr The expression that may be moved.
* @param exprParent The parent of {@code expr}.
* @param value The value of this expression, expressed as a node. Each expression may have
* multiple values, so this function may be called multiple times for the same expression. For
* example,
* a = true;
* is equal to the name "a" and the boolean "true".
* @return Whether the expression was collapsed successfully.
*/
private boolean collapseAssignEqualTo(Node expr, Node exprParent, Node value) {
Node assign = expr.getFirstChild();
Node parent = exprParent;
Node next = expr.getNext();
while (next != null) {
switch (next.getToken()) {
case AND:
case OR:
case HOOK:
case IF:
case RETURN:
case EXPR_RESULT:
case COALESCE:
// Dive down the left side
parent = next;
next = next.getFirstChild();
break;
case CONST:
case LET:
case VAR:
if (next.getFirstChild().hasChildren()) {
parent = next.getFirstChild();
next = parent.getFirstChild();
break;
}
return false;
// `OPTCHAIN_GETPROP` cannot be matched with a value seen before on the RHS of an assign
// as it is not a qualified name. Consequently `b = a.x; a = a.x` optimizes to `a = b =
// a.x` but `b = a?.x; a = a?.x` does not.
case OPTCHAIN_GETPROP:
return false;
case GETPROP:
case NAME:
if (next.isQualifiedName()) {
if (value.isQualifiedName() &&
next.matchesQualifiedName(value)) {
// If the previous expression evaluates to value of a
// qualified name, and that qualified name is used again
// shortly, then we can exploit the assign here.
// Verify the assignment doesn't change its own value.
if (!isSafeReplacement(next, assign)) {
return false;
}
expr.detach();
assign.detach();
next.replaceWith(assign);
reportChangeToEnclosingScope(parent);
return true;
}
}
return false;
case ASSIGN:
// Assigns are really tricky. In lots of cases, we want to inline
// into the right side of the assign. But the left side of the
// assign is evaluated first, and it may have convoluted logic:
// a = null;
// (a = b).c = null;
// We don't want to exploit the first assign. Similarly:
// a.b = null;
// a.b.c = null;
// We don't want to exploit the first assign either.
//
// To protect against this, we simply only inline when the left side
// is guaranteed to evaluate to the same L-value no matter what.
Node leftSide = next.getFirstChild();
if (leftSide.isName() || (leftSide.isGetProp() && leftSide.getFirstChild().isThis())) {
// Dive down the right side of the assign.
parent = next;
next = leftSide.getNext();
break;
} else {
return false;
}
default:
if (NodeUtil.isImmutableValue(next)
&& next.isEquivalentTo(value)) {
// If the r-value of the expr assign is an immutable value,
// and the value is used again shortly, then we can exploit
// the assign here.
expr.detach();
assign.detach();
next.replaceWith(assign);
reportChangeToEnclosingScope(parent);
return true;
}
// Return without inlining a thing
return false;
}
}
return false;
}
/**
* Checks name referenced in node to determine if it might have
* changed.
* @return Whether the replacement can be made.
*/
private boolean isSafeReplacement(Node node, Node replacement) {
// No checks are needed for simple names.
if (node.isName()) {
return true;
}
checkArgument(node.isGetProp());
while (node.isGetProp()) {
node = node.getFirstChild();
}
return !(node.isName() && isNameAssignedTo(node.getString(), replacement));
}
/**
* @return Whether name is assigned in the expression rooted at node.
*/
private static boolean isNameAssignedTo(String name, Node node) {
for (Node c = node.getFirstChild(); c != null; c = c.getNext()) {
if (isNameAssignedTo(name, c)) {
return true;
}
}
if (node.isName()) {
Node parent = node.getParent();
if (parent.isAssign() && parent.getFirstChild() == node) {
if (name.equals(node.getString())) {
return true;
}
}
}
return false;
}
}