com.google.javascript.jscomp.MinimizeExitPoints Maven / Gradle / Ivy
/*
* 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 static com.google.common.base.Preconditions.checkState;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import com.google.javascript.rhino.jstype.TernaryValue;
import javax.annotation.Nullable;
/**
* Transform the structure of the AST so that the number of explicit exits
* are minimized and instead flows to implicit exits conditions.
*/
class MinimizeExitPoints extends AbstractPeepholeOptimization {
@Override
Node optimizeSubtree(Node n) {
switch (n.getToken()) {
case LABEL:
tryMinimizeExits(
n.getLastChild(), Token.BREAK, n.getFirstChild().getString());
break;
case FOR:
case FOR_IN:
case FOR_OF:
case FOR_AWAIT_OF:
case WHILE:
tryMinimizeExits(NodeUtil.getLoopCodeBlock(n), Token.CONTINUE, null);
break;
case DO:
tryMinimizeExits(NodeUtil.getLoopCodeBlock(n), Token.CONTINUE, null);
Node cond = NodeUtil.getConditionExpression(n);
if (getSideEffectFreeBooleanValue(cond) == TernaryValue.FALSE) {
// Normally, we wouldn't be able to optimize BREAKs inside a loop
// but as we know the condition will always be false, we can treat them
// as we would a CONTINUE.
tryMinimizeExits(n.getFirstChild(), Token.BREAK, null);
}
break;
case BLOCK:
if (n.getParent() != null && n.getParent().isFunction()) {
tryMinimizeExits(n, Token.RETURN, null);
}
break;
case SWITCH:
tryMinimizeSwitchExits(n, Token.BREAK, null);
break;
// TODO(johnlenz): Minimize any block that ends in a optimizable statements:
// break, continue, return
default:
break;
}
return n;
}
/**
* Attempts to minimize the number of explicit exit points in a control
* structure to take advantage of the implied exit at the end of the
* structure. This is accomplished by removing redundant statements, and
* moving statements following a qualifying IF node into that node.
* For example:
*
* function () {
* if (x) return;
* else blah();
* foo();
* }
*
* becomes:
*
* function () {
* if (x) ;
* else {
* blah();
* foo();
* }
*
* @param n The execution node of a parent to inspect.
* @param exitType The type of exit to look for.
* @param labelName If parent is a label the name of the label to look for,
* null otherwise. Non-null only for breaks within labels.
*/
void tryMinimizeExits(Node n, Token exitType, @Nullable String labelName) {
// Just an 'exit'.
if (matchingExitNode(n, exitType, labelName)) {
reportChangeToEnclosingScope(n);
NodeUtil.removeChild(n.getParent(), n);
return;
}
// Just an 'if'.
if (n.isIf()) {
Node ifBlock = n.getSecondChild();
tryMinimizeExits(ifBlock, exitType, labelName);
Node elseBlock = ifBlock.getNext();
if (elseBlock != null) {
tryMinimizeExits(elseBlock, exitType, labelName);
}
return;
}
// Just a 'try/catch/finally'.
if (n.isTry()) {
Node tryBlock = n.getFirstChild();
tryMinimizeExits(tryBlock, exitType, labelName);
Node allCatchNodes = NodeUtil.getCatchBlock(n);
if (NodeUtil.hasCatchHandler(allCatchNodes)) {
checkState(allCatchNodes.hasOneChild());
Node catchNode = allCatchNodes.getFirstChild();
Node catchCodeBlock = catchNode.getLastChild();
tryMinimizeExits(catchCodeBlock, exitType, labelName);
}
/* Don't try to minimize the exits of finally blocks, as this
* can cause problems if it changes the completion type of the finally
* block. See ECMA 262 Sections 8.9 & 12.14
*/
}
// Just a 'label'.
if (n.isLabel()) {
Node labelBlock = n.getLastChild();
tryMinimizeExits(labelBlock, exitType, labelName);
}
// We can only minimize switch cases if we are not trying to remove unlabeled breaks.
if (n.isSwitch() && (exitType != Token.BREAK || labelName != null)) {
tryMinimizeSwitchExits(n, exitType, labelName);
return;
}
// The rest assumes a block with at least one child, bail on anything else.
if (!n.isBlock() || !n.hasChildren()) {
return;
}
// Multiple if-exits can be converted in a single pass.
// Convert "if (blah) break; if (blah2) break; other_stmt;" to
// become "if (blah); else { if (blah2); else { other_stmt; } }"
// which will get converted to "if (!blah && !blah2) { other_stmt; }".
for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
// An 'if' block to process below.
if (c.isIf()) {
Node ifTree = c;
// First, the true condition block.
Node trueBlock = ifTree.getSecondChild();
Node falseBlock = trueBlock.getNext();
tryMinimizeIfBlockExits(trueBlock, falseBlock,
ifTree, exitType, labelName);
// Now the else block.
// The if blocks may have changed, get them again.
trueBlock = ifTree.getSecondChild();
falseBlock = trueBlock.getNext();
if (falseBlock != null) {
tryMinimizeIfBlockExits(falseBlock, trueBlock,
ifTree, exitType, labelName);
}
}
if (c == n.getLastChild()) {
break;
}
}
// Now try to minimize the exits of the last child, if it is removed
// look at what has become the last child.
for (Node c = n.getLastChild(); c != null; c = n.getLastChild()) {
tryMinimizeExits(c, exitType, labelName);
// If the node is still the last child, we are done.
if (c == n.getLastChild()) {
break;
}
}
}
void tryMinimizeSwitchExits(Node n, Token exitType, @Nullable String labelName) {
checkState(n.isSwitch());
// Skipping the switch condition, visit all the children.
for (Node c = n.getSecondChild(); c != null; c = c.getNext()) {
if (c != n.getLastChild()) {
tryMinimizeSwitchCaseExits(c, exitType, labelName);
} else {
// Last case, the last case block can be optimized more aggressively.
tryMinimizeExits(c.getLastChild(), exitType, labelName);
}
}
}
/**
* Attempt to remove explicit exits from switch cases that also occur implicitly
* after the switch.
*/
void tryMinimizeSwitchCaseExits(Node n, Token exitType, @Nullable String labelName) {
checkState(NodeUtil.isSwitchCase(n));
checkState(n != n.getParent().getLastChild());
Node block = n.getLastChild();
Node maybeBreak = block.getLastChild();
if (maybeBreak == null || !maybeBreak.isBreak() || maybeBreak.hasChildren()) {
// Can not minimize exits from a case without an explicit break from the switch.
return;
}
// Now try to minimize the exits of the last child before the break, if it is removed
// look at what has become the child before the break.
Node childBeforeBreak = maybeBreak.getPrevious();
while (childBeforeBreak != null) {
Node c = childBeforeBreak;
tryMinimizeExits(c, exitType, labelName);
// If the node is still the last child, we are done.
childBeforeBreak = maybeBreak.getPrevious();
if (c == childBeforeBreak) {
break;
}
}
}
/**
* Look for exits (returns, breaks, or continues, depending on the context) at
* the end of a block and removes them by moving the if node's siblings,
* if any, into the opposite condition block.
*
* @param srcBlock The block to inspect.
* @param destBlock The block to move sibling nodes into.
* @param ifNode The if node to work with.
* @param exitType The type of exit to look for.
* @param labelName The name associated with the exit, if any. null for anything excepted for
* named-break associated with a label.
*/
private void tryMinimizeIfBlockExits(Node srcBlock, Node destBlock,
Node ifNode, Token exitType, @Nullable String labelName) {
Node exitNodeParent = null;
Node exitNode = null;
// Pick an exit node candidate.
if (srcBlock.isBlock()) {
if (!srcBlock.hasChildren()) {
return;
}
exitNodeParent = srcBlock;
exitNode = exitNodeParent.getLastChild();
} else {
// Just a single statement, if it isn't an exit bail.
exitNodeParent = ifNode;
exitNode = srcBlock;
}
// Verify the candidate.
if (!matchingExitNode(exitNode, exitType, labelName)) {
return;
}
// Ensure no block-scoped declarations are moved into an inner block.
if (!tryConvertAllBlockScopedFollowing(ifNode)) {
return;
}
// Take case of the if nodes siblings, if any.
if (ifNode.getNext() != null) {
// Move siblings of the if block into the opposite
// logic block of the exit.
Node newDestBlock = IR.block().srcref(ifNode);
if (destBlock == null) {
// Only possible if this is the false block.
ifNode.addChildToBack(newDestBlock);
} else if (destBlock.isEmpty()) {
// Use the new block.
ifNode.replaceChild(destBlock, newDestBlock);
} else if (destBlock.isBlock()) {
// Reuse the existing block.
newDestBlock = destBlock;
} else {
// Add the existing statement to the new block.
ifNode.replaceChild(destBlock, newDestBlock);
newDestBlock.addChildToBack(destBlock);
}
// Move all the if node's following siblings.
moveAllFollowing(ifNode, ifNode.getParent(), newDestBlock);
reportChangeToEnclosingScope(ifNode);
}
}
/**
* Determines if n matches the type and name for the following types of
* "exits":
* - return without values
* - continues and breaks with or without names.
* @param n The node to inspect.
* @param type The Token type to look for.
* @param labelName The name that must be associated with the exit type.
* non-null only for breaks associated with labels.
* @return Whether the node matches the specified block-exit type.
*/
private static boolean matchingExitNode(Node n, Token type, @Nullable String labelName) {
if (n.getToken() == type) {
if (type == Token.RETURN) {
// only returns without expressions.
return !n.hasChildren();
} else {
if (labelName == null) {
return !n.hasChildren();
} else {
return n.hasChildren()
&& labelName.equals(n.getFirstChild().getString());
}
}
}
return false;
}
/**
* Move all the child nodes following start in srcParent to the end of
* destParent's child list.
* @param start The start point in the srcParent child list.
* @param srcParent The parent node of start.
* @param destParent The destination node.
*/
private static void moveAllFollowing(
Node start, Node srcParent, Node destParent) {
for (Node n = start.getNext(); n != null; n = start.getNext()) {
boolean isFunctionDeclaration = NodeUtil.isFunctionDeclaration(n);
srcParent.removeChild(n);
if (isFunctionDeclaration) {
destParent.addChildToFront(n);
} else {
destParent.addChildToBack(n);
}
}
}
/**
* Convert all let/const declarations following the start node to var declarations if possible.
*
* See the unit tests for examples of why this is necessary before moving code into an inner
* block, and why this is unsafe to do to declarations inside a loop.
*
* @param start The start point
* @return Whether all block-scoped declarations have been converted.
*/
private static boolean tryConvertAllBlockScopedFollowing(Node start) {
if (NodeUtil.isWithinLoop(start)) {
// If in a loop, don't convert anything to a var. Return true only if there are no let/consts.
return !hasBlockScopedVarsFollowing(start);
}
for (Node n = start.getNext(); n != null; n = n.getNext()) {
if (n.isLet() || n.isConst()) {
n.setToken(Token.VAR);
}
}
return true;
}
/**
* Detect any block-scoped declarations that are younger siblings of the given starting point.
*
* @param start The start point
*/
private static boolean hasBlockScopedVarsFollowing(Node start) {
for (Node n = start.getNext(); n != null; n = n.getNext()) {
if (n.isLet() || n.isConst()) {
return true;
}
}
return false;
}
}