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/*
* Copyright 2008 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.jscomp.ControlFlowGraph.Branch;
import com.google.javascript.jscomp.NodeTraversal.Callback;
import com.google.javascript.jscomp.NodeTraversal.ChangeScopeRootCallback;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;
import com.google.javascript.jscomp.graph.GraphReachability;
import com.google.javascript.rhino.Node;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* Removes dead code from a parse tree. The kinds of dead code that this pass
* removes are:
* - Any code following a return statement, such as the alert
* call in: if (x) { return; alert('unreachable'); }.
* - Statements that have no side effects, such as:
* a.b.MyClass.prototype.propertyName; or true;.
* That first kind of statement sometimes appears intentionally, so that
* prototype properties can be annotated using JSDoc without actually
* being initialized.
*/
// TODO(dimvar): Besides dead code after returns, this pass removes useless live
// code such as breaks/continues/returns and stms w/out side effects.
// These things don't require reachability info, consider making them their own
// pass or putting them in some other, more related pass.
class UnreachableCodeElimination implements CompilerPass {
private static final Logger logger =
Logger.getLogger(UnreachableCodeElimination.class.getName());
private final AbstractCompiler compiler;
private boolean codeChanged;
UnreachableCodeElimination(AbstractCompiler compiler) {
this.compiler = compiler;
}
@Override
public void process(Node externs, Node toplevel) {
checkState(compiler.getLifeCycleStage().isNormalized());
NodeTraversal.traverseChangedFunctions(compiler, new ChangeScopeRootCallback() {
@Override
public void enterChangeScopeRoot(AbstractCompiler compiler, Node root) {
// Computes the control flow graph.
ControlFlowAnalysis cfa =
new ControlFlowAnalysis(compiler, false, false);
cfa.process(null, root);
ControlFlowGraph cfg = cfa.getCfg();
new GraphReachability<>(cfg)
.compute(cfg.getEntry().getValue());
if (root.isFunction()) {
root = root.getLastChild();
}
do {
codeChanged = false;
NodeTraversal.traverse(compiler, root, new EliminationPass(cfg));
} while (codeChanged);
}
});
}
private class EliminationPass implements Callback {
private final ControlFlowGraph cfg;
private EliminationPass(ControlFlowGraph cfg) {
this.cfg = cfg;
}
@Override
public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
if (parent == null) {
return true;
} else if (n.isExport()) {
// TODO(b/129564961): We should be exploring EXPORTs. We don't because their descendants
// have side-effects that `NodeUtil::mayHaveSideEffects` doesn't recognize. Since this pass
// currently runs after exports are removed anyway, this isn't yet an issue.
return false;
} else if (parent.isFunction()) {
// We only want to traverse the name of a function.
return n.isFirstChildOf(parent);
}
return true;
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (parent == null || n.isFunction() || n.isScript()) {
return;
}
DiGraphNode gNode = cfg.getNode(n);
if (gNode == null) { // Not in CFG.
return;
}
if (gNode.getAnnotation() != GraphReachability.REACHABLE
|| !compiler.getAstAnalyzer().mayHaveSideEffects(n)) {
removeDeadExprStatementSafely(n);
return;
}
tryRemoveUnconditionalBranching(n);
}
/**
* Tries to remove n if it is an unconditional branch node (break, continue,
* or return) and the target of n is the same as the follow of n.
* That is, if removing n preserves the control flow. Also if n targets
* another unconditional branch, this function will recursively try to
* remove the target branch as well. The reason why we want to cascade this
* removal is because we only run this pass once. If we have code such as
*
* break -> break -> break
*
* where all 3 breaks are useless, then the order of removal matters. When
* we first look at the first break, we see that it branches to the 2nd
* break. However, if we remove the last break, the 2nd break becomes
* useless and finally the first break becomes useless as well.
*/
@SuppressWarnings("fallthrough")
private void tryRemoveUnconditionalBranching(Node n) {
/*
* For each unconditional branching control flow node, check to see
* if the ControlFlowAnalysis.computeFollowNode of that node is same as
* the branching target. If it is, the branch node is safe to be removed.
*
* This is not as clever as MinimizeExitPoints because it doesn't do any
* if-else conversion but it handles more complicated switch statements
* much more nicely.
*/
// If n is null the target is the end of the function, nothing to do.
if (n == null) {
return;
}
DiGraphNode gNode = cfg.getNode(n);
if (gNode == null) {
return;
}
switch (n.getToken()) {
case RETURN:
if (n.hasChildren()) {
break;
}
case BREAK:
case CONTINUE:
// We are looking for a control flow changing statement that always
// branches to the same node. If after removing it control still
// branches to the same node, it is safe to remove.
List> outEdges = gNode.getOutEdges();
if (outEdges.size() == 1
&&
// If there is a next node, this jump is not useless.
(n.getNext() == null || n.getNext().isFunction())) {
checkState(outEdges.get(0).getValue() == Branch.UNCOND);
Node fallThrough = computeFollowing(n);
Node nextCfgNode = outEdges.get(0).getDestination().getValue();
if (nextCfgNode == fallThrough && !inFinally(n.getParent(), n)) {
removeNode(n);
}
}
break;
default:
break;
}
}
private boolean inFinally(Node parent, Node child) {
if (parent == null || parent.isFunction()) {
return false;
} else if (NodeUtil.isTryFinallyNode(parent, child)) {
return true;
} else {
return inFinally(parent.getParent(), parent);
}
}
private Node computeFollowing(Node n) {
Node next = ControlFlowAnalysis.computeFollowNode(n);
while (next != null && next.isBlock()) {
if (next.hasChildren()) {
next = next.getFirstChild();
} else {
next = computeFollowing(next);
}
}
return next;
}
private void removeDeadExprStatementSafely(Node n) {
Node parent = n.getParent();
if (n.isEmpty() || (n.isBlock() && !n.hasChildren())) {
// Not always trivial to remove, let FoldConstants work its magic later.
return;
}
// Every expression in a FOR-IN or FOR-OF header looks side effect free on its own.
if (NodeUtil.isEnhancedFor(parent)) {
return;
}
switch (n.getToken()) {
// In the CFG, the only incoming edges of the DO node are from
// breaks/continues and the condition. The edge from the previous
// statement connects directly to the body of the DO.
//
// Removing an unreachable DO node is messy b/c it means we still have
// to execute one iteration of the body. If the DO's body has breaks in
// the middle, it can get even more tricky and code size might actually
// increase.
case DO:
case EXPORT:
return;
case BLOCK:
// BLOCKs are used in several ways including wrapping CATCH
// blocks in TRYs
if (parent.isTry() && NodeUtil.isTryCatchNodeContainer(n)) {
return;
}
break;
case CATCH:
Node tryNode = parent.getParent();
NodeUtil.maybeAddFinally(tryNode);
break;
default:
break;
}
if (n.isVar() && !n.getFirstChild().hasChildren()) {
// Very unlikely case, Consider this:
// File 1: {throw 1}
// File 2: {var x}
// The node var x is unreachable in the global scope.
// Before we remove the node, redeclareVarsInsideBranch
// would basically move var x to the beginning of File 2,
// which resulted in zero changes to the AST but triggered
// reportCodeChange().
// Instead, we should just ignore dead variable declarations.
return;
}
removeNode(n);
}
private void removeNode(Node n) {
codeChanged = true;
NodeUtil.redeclareVarsInsideBranch(n);
compiler.reportChangeToEnclosingScope(n);
if (logger.isLoggable(Level.FINE)) {
logger.fine("Removing " + n);
}
NodeUtil.removeChild(n.getParent(), n);
NodeUtil.markFunctionsDeleted(n, compiler);
}
}
}
