com.google.javascript.jscomp.UnreachableCodeElimination Maven / Gradle / Ivy
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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.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.ArrayDeque;
import java.util.Deque;
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 {@code alert} call in:
* {@code if (x) { return; alert('unreachable'); }}.
* - Statements that have no side effects, such as:
* {@code a.b.MyClass.prototype.propertyName;} or {@code 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.traverse(compiler, compiler.getJsRoot(), new EliminationInChangedFunctionsPass());
}
private final class EliminationInChangedFunctionsPass
extends NodeTraversal.AbstractChangedScopeCallback {
@Override
public void enterChangedScopeRoot(AbstractCompiler compiler, Node root) {
// Computes the control flow graph.
ControlFlowGraph cfg =
ControlFlowAnalysis.builder().setCompiler(compiler).setCfgRoot(root).computeCfg();
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 NodeTraversal.Callback {
private final ControlFlowGraph cfg;
/**
* Keep track of nodes that contain a sequence of statements.
*
* As soon as we find one statement is unreachable, we can skip traversing the rest.
*/
private final Deque statementSequenceParentContextStack =
new ArrayDeque<>();
private EliminationPass(ControlFlowGraph cfg) {
this.cfg = cfg;
}
@Override
public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
if (n.isExport()) {
// TODO(b/129564961): We should be exploring EXPORTs. We don't because their descendants
// have side-effects that `AstAnalyzer.mayHaveSideEffects` doesn't recognize. Since this
// pass currently runs after exports are removed anyway, this isn't yet an issue.
return false;
} else if (n.isFunction()) {
// Do not descend into function scopes, because they won't be included in our
// current CFG.
return false;
}
StatementSequenceParentContext statementSequenceParentContext =
statementSequenceParentContextStack.peek();
if (statementSequenceParentContext != null
&& statementSequenceParentContext.statementParentNode == parent) {
// We're looking at a statement node in the current statement parent
if (statementSequenceParentContext.firstUnreachableStatementNode != null) {
// A previous statement is unreachable, so there's no point looking at this one.
return false;
}
if (isDefinitelyUnreachable(n)) {
statementSequenceParentContext.firstUnreachableStatementNode = n;
return false;
}
}
if (isStatementSequenceParent(n)) {
statementSequenceParentContextStack.push(new StatementSequenceParentContext(n));
}
return true;
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
StatementSequenceParentContext statementSequenceParentContext =
statementSequenceParentContextStack.peek();
if (statementSequenceParentContext != null
&& statementSequenceParentContext.statementParentNode == n) {
// We're now visiting the statement parent, itself.
statementSequenceParentContextStack.pop();
Node unreachableStatementNode =
statementSequenceParentContext.firstUnreachableStatementNode;
while (unreachableStatementNode != null) {
final Node nextStatement = unreachableStatementNode.getNext();
removeStatementNode(unreachableStatementNode);
unreachableStatementNode = nextStatement;
}
return;
}
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);
}
private boolean isDefinitelyUnreachable(Node n) {
DiGraphNode gNode = getCfgNodeForStatement(n);
if (gNode == null) {
// Not in CFG.
// We may have traversed into a scope not covered by the CFG,
// or maybe just looking at a node the CFG doesn't consider part of the control flow.
return false;
}
return gNode.getAnnotation() != GraphReachability.REACHABLE;
}
private DiGraphNode getCfgNodeForStatement(Node statement) {
switch (statement.getToken()) {
case DO:
// CFG flows first into the statement within the do {} while ();
// So we should consider that CFG node to represent the whole statement.
return cfg.getNode(statement.getFirstChild());
case LABEL:
// A LABEL is never actually executed, so get what it labels.
// We use recursion because it is possible to label a label.
return getCfgNodeForStatement(statement.getLastChild());
default:
return cfg.getNode(statement);
}
}
/**
* 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)) {
logicallyRemoveNode(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;
}
logicallyRemoveNode(n);
}
/**
* Logically, put possibly not actually, remove a node.
*
* This method uses {@code NodeUtil.removeChild()} which has a lot of logic to handle
* attempts to remove nodes that are structurally required by the AST. It will make a change
* that has the behavior of the node being removed, even though what actually is done to the AST
* may not be simple removal of the node.
*/
private void logicallyRemoveNode(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);
}
}
/**
* Remove a statement that is part of a sequence of statements.
*
*
Unlike {@code logicallyRemoveNode()}, this method will always remove the node.
*/
private void removeStatementNode(Node statementNode) {
codeChanged = true;
NodeUtil.redeclareVarsInsideBranch(statementNode);
compiler.reportChangeToEnclosingScope(statementNode);
if (logger.isLoggable(Level.FINE)) {
logger.fine("Removing " + statementNode);
}
// Since we know we have a statement within a statement sequence here, simply detaching it is
// always safe.
statementNode.detach();
NodeUtil.markFunctionsDeleted(statementNode, compiler);
}
/** Is {@code n} a {@code Node} that has a sequence of statements as its children? */
private static boolean isStatementSequenceParent(Node n) {
// A LABEL is a statement parent, but only for a single statement.
// For historical reasons, the second child of a TRY is a BLOCK with a single CATCH child.
// We don't want to treat the CATCH as if it were a statement.
return NodeUtil.isStatementParent(n) && !n.isLabel() && !NodeUtil.isTryCatchNodeContainer(n);
}
/** One of these is created for each node whose children are a sequence of statements. */
private static class StatementSequenceParentContext {
final Node statementParentNode;
/** Set non-null only if we discover that some statements are unreachable. */
Node firstUnreachableStatementNode = null;
public StatementSequenceParentContext(Node statementParentNode) {
this.statementParentNode = statementParentNode;
}
}
}