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Closure Compiler is a JavaScript optimizing compiler. It parses your
JavaScript, analyzes it, removes dead code and rewrites and minimizes
what's left. It also checks syntax, variable references, and types, and
warns about common JavaScript pitfalls. It is used in many of Google's
JavaScript apps, including Gmail, Google Web Search, Google Maps, and
Google Docs.
/*
* 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import com.google.javascript.jscomp.ControlFlowGraph.Branch;
import com.google.javascript.jscomp.DataFlowAnalysis.LinearFlowState;
import com.google.javascript.jscomp.LiveVariablesAnalysis.LiveVariableLattice;
import com.google.javascript.jscomp.NodeUtil.AllVarsDeclaredInFunction;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Map;
/**
* Removes local variable assignments that are useless based on information from {@link
* LiveVariablesAnalysis}. If there is an assignment to variable {@code x} and {@code x} is dead
* after this assignment, we know that the current content of {@code x} will not be read and this
* assignment is useless.
*/
class DeadAssignmentsElimination extends NodeTraversal.AbstractCfgCallback implements CompilerPass {
private final AbstractCompiler compiler;
private LiveVariablesAnalysis liveness;
private final Deque functionStack;
private static final class BailoutInformation {
boolean containsFunction;
boolean containsRemovableAssign;
}
public DeadAssignmentsElimination(AbstractCompiler compiler) {
this.compiler = compiler;
this.functionStack = new ArrayDeque<>();
}
@Override
public void process(Node externs, Node root) {
checkNotNull(externs);
checkNotNull(root);
checkState(compiler.getLifeCycleStage().isNormalized());
NodeTraversal.traverse(compiler, root, this);
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (functionStack.isEmpty()) {
return;
}
if (n.isFunction()) {
functionStack.peekFirst().containsFunction = true;
} else if (isRemovableAssign(n)) {
functionStack.peekFirst().containsRemovableAssign = true;
}
}
@Override
public void enterScopeWithCfg(NodeTraversal t) {
if (t.inFunctionBlockScope()) {
functionStack.addFirst(new BailoutInformation());
}
}
@Override
public void exitScopeWithCfg(NodeTraversal t) {
if (t.inFunctionBlockScope()) {
eliminateDeadAssignments(t);
functionStack.removeFirst();
}
}
private void eliminateDeadAssignments(NodeTraversal t) {
checkArgument(t.inFunctionBlockScope());
checkState(!functionStack.isEmpty());
// Skip unchanged functions (note that the scope root is the function block, not the function).
if (!compiler.hasScopeChanged(t.getScopeRoot().getParent())) {
return;
}
BailoutInformation currentFunction = functionStack.peekFirst();
// We are not going to do any dead assignment elimination in when there is
// at least one inner function because in most browsers, when there is a
// closure, ALL the variables are saved (escaped).
if (currentFunction.containsFunction) {
return;
}
// We don't do any dead assignment elimination if there are no assigns
// to eliminate. :)
if (!currentFunction.containsRemovableAssign) {
return;
}
Scope blockScope = t.getScope();
Scope functionScope = blockScope.getParent();
if (LiveVariablesAnalysis.MAX_VARIABLES_TO_ANALYZE
< blockScope.getVarCount() + functionScope.getVarCount()) {
return;
}
// Computes liveness information first.
ControlFlowGraph cfg = getControlFlowGraph(compiler);
SyntacticScopeCreator scopeCreator = new SyntacticScopeCreator(compiler);
AllVarsDeclaredInFunction allVarsDeclaredInFunction =
NodeUtil.getAllVarsDeclaredInFunction(compiler, scopeCreator, functionScope);
liveness =
new LiveVariablesAnalysis(
cfg, functionScope, blockScope, compiler, scopeCreator, allVarsDeclaredInFunction);
liveness.analyze();
Map allVarsInFn = liveness.getAllVariables();
tryRemoveDeadAssignments(t, cfg, allVarsInFn);
}
// Matches all assignment operators and increment/decrement operators.
// Does *not* match VAR initialization, since RemoveUnusedVariables
// will already remove variables that are initialized but unused.
boolean isRemovableAssign(Node n) {
return (NodeUtil.isAssignmentOp(n) && n.getFirstChild().isName()) || n.isInc() || n.isDec();
}
/**
* Try to remove useless assignments from a control flow graph that has been
* annotated with liveness information.
*
* @param t The node traversal.
* @param cfg The control flow graph of the program annotated with liveness
* information.
*/
private void tryRemoveDeadAssignments(NodeTraversal t,
ControlFlowGraph cfg,
Map allVarsInFn) {
Iterable> nodes = cfg.getNodes();
for (DiGraphNode cfgNode : nodes) {
LinearFlowState state = cfgNode.getAnnotation();
Node n = cfgNode.getValue();
if (n == null) {
continue;
}
switch (n.getToken()) {
case IF:
case WHILE:
case DO:
tryRemoveAssignment(t, NodeUtil.getConditionExpression(n), state, allVarsInFn);
continue;
case FOR:
case FOR_IN:
case FOR_OF:
case FOR_AWAIT_OF:
if (n.isVanillaFor()) {
tryRemoveAssignment(t, NodeUtil.getConditionExpression(n), state, allVarsInFn);
}
continue;
case SWITCH:
case CASE:
case RETURN:
if (n.hasChildren()) {
tryRemoveAssignment(t, n.getFirstChild(), state, allVarsInFn);
}
continue;
// TODO(user): case VAR: Remove var a=1;a=2;.....
default:
break;
}
tryRemoveAssignment(t, n, state, allVarsInFn);
}
}
private void tryRemoveAssignment(
NodeTraversal t,
Node n,
LinearFlowState state,
Map allVarsInFn) {
tryRemoveAssignment(t, n, n, state, allVarsInFn);
}
/**
* Determines if any local variables are dead after the instruction {@code n} and are assigned
* within the subtree of {@code n}. Removes those assignments if there are any.
*
* @param n Target instruction.
* @param exprRoot The CFG node where the liveness information in state is still correct.
* @param state The liveness information at {@code n}.
*/
private void tryRemoveAssignment(
NodeTraversal t,
Node n,
Node exprRoot,
LinearFlowState state,
Map allVarsInFn) {
Node parent = n.getParent();
boolean isDeclarationNode = NodeUtil.isNameDeclaration(parent);
if (NodeUtil.isAssignmentOp(n) || n.isInc() || n.isDec() || isDeclarationNode) {
if (parent.isConst()) {
// Removing the RHS of a const produces as invalid AST.
return;
}
Node lhs = isDeclarationNode ? n : n.getFirstChild();
Node rhs = NodeUtil.getRValueOfLValue(lhs);
// Recurse first. Example: dead_x = dead_y = 1; We try to clean up dead_y
// first.
if (rhs != null) {
tryRemoveAssignment(t, rhs, exprRoot, state, allVarsInFn);
rhs = NodeUtil.getRValueOfLValue(lhs);
}
// Multiple declarations should be processed from right-to-left to ensure side-effects
// are run in the correct order.
if (isDeclarationNode && lhs.getNext() != null) {
tryRemoveAssignment(t, lhs.getNext(), exprRoot, state, allVarsInFn);
}
// Ignore declarations that don't initialize a value. Dead code removal will kill those nodes.
// Also ignore the var declaration if it's in a for-loop instantiation since there's not a
// safe place to move the side-effects.
if (isDeclarationNode && (rhs == null || NodeUtil.isAnyFor(parent.getParent()))) {
return;
}
if (!lhs.isName()) {
return; // Not a local variable assignment.
}
String name = lhs.getString();
Scope scope = t.getScope();
checkState(scope.isFunctionBlockScope() || scope.isBlockScope());
if (!allVarsInFn.containsKey(name)) {
return;
}
Var var = allVarsInFn.get(name);
if (liveness.getEscapedLocals().contains(var)) {
return; // Local variable that might be escaped due to closures.
}
// If we have an identity assignment such as a=a, always remove it
// regardless of what the liveness results because it
// does not change the result afterward.
if (rhs != null && rhs.isName() && rhs.getString().equals(var.getName()) && n.isAssign()) {
rhs.detach();
n.replaceWith(rhs);
compiler.reportChangeToEnclosingScope(rhs);
return;
}
int index = liveness.getVarIndex(var.getName());
if (state.getOut().isLive(index)) {
return; // Variable not dead.
}
if (state.getIn().isLive(index)
&& isVariableStillLiveWithinExpression(n, exprRoot, var.getName())) {
// The variable is killed here but it is also live before it.
// This is possible if we have say:
// if (X = a && a = C) {..} ; .......; a = S;
// In this case we are safe to remove "a = C" because it is dead.
// However if we have:
// if (a = C && X = a) {..} ; .......; a = S;
// removing "a = C" is NOT correct, although the live set at the node
// is exactly the same.
// TODO(user): We need more fine grain CFA or we need to keep track
// of GEN sets when we recurse here.
return;
}
if (n.isAssign()) {
rhs.detach();
n.replaceWith(rhs);
} else if (NodeUtil.isAssignmentOp(n)) {
rhs.detach();
lhs.detach();
Node op = new Node(NodeUtil.getOpFromAssignmentOp(n), lhs, rhs);
n.replaceWith(op);
} else if (n.isInc() || n.isDec()) {
if (parent.isExprResult()) {
n.replaceWith(IR.voidNode(IR.number(0).srcref(n)));
} else if (n.isComma() && n != parent.getLastChild()) {
n.detach();
} else if (parent.isVanillaFor() && NodeUtil.getConditionExpression(parent) != n) {
n.replaceWith(IR.empty());
} else {
// Cannot replace x = a++ with x = a because that's not valid
// when a is not a number.
return;
}
} else if (isDeclarationNode) {
rhs.detach();
IR.exprResult(rhs).insertAfter(parent);
rhs.getParent().srcref(rhs);
} else {
// Not reachable.
throw new IllegalStateException("Unknown statement");
}
compiler.reportChangeToEnclosingScope(parent);
return;
} else {
for (Node c = n.getFirstChild(); c != null;) {
Node next = c.getNext();
if (!ControlFlowGraph.isEnteringNewCfgNode(c)) {
tryRemoveAssignment(t, c, exprRoot, state, allVarsInFn);
}
c = next;
}
return;
}
}
/**
* Given a variable, node n in the tree and a sub-tree denoted by exprRoot as
* the root, this function returns true if there exists a read of that
* variable before a write to that variable that is on the right side of n.
*
* For example, suppose the node is x = 1:
*
* y = 1, x = 1; // false, there is no reads at all.
* y = 1, x = 1, print(x) // true, there is a read right of n.
* y = 1, x = 1, x = 2, print(x) // false, there is a read right of n but
* // it is after a write.
*
* @param n The current node we should look at.
* @param exprRoot The node
*/
private boolean isVariableStillLiveWithinExpression(
Node n, Node exprRoot, String variable) {
while (n != exprRoot) {
VariableLiveness state = VariableLiveness.MAYBE_LIVE;
switch (n.getParent().getToken()) {
case OR:
case AND:
case COALESCE:
// If the currently node is the first child of
// AND/OR, be conservative only consider the READs
// of the second operand.
if (n.getNext() != null) {
state = isVariableReadBeforeKill(
n.getNext(), variable);
if (state == VariableLiveness.KILL) {
state = VariableLiveness.MAYBE_LIVE;
}
}
break;
case HOOK:
// If current node is the condition, check each following
// branch, otherwise it is a conditional branch and the
// other branch can be ignored.
if (n.getNext() != null && n.getNext().getNext() != null) {
state = checkHookBranchReadBeforeKill(
n.getNext(), n.getNext().getNext(), variable);
}
break;
default:
for (Node sibling = n.getNext(); sibling != null;
sibling = sibling.getNext()) {
state = isVariableReadBeforeKill(sibling, variable);
if (state != VariableLiveness.MAYBE_LIVE) {
break;
}
}
}
// If we see a READ or KILL there is no need to continue.
if (state == VariableLiveness.READ) {
return true;
} else if (state == VariableLiveness.KILL) {
return false;
}
n = n.getParent();
}
return false;
}
// The current liveness of the variable
private enum VariableLiveness {
MAYBE_LIVE, // May be still live in the current expression tree.
READ, // Known there is a read left of it.
KILL, // Known there is a write before any read.
}
/**
* Give an expression and a variable. It returns READ, if the first
* reference of that variable is a read. It returns KILL, if the first
* reference of that variable is an assignment. It returns MAY_LIVE otherwise.
*/
private VariableLiveness isVariableReadBeforeKill(
Node n, String variable) {
if (ControlFlowGraph.isEnteringNewCfgNode(n)) { // Not a FUNCTION
return VariableLiveness.MAYBE_LIVE;
}
if (n.isName() && variable.equals(n.getString())) {
if (NodeUtil.isNameDeclOrSimpleAssignLhs(n, n.getParent())) {
checkState(n.getParent().isAssign(), n.getParent());
// The expression to which the assignment is made is evaluated before
// the RHS is evaluated (normal left to right evaluation) but the KILL
// occurs after the RHS is evaluated.
Node rhs = n.getNext();
VariableLiveness state = isVariableReadBeforeKill(rhs, variable);
if (state == VariableLiveness.READ) {
return state;
}
return VariableLiveness.KILL;
} else {
return VariableLiveness.READ;
}
}
switch (n.getToken()) {
// Conditionals
case OR:
case AND:
case COALESCE:
VariableLiveness v1 = isVariableReadBeforeKill(
n.getFirstChild(), variable);
VariableLiveness v2 = isVariableReadBeforeKill(
n.getLastChild(), variable);
// With a AND/OR/COALESCE the first branch always runs, but the second is
// may not.
if (v1 != VariableLiveness.MAYBE_LIVE) {
return v1;
} else if (v2 == VariableLiveness.READ) {
return VariableLiveness.READ;
} else {
return VariableLiveness.MAYBE_LIVE;
}
case HOOK:
VariableLiveness first = isVariableReadBeforeKill(
n.getFirstChild(), variable);
if (first != VariableLiveness.MAYBE_LIVE) {
return first;
}
return checkHookBranchReadBeforeKill(
n.getSecondChild(), n.getLastChild(), variable);
default:
// Expressions are evaluated left-right, depth first.
for (Node child = n.getFirstChild();
child != null; child = child.getNext()) {
VariableLiveness state = isVariableReadBeforeKill(child, variable);
if (state != VariableLiveness.MAYBE_LIVE) {
return state;
}
}
}
return VariableLiveness.MAYBE_LIVE;
}
private VariableLiveness checkHookBranchReadBeforeKill(
Node trueCase, Node falseCase, String variable) {
VariableLiveness v1 = isVariableReadBeforeKill(
trueCase, variable);
VariableLiveness v2 = isVariableReadBeforeKill(
falseCase, variable);
// With a hook it is unknown which branch will run, so
// we must be conservative. A read by either is a READ, and
// a KILL is only considered if both KILL.
if (v1 == VariableLiveness.READ || v2 == VariableLiveness.READ) {
return VariableLiveness.READ;
} else if (v1 == VariableLiveness.KILL && v2 == VariableLiveness.KILL) {
return VariableLiveness.KILL;
} else {
return VariableLiveness.MAYBE_LIVE;
}
}
}
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