com.google.javascript.jscomp.LiveVariablesAnalysis 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 com.google.common.base.Preconditions;
import com.google.javascript.jscomp.ControlFlowGraph.Branch;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.LatticeElement;
import com.google.javascript.rhino.Node;
import java.util.BitSet;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
/**
* Compute the "liveness" of all local variables. A variable is "live" at a
* point of a program if the value it is currently holding might be read later.
* Otherwise, the variable is considered "dead" if we know for sure that it will
* no longer be read. Dead variables are candidates for dead assignment
* elimination and variable name sharing. The worst case safe assumption is to
* assume that all variables are live. In that case, we will have no opportunity
* for optimizations. This is especially the case within a TRY block when an
* assignment is not guaranteed to take place. We bail out by assuming that
* all variables are live.
*
* Due to the possibility of inner functions and closures, certain "local"
* variables can escape the function. These variables will be considered as
* global and they can be retrieved with {@link #getEscapedLocals()}.
*
*/
class LiveVariablesAnalysis extends
DataFlowAnalysis {
// 100 = ((# of original Power Rangers) ^
// (# years of Warren Harding in office)) *
// (# of Ninja Turtles)
static final int MAX_VARIABLES_TO_ANALYZE = 100;
public static final String ARGUMENT_ARRAY_ALIAS = "arguments";
private static class LiveVariableJoinOp
implements JoinOp {
@Override
public LiveVariableLattice apply(List in) {
LiveVariableLattice result = new LiveVariableLattice(in.get(0));
for (int i = 1; i < in.size(); i++) {
result.liveSet.or(in.get(i).liveSet);
}
return result;
}
}
/**
* The lattice that stores the liveness of all local variables at a given
* point in the program. The whole lattice is the power set of all local
* variables and a variable is live if it is in the set.
*/
static class LiveVariableLattice implements LatticeElement {
private final BitSet liveSet;
/**
* @param numVars Number of all local variables.
*/
private LiveVariableLattice(int numVars) {
this.liveSet = new BitSet(numVars);
}
private LiveVariableLattice(LiveVariableLattice other) {
Preconditions.checkNotNull(other);
this.liveSet = (BitSet) other.liveSet.clone();
}
@Override
public boolean equals(Object other) {
Preconditions.checkNotNull(other);
return (other instanceof LiveVariableLattice) &&
this.liveSet.equals(((LiveVariableLattice) other).liveSet);
}
public boolean isLive(Var v) {
Preconditions.checkNotNull(v);
return liveSet.get(v.index);
}
public boolean isLive(int index) {
return liveSet.get(index);
}
@Override
public String toString() {
return liveSet.toString();
}
@Override
public int hashCode() {
return liveSet.hashCode();
}
}
// The scope of the function that we are analyzing.
private final Scope jsScope;
private final Set escaped;
LiveVariablesAnalysis(ControlFlowGraph cfg, Scope jsScope,
AbstractCompiler compiler) {
super(cfg, new LiveVariableJoinOp());
this.jsScope = jsScope;
this.escaped = new HashSet<>();
computeEscaped(jsScope, escaped, compiler);
}
public Set getEscapedLocals() {
return escaped;
}
public int getVarIndex(String var) {
return jsScope.getVar(var).index;
}
@Override
boolean isForward() {
return false;
}
@Override
LiveVariableLattice createEntryLattice() {
return new LiveVariableLattice(jsScope.getVarCount());
}
@Override
LiveVariableLattice createInitialEstimateLattice() {
return new LiveVariableLattice(jsScope.getVarCount());
}
@Override
LiveVariableLattice flowThrough(Node node, LiveVariableLattice input) {
final BitSet gen = new BitSet(input.liveSet.size());
final BitSet kill = new BitSet(input.liveSet.size());
// Make kills conditional if the node can end abruptly by an exception.
boolean conditional = false;
List> edgeList = getCfg().getOutEdges(node);
for (DiGraphEdge edge : edgeList) {
if (Branch.ON_EX.equals(edge.getValue())) {
conditional = true;
}
}
computeGenKill(node, gen, kill, conditional);
LiveVariableLattice result = new LiveVariableLattice(input);
// L_in = L_out - Kill + Gen
result.liveSet.andNot(kill);
result.liveSet.or(gen);
return result;
}
/**
* Computes the GEN and KILL set.
*
* @param n Root node.
* @param gen Local variables that are live because of the instruction at
* {@code n} will be added to this set.
* @param kill Local variables that are killed because of the instruction at
* {@code n} will be added to this set.
* @param conditional {@code true} if any assignments encountered are
* conditionally executed. These assignments might not kill a variable.
*/
private void computeGenKill(Node n, BitSet gen, BitSet kill,
boolean conditional) {
switch (n.getToken()) {
case SCRIPT:
case BLOCK:
case ROOT:
case FUNCTION:
return;
case WHILE:
case DO:
case IF:
computeGenKill(NodeUtil.getConditionExpression(n), gen, kill,
conditional);
return;
case FOR:
computeGenKill(NodeUtil.getConditionExpression(n), gen, kill, conditional);
return;
case FOR_IN: {
// for(x in y) {...}
Node lhs = n.getFirstChild();
if (lhs.isVar()) {
// for(var x in y) {...}
lhs = lhs.getLastChild();
}
if (lhs.isName()) {
addToSetIfLocal(lhs, kill);
addToSetIfLocal(lhs, gen);
} else {
computeGenKill(lhs, gen, kill, conditional);
}
// rhs is executed only once so we don't go into it every loop.
return;
}
case VAR:
for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
if (c.hasChildren()) {
computeGenKill(c.getFirstChild(), gen, kill, conditional);
if (!conditional) {
addToSetIfLocal(c, kill);
}
}
}
return;
case AND:
case OR:
computeGenKill(n.getFirstChild(), gen, kill, conditional);
// May short circuit.
computeGenKill(n.getLastChild(), gen, kill, true);
return;
case HOOK:
computeGenKill(n.getFirstChild(), gen, kill, conditional);
// Assume both sides are conditional.
computeGenKill(n.getSecondChild(), gen, kill, true);
computeGenKill(n.getLastChild(), gen, kill, true);
return;
case NAME:
if (isArgumentsName(n)) {
markAllParametersEscaped();
} else {
addToSetIfLocal(n, gen);
}
return;
default:
if (NodeUtil.isAssignmentOp(n) && n.getFirstChild().isName()) {
Node lhs = n.getFirstChild();
if (!conditional) {
addToSetIfLocal(lhs, kill);
}
if (!n.isAssign()) {
// assignments such as a += 1 reads a.
addToSetIfLocal(lhs, gen);
}
computeGenKill(lhs.getNext(), gen, kill, conditional);
} else {
for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
computeGenKill(c, gen, kill, conditional);
}
}
return;
}
}
private void addToSetIfLocal(Node node, BitSet set) {
Preconditions.checkState(node.isName());
String name = node.getString();
if (!jsScope.isDeclared(name, false)) {
return;
}
Var var = jsScope.getVar(name);
if (!escaped.contains(var)) {
set.set(var.index);
}
}
/**
* Give up computing liveness of formal parameter by putting all the parameter
* names in the escaped set.
*/
void markAllParametersEscaped() {
Node lp = jsScope.getRootNode().getSecondChild();
for (Node arg = lp.getFirstChild(); arg != null; arg = arg.getNext()) {
escaped.add(jsScope.getVar(arg.getString()));
}
}
private boolean isArgumentsName(Node n) {
return n.isName() && n.getString().equals(ARGUMENT_ARRAY_ALIAS)
&& !jsScope.isDeclared(ARGUMENT_ARRAY_ALIAS, false);
}
}