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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.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static java.util.Comparator.comparingInt;
import com.google.common.base.Joiner;
import com.google.javascript.jscomp.AbstractCompiler.LifeCycleStage;
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.jscomp.graph.GraphColoring;
import com.google.javascript.jscomp.graph.GraphColoring.GreedyGraphColoring;
import com.google.javascript.jscomp.graph.GraphNode;
import com.google.javascript.jscomp.graph.LinkedUndirectedGraph;
import com.google.javascript.jscomp.graph.UndiGraph;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Comparator;
import java.util.Deque;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;
import org.jspecify.nullness.Nullable;
/**
* Reuse variable names if possible.
*
* For example, from
*
* var x = 1; print(x); var y = 2; print(y);
*
* to
*
* var x = 1; print(x); x = 2; print(x)
*
* The benefits are slightly shorter code because of the removed var declaration,
* less unique variables in hope for better renaming, and finally better gzip
* compression.
*
* The pass operates similar to a typical register allocator found in an
* optimizing compiler by first computing live ranges with
* {@link LiveVariablesAnalysis} and a variable interference graph. Then it uses
* graph coloring in {@link GraphColoring} to determine which two variables can
* be merge together safely.
*/
class CoalesceVariableNames extends NodeTraversal.AbstractCfgCallback implements CompilerPass {
private final AbstractCompiler compiler;
private final MemoizedScopeCreator scopeCreator;
private final Deque> colorings;
private final Deque liveAnalyses;
private final boolean usePseudoNames;
private final AstFactory astFactory;
private LiveVariablesAnalysis liveness;
private final Comparator coloringTieBreaker =
comparingInt((Var arg) -> liveness.getVarIndex(arg.getName()));
/** A stack of shouldOptimizeScope results. */
private final Deque shouldOptimizeScopeStack = new ArrayDeque<>();
/**
* @param usePseudoNames For debug purposes, when merging variable foo and bar to foo, rename both
* variable to foo_bar.
*/
CoalesceVariableNames(AbstractCompiler compiler, boolean usePseudoNames) {
// The code is normalized at this point in the compilation process. This allows us to use the
// fact that all variables have been given unique names. We can hoist coalesced variables to
// VARS because we know that shadowing can't occur.
checkState(compiler.getLifeCycleStage().isNormalized());
this.compiler = compiler;
colorings = new ArrayDeque<>();
liveAnalyses = new ArrayDeque<>();
this.usePseudoNames = usePseudoNames;
this.astFactory = compiler.createAstFactory();
this.scopeCreator = new MemoizedScopeCreator(new SyntacticScopeCreator(compiler));
}
@Override
public void process(Node externs, Node root) {
checkNotNull(externs);
checkNotNull(root);
NodeTraversal.builder()
.setCompiler(compiler)
.setCallback(this)
.setScopeCreator(this.scopeCreator)
.traverse(root);
compiler.setLifeCycleStage(LifeCycleStage.RAW);
}
/** Returns populated AllVarsDeclaredInFunction object iff shouldOptimizeScope is true. */
private static @Nullable AllVarsDeclaredInFunction shouldOptimizeScope(NodeTraversal t) {
// TODO(user): We CAN do this in the global scope, just need to be
// careful when something is exported. Liveness uses bit-vector for live
// sets so I don't see compilation time will be a problem for running this
// pass in the global scope.
if (t.getScopeRoot().isFunction()) {
AllVarsDeclaredInFunction allVarsDeclaredInFunction =
NodeUtil.getAllVarsDeclaredInFunction(t.getCompiler(), t.getScopeCreator(), t.getScope());
if (LiveVariablesAnalysis.MAX_VARIABLES_TO_ANALYZE
> allVarsDeclaredInFunction.getAllVariablesInOrder().size()) {
return allVarsDeclaredInFunction;
}
}
return null;
}
@Override
public void enterScopeWithCfg(NodeTraversal t) {
AllVarsDeclaredInFunction allVarsDeclaredInFunction = shouldOptimizeScope(t);
if (allVarsDeclaredInFunction == null) {
shouldOptimizeScopeStack.push(false);
return;
}
shouldOptimizeScopeStack.push(true);
Scope scope = t.getScope();
checkState(scope.isFunctionScope(), scope);
// live variables analysis is based off of the control flow graph
ControlFlowGraph cfg = getControlFlowGraph(compiler);
liveness =
new LiveVariablesAnalysis(
cfg, scope, null, compiler, this.scopeCreator, allVarsDeclaredInFunction);
if (FeatureSet.ES3.contains(compiler.getOptions().getOutputFeatureSet())) {
// If the function has exactly 2 params, mark them as escaped. This is a work-around for a
// bug in IE 8 and below, where it throws an exception if you write to the parameters of the
// callback in a sort(). See http://blickly.github.io/closure-compiler-issues/#58 and
// https://www.zachleat.com/web/array-sort/
Node enclosingFunction = scope.getRootNode();
if (NodeUtil.getFunctionParameters(enclosingFunction).hasTwoChildren()) {
liveness.markAllParametersEscaped();
}
}
liveness.analyze();
liveAnalyses.push(liveness);
// The interference graph has the function's variables as its nodes and any interference
// between the variables as the edges. Interference between two variables means that they are
// alive at overlapping times, which means that their variable names cannot be coalesced.
UndiGraph interferenceGraph =
computeVariableNamesInterferenceGraph(cfg, liveness.getEscapedLocals());
// Color any interfering variables with different colors and any variables that can be safely
// coalesced wih the same color.
GraphColoring coloring =
new GreedyGraphColoring<>(interferenceGraph, coloringTieBreaker);
coloring.color();
colorings.push(coloring);
}
@Override
public void exitScopeWithCfg(NodeTraversal t) {
if (!shouldOptimizeScopeStack.pop()) {
return;
}
colorings.pop();
liveAnalyses.pop();
liveness = liveAnalyses.peek();
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (colorings.isEmpty() || !n.isName() || parent.isFunction()) {
// Don't rename named functions.
return;
}
Var var = liveness.getAllVariables().get(n.getString());
GraphNode vNode = colorings.peek().getGraph().getNode(var);
if (vNode == null) {
// This is not a local.
return;
}
Var coalescedVar = colorings.peek().getPartitionSuperNode(var);
if (!usePseudoNames) {
if (vNode.getValue().equals(coalescedVar)) {
// The coalesced name is itself, nothing to do.
return;
}
// Rename.
n.setString(coalescedVar.getName());
compiler.reportChangeToEnclosingScope(n);
if (NodeUtil.isNameDeclaration(parent)
|| (NodeUtil.getEnclosingType(n, Token.DESTRUCTURING_LHS) != null
&& NodeUtil.isLhsByDestructuring(n))) {
makeDeclarationVar(coalescedVar);
removeVarDeclaration(n);
}
} else {
// This code block is slow but since usePseudoName is for debugging,
// we should not sacrifice performance for non-debugging compilation to
// make this fast.
Set allMergedNames = new TreeSet<>();
for (Var iVar : liveness.getAllVariablesInOrder()) {
// Look for all the variables that can be merged (in the graph by now)
// and it is merged with the current coalescedVar.
if (colorings.peek().getGraph().getNode(iVar) != null
&& colorings.peek().haveSameColor(iVar, coalescedVar)) {
allMergedNames.add(iVar.getName());
}
}
// Keep its original name.
if (allMergedNames.size() == 1) {
return;
}
String pseudoName = Joiner.on("_").join(allMergedNames);
while (t.getScope().hasSlot(pseudoName)) {
pseudoName += "$";
}
// Rename.
n.setString(pseudoName);
compiler.reportChangeToEnclosingScope(n);
if (!vNode.getValue().equals(coalescedVar)
&& (NodeUtil.isNameDeclaration(parent)
|| (NodeUtil.getEnclosingType(n, Token.DESTRUCTURING_LHS) != null
&& NodeUtil.isLhsByDestructuring(n)))) {
makeDeclarationVar(coalescedVar);
removeVarDeclaration(n);
}
}
}
/**
* In order to determine when it is appropriate to coalesce two variables, we use a live variables
* analysis to make sure they are not alive at the same time. We take every CFG node and determine
* which pairs of variables are alive at the same time. These pairs are set to true in a bit map.
* We take every pairing of variables and use the bit map to check if the two variables are alive
* at the same time. If two variables are alive at the same time, we create an edge between them
* in the interference graph. The interference graph is the input to a graph coloring algorithm
* that ensures any interfering variables are marked in different color groups, while variables
* that can safely be coalesced are assigned the same color group.
*
* @param escaped we don't want to coalesce any escaped variables
* @return graph with variable nodes and edges representing variable interference
*/
private UndiGraph computeVariableNamesInterferenceGraph(
ControlFlowGraph cfg, Set escaped) {
UndiGraph interferenceGraph = LinkedUndirectedGraph.create();
// First create a node for each non-escaped variable. We add these nodes in the order in which
// they appear in the code because we want the names that appear earlier in the code to be used
// when coalescing to variables that appear later in the code.
Var[] orderedVariables = liveness.getAllVariablesInOrder().toArray(new Var[0]);
// index i in interferenceGraphNodes is set to true when interferenceGraph
// has node orderedVariables[i]
BitSet interferenceGraphNodes = new BitSet();
// interferenceBitSet[i] = indices of all variables that should have an edge with
// orderedVariables[i]
BitSet[] interferenceBitSet = new BitSet[orderedVariables.length];
Arrays.setAll(interferenceBitSet, i -> new BitSet());
int vIndex = -1;
for (Var v : orderedVariables) {
vIndex++;
if (escaped.contains(v)) {
continue;
}
// NOTE(user): In theory, we CAN coalesce function names just like any variables. Our
// Liveness analysis captures this just like it as described in the specification. However, we
// saw some zipped and unzipped size increase after this. We are not totally sure why
// that is but, for now, we will respect the dead functions and not play around with it
if (v.getParentNode().isFunction()) {
continue;
}
// NOTE: we skip class declarations for a combination of two reasons:
// 1. they are block-scoped, so we would need to rewrite them as class expressions
// e.g. `class C {}` -> `var C = class {}` to avoid incorrect semantics
// (see testDontCoalesceClassDeclarationsWithDestructuringDeclaration).
// This is possible but increases pre-gzip code size and complexity.
// 2. since function declaration coalescing seems to cause a size regression (as discussed
// above) we assume that coalescing class names may cause a similar size regression.
if (v.getParentNode().isClass()) {
continue;
}
// Skip lets and consts that have multiple variables declared in them, otherwise this produces
// incorrect semantics. See test case "testCapture".
// Skipping vars technically isn't needed for correct semantics, but works around a Safari
// bug for var redeclarations (https://github.com/google/closure-compiler/issues/3164)
if (isInMultipleLvalueDecl(v)) {
continue;
}
interferenceGraph.createNode(v);
interferenceGraphNodes.set(vIndex);
}
// Go through every CFG node in the program and look at variables that are live.
// Set the pair of live variables in interferenceBitSet so we can add an edge between them.
for (DiGraphNode cfgNode : cfg.getNodes()) {
if (cfg.isImplicitReturn(cfgNode)) {
continue;
}
LinearFlowState state = cfgNode.getAnnotation();
// Check the live states and add edge when possible. An edge between two variables
// means that they are alive at overlapping times, which means that their
// variable names cannot be coalesced.
LiveVariableLattice livein = state.getIn();
for (int i = livein.nextSetBit(0); i >= 0; i = livein.nextSetBit(i + 1)) {
for (int j = livein.nextSetBit(i); j >= 0; j = livein.nextSetBit(j + 1)) {
interferenceBitSet[i].set(j);
}
}
LiveVariableLattice liveout = state.getOut();
for (int i = liveout.nextSetBit(0); i >= 0; i = liveout.nextSetBit(i + 1)) {
for (int j = liveout.nextSetBit(i); j >= 0; j = liveout.nextSetBit(j + 1)) {
interferenceBitSet[i].set(j);
}
}
LiveRangeChecker liveRangeChecker =
new LiveRangeChecker(cfgNode.getValue(), orderedVariables, state);
liveRangeChecker.check(cfgNode.getValue());
liveRangeChecker.setCrossingVariables(interferenceBitSet);
}
// Go through each variable and try to connect them.
int v1Index = -1;
for (Var v1 : orderedVariables) {
v1Index++;
int v2Index = -1;
for (Var v2 : orderedVariables) {
v2Index++;
// Skip duplicate pairs. Also avoid merging a variable with itself.
if (v1Index > v2Index) {
continue;
}
if (!interferenceGraphNodes.get(v1Index) || !interferenceGraphNodes.get(v2Index)) {
// Skip nodes that were not added. They are globals and escaped locals.
continue;
}
if ((v1.isParam() && v2.isParam()) || interferenceBitSet[v1Index].get(v2Index)) {
// Add an edge between variable pairs that are both parameters
// because we don't want parameters to share a name.
interferenceGraph.connectIfNotFound(v1, null, v2);
}
}
}
return interferenceGraph;
}
/**
* Returns whether this variable's declaration also declares other names.
*
* For example, this would return true for `x` in `let [x, y, z] = []`;
*/
private boolean isInMultipleLvalueDecl(Var v) {
Token declarationType = v.declarationType();
switch (declarationType) {
case LET:
case CONST:
case VAR:
Node nameDecl = NodeUtil.getEnclosingNode(v.getNode(), NodeUtil::isNameDeclaration);
int[] count = {0}; // for lambda access
NodeUtil.visitLhsNodesInNode(nameDecl, (lhs) -> count[0]++);
return count[0] > 1;
default:
return false;
}
}
/**
* Remove variable declaration if the variable has been coalesced with another variable that has
* already been declared.
*
*
A precondition is that if the variable has already been declared, it must be the only lvalue
* in said declaration. For example, this method will not accept `var x = 1, y = 2`. In theory we
* could leave in the `var` declaration, but var shadowing of params triggers a Safari bug:
* https://bugs.webkit.org/show_bug.cgi?id=182414 Another
*
* @param name name node of the variable being coalesced
*/
private static void removeVarDeclaration(Node name) {
Node var = NodeUtil.getEnclosingNode(name, NodeUtil::isNameDeclaration);
Node parent = var.getParent();
if (var.getFirstChild().isDestructuringLhs()) {
// convert `const [x] = arr` to `[x] = arr`
// a precondition for this method is that `x` is the only lvalue in the destructuring pattern
Node destructuringLhs = var.getFirstChild();
Node pattern = destructuringLhs.removeFirstChild();
if (NodeUtil.isEnhancedFor(parent)) {
var.replaceWith(pattern);
} else {
Node rvalue = var.getFirstFirstChild().detach();
var.replaceWith(NodeUtil.newExpr(IR.assign(pattern, rvalue).srcref(var)));
}
} else if (NodeUtil.isEnhancedFor(parent)) {
// convert `for (let x of ...` to `for (x of ...`
var.replaceWith(name.detach());
} else {
// either `var x = 0;` or `var x;`
checkState(var.hasOneChild() && var.getFirstChild() == name, var);
if (name.hasChildren()) {
// convert `let x = 0;` to `x = 0;`
Node value = name.removeFirstChild();
name.detach();
Node assign = IR.assign(name, value).srcref(name);
// We don't need to wrapped it with EXPR node if it is within a FOR.
if (!parent.isVanillaFor()) {
assign = NodeUtil.newExpr(assign);
}
var.replaceWith(assign);
} else {
// convert `let x;` to ``
// and `for (let x;;) {}` to `for (;;) {}`
// We can expect uninitialized declarations at this point and it's okay to remove them
// and they've been coalesced with another declaration.
NodeUtil.removeChild(parent, var);
}
}
}
/**
* Convert `const` or `let` declarations to `var` declarations.
*
*
This method should be called on the first declared variable of a group that are being
* coalesced.
*
*
Because the code has already been normalized by the time this pass runs, we can safely
* redeclare any let and const coalesced variables as vars
*/
private void makeDeclarationVar(Var coalescedName) {
if (coalescedName.isConst() || coalescedName.isLet()) {
Node nameNode = checkNotNull(coalescedName.getNameNode(), coalescedName);
if (isUninitializedLetNameInLoopBody(nameNode)) {
// We need to make sure that within a loop:
//
// `let x;`
// becomes
// `var x = void 0;`
//
// If we don't we won't be correctly resetting the variable to undefined on each loop
// iteration once we turn it into a var declaration.
//
// Note that all other cases will already have an initializer.
// const x = 1; // constant requires an initializer
// let {x, y} = obj; // destructuring requires an initializer
// let [x, y] = iterable; // destructuring requires an initializer
Node undefinedValue = astFactory.createUndefinedValue().srcrefTree(nameNode);
nameNode.addChildToFront(undefinedValue);
}
// find the declaration node in a way that works normal and destructuring declarations.
Node declNode = NodeUtil.getEnclosingNode(nameNode.getParent(), NodeUtil::isNameDeclaration);
// normalization ensures that all variables in a function are uniquely named, so it's OK
// to turn a `const` or `let` into a `var`.
declNode.setToken(Token.VAR);
}
}
private static boolean isUninitializedLetNameInLoopBody(Node nameNode) {
checkState(nameNode.isName(), nameNode);
Node letNode = nameNode.getParent();
if (!letNode.isLet()) {
// We're looking for `let name;`
// Note that in the case of destructuring an initializer always exists.
// `let {name} = initializerRequiredHere;
return false;
}
if (nameNode.hasOneChild()) {
// `let name = child;` has an initializer
return false;
}
Node letParent = letNode.getParent();
if (NodeUtil.isLoopStructure(letParent)) {
// `for (let x; ...`
// `for (let x in ...`
// `for (let x of ...`
// `for await (let x of ...`
// In all these cases the variable gets initialized on each loop iteration
return false;
}
// Inside a loop body, but not the loop control node itself
return NodeUtil.isWithinLoop(letParent);
}
/**
* Used to find written and read variables in the same CFG node so that the variable pairs can be
* marked as interfering in an interference bit map. Indices of written and read variables are put
* in a list. These two lists are used to mark each written variable as "crossing" all read
* variables.
*/
private static class LiveRangeChecker {
private final Node root;
private final LinearFlowState state;
private final Var[] orderedVariables;
private final List isAssignToList = new ArrayList<>(); // indices of written variables
private final List isReadFromList = new ArrayList<>(); // indices of read variables
LiveRangeChecker(
Node root, Var[] orderedVariables, LinearFlowState state) {
this.root = root;
this.orderedVariables = orderedVariables;
this.state = state;
}
void check(Node n) {
// For most AST nodes, traverse the subtree in postorder because that's how the expressions
// are evaluated.
if (n == root || !ControlFlowGraph.isEnteringNewCfgNode(n)) {
if ((n.isDestructuringLhs() && n.hasTwoChildren())
|| (n.isAssign() && n.getFirstChild().isDestructuringPattern())
|| n.isDefaultValue()) {
// Evaluate the rhs of a destructuring assignment/declaration before the lhs.
check(n.getSecondChild());
check(n.getFirstChild());
} else {
for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
check(c);
}
}
if (shouldVisit(n)) {
visit(n, n.getParent());
}
}
}
void visit(Node n, Node parent) {
for (int iVar = 0; iVar < orderedVariables.length; iVar++) {
if (isAssignTo(orderedVariables[iVar], n, parent)) {
isAssignToList.add(iVar);
}
}
if (!isAssignToList.isEmpty()) {
for (int iVar = 0; iVar < orderedVariables.length; iVar++) {
boolean varOutLive = state.getOut().isLive(iVar);
if (varOutLive || isReadFrom(orderedVariables[iVar], n)) {
isReadFromList.add(iVar);
}
}
}
}
void setCrossingVariables(BitSet[] interferenceBitSet) {
for (Integer iWrittenVar : isAssignToList) {
for (Integer iReadVar : isReadFromList) {
interferenceBitSet[iWrittenVar].set(iReadVar);
interferenceBitSet[iReadVar].set(iWrittenVar);
}
}
}
/** Returns whether any LiveRangeChecker would be interested in the node. */
public static boolean shouldVisit(Node n) {
return (n.isName() || (n.hasChildren() && n.getFirstChild().isName()));
}
static boolean isAssignTo(Var var, Node n, Node parent) {
if (n.isName()) {
if (parent.isParamList()) {
// In a function declaration, the formal parameters are assigned.
return var.getName().equals(n.getString());
} else if (NodeUtil.isNameDeclaration(parent) && n.hasChildren()) {
// If this is a VAR declaration, if the name node has a child, we are
// assigning to that name.
return var.getName().equals(n.getString());
} else if (NodeUtil.isLhsByDestructuring(n)) {
return var.getName().equals(n.getString());
}
} else if (NodeUtil.isAssignmentOp(n)) {
// Lastly, any assignmentOP is also an assign.
Node name = n.getFirstChild();
return name.isName() && var.getName().equals(name.getString());
}
return false; // Definitely a read.
}
static boolean isReadFrom(Var var, Node name) {
return name.isName()
&& var.getName().equals(name.getString())
&& !NodeUtil.isNameDeclOrSimpleAssignLhs(name, name.getParent());
}
}
}