com.google.javascript.jscomp.CoalesceVariableNames Maven / Gradle / Ivy
/*
* 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 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.FlowState;
import com.google.javascript.jscomp.LiveVariablesAnalysis.LiveVariableLattice;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.NodeTraversal.ScopedCallback;
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.Comparator;
import java.util.Deque;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;
import javax.annotation.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 AbstractPostOrderCallback implements
CompilerPass, ScopedCallback {
private final AbstractCompiler compiler;
private final Deque> colorings;
private final Deque liveAnalyses;
private final boolean usePseudoNames;
private LiveVariablesAnalysis liveness;
private final Comparator coloringTieBreaker =
new Comparator() {
@Override
public int compare(Var v1, Var v2) {
return liveness.getVarIndex(v1.getName()) - liveness.getVarIndex(v2.getName());
}
};
/**
* @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;
}
@Override
public void process(Node externs, Node root) {
checkNotNull(externs);
checkNotNull(root);
NodeTraversal.traverse(compiler, root, this);
compiler.setLifeCycleStage(LifeCycleStage.RAW);
}
private static boolean 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()) {
return false;
}
Map allVarsInFn = new HashMap<>();
List orderedVars = new ArrayList<>();
NodeUtil.getAllVarsDeclaredInFunction(
allVarsInFn, orderedVars, t.getCompiler(), t.getScopeCreator(), t.getScope());
return LiveVariablesAnalysis.MAX_VARIABLES_TO_ANALYZE > orderedVars.size();
}
@Override
public void enterScope(NodeTraversal t) {
Scope scope = t.getScope();
if (!shouldOptimizeScope(t)) {
return;
}
checkState(scope.isFunctionScope(), scope);
// live variables analysis is based off of the control flow graph
ControlFlowGraph cfg = t.getControlFlowGraph();
liveness =
new LiveVariablesAnalysis(cfg, scope, null, compiler, new SyntacticScopeCreator(compiler));
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 exitScope(NodeTraversal t) {
if (!shouldOptimizeScope(t)) {
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
&& coalescedVar.equals(colorings.peek().getPartitionSuperNode(iVar))) {
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 pairing of variables
* and for every CFG node, determine whether 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 cfg
* @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.
List orderedVariables = liveness.getAllVariablesInOrder();
for (Var v : orderedVariables) {
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);
}
// Go through each variable and try to connect them.
int v1Index = -1;
for (Var v1 : orderedVariables) {
v1Index++;
int v2Index = -1;
NEXT_VAR_PAIR:
for (Var v2 : orderedVariables) {
v2Index++;
// Skip duplicate pairs.
if (v1Index > v2Index) {
continue;
}
if (!interferenceGraph.hasNode(v1) || !interferenceGraph.hasNode(v2)) {
// Skip nodes that were not added. They are globals and escaped
// locals. Also avoid merging a variable with itself.
continue NEXT_VAR_PAIR;
}
if (v1.isParam() && v2.isParam()) {
interferenceGraph.connectIfNotFound(v1, null, v2);
continue NEXT_VAR_PAIR;
}
// Go through every CFG node in the program and look at
// this variable pair. If they are both live at the same
// time, add an edge between them and continue to the next pair.
NEXT_CROSS_CFG_NODE:
for (DiGraphNode cfgNode : cfg.getNodes()) {
if (cfg.isImplicitReturn(cfgNode)) {
continue NEXT_CROSS_CFG_NODE;
}
FlowState state = cfgNode.getAnnotation();
// Check the live states and add edge when possible.
if ((state.getIn().isLive(v1Index) && state.getIn().isLive(v2Index))
|| (state.getOut().isLive(v1Index) && state.getOut().isLive(v2Index))) {
interferenceGraph.connectIfNotFound(v1, null, v2);
continue NEXT_VAR_PAIR;
}
}
// v1 and v2 might not have an edge between them! woohoo. there's
// one last sanity check that we have to do: we have to check
// if there's a collision *within* the cfg node.
NEXT_INTRA_CFG_NODE:
for (DiGraphNode cfgNode : cfg.getNodes()) {
if (cfg.isImplicitReturn(cfgNode)) {
continue NEXT_INTRA_CFG_NODE;
}
FlowState state = cfgNode.getAnnotation();
boolean v1OutLive = state.getOut().isLive(v1Index);
boolean v2OutLive = state.getOut().isLive(v2Index);
CombinedLiveRangeChecker checker = new CombinedLiveRangeChecker(
cfgNode.getValue(),
new LiveRangeChecker(v1, v2OutLive ? null : v2),
new LiveRangeChecker(v2, v1OutLive ? null : v1));
checker.check(cfgNode.getValue());
if (checker.connectIfCrossed(interferenceGraph)) {
continue NEXT_VAR_PAIR;
}
}
}
}
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);
return NodeUtil.findLhsNodesInNode(nameDecl).size() > 1;
default:
return false;
}
}
/**
* A simple wrapper to call two LiveRangeChecker callbacks during the same traversal.
*/
private static class CombinedLiveRangeChecker {
private final Node root;
private final LiveRangeChecker callback1;
private final LiveRangeChecker callback2;
CombinedLiveRangeChecker(
Node root,
LiveRangeChecker callback1,
LiveRangeChecker callback2) {
this.root = root;
this.callback1 = callback1;
this.callback2 = callback2;
}
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);
}
}
visit(n, n.getParent());
}
}
void visit(Node n, Node parent) {
if (LiveRangeChecker.shouldVisit(n)) {
callback1.visit(n, parent);
callback2.visit(n, parent);
}
}
boolean connectIfCrossed(UndiGraph interferenceGraph) {
if (callback1.crossed || callback2.crossed) {
Var v1 = callback1.def;
Var v2 = callback2.def;
interferenceGraph.connectIfNotFound(v1, null, v2);
return true;
}
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 ...`
parent.replaceChild(var, 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();
var.removeChild(name);
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);
}
parent.replaceChild(var, assign);
} else {
// convert `let x;` to ``
// and `for (let x;;) {}` to `for (;;) {}`
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 =
compiler.createAstFactory().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);
}
private static class LiveRangeChecker {
boolean defFound = false;
boolean crossed = false;
private final Var def;
@Nullable
private final Var use;
public LiveRangeChecker(Var def, Var use) {
this.def = checkNotNull(def);
this.use = use;
}
/**
* @return Whether any LiveRangeChecker would be interested in the node.
*/
public static boolean shouldVisit(Node n) {
return (n.isName() || (n.hasChildren() && n.getFirstChild().isName()));
}
void visit(Node n, Node parent) {
if (!defFound && isAssignTo(def, n, parent)) {
defFound = true;
}
if (defFound && (use == null || isReadFrom(use, n))) {
crossed = true;
}
}
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());
}
}
}