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 com.google.common.base.Joiner;
import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.ControlFlowGraph.AbstractCfgNodeTraversalCallback;
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.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.Comparator;
import java.util.Deque;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Set;
import java.util.TreeSet;
/**
* 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 boolean usePseudoNames;
private static final Comparator coloringTieBreaker =
new Comparator() {
@Override
public int compare(Var v1, Var v2) {
return v1.index - v2.index;
}
};
/**
* @param usePseudoNames For debug purposes, when merging variable foo and bar
* to foo, rename both variable to foo_bar.
*/
CoalesceVariableNames(AbstractCompiler compiler, boolean usePseudoNames) {
Preconditions.checkState(!compiler.getLifeCycleStage().isNormalized());
this.compiler = compiler;
colorings = new LinkedList<>();
this.usePseudoNames = usePseudoNames;
}
@Override
public void process(Node externs, Node root) {
NodeTraversal.traverse(compiler, root, this);
}
private static boolean shouldOptimizeScope(Scope scope) {
// 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 (scope.isGlobal()) {
return false;
}
return LiveVariablesAnalysis.MAX_VARIABLES_TO_ANALYZE >= scope.getVarCount();
}
@Override
public void enterScope(NodeTraversal t) {
Scope scope = t.getScope();
if (!shouldOptimizeScope(scope)) {
return;
}
ControlFlowGraph cfg = t.getControlFlowGraph();
LiveVariablesAnalysis liveness =
new LiveVariablesAnalysis(cfg, scope, compiler);
// If the function has exactly 2 params, mark them as escaped. This is
// a work-around for an IE bug where it throws an exception if you
// write to the parameters of the callback in a sort(). See:
// http://code.google.com/p/closure-compiler/issues/detail?id=58
if (scope.getRootNode().getSecondChild().getChildCount() == 2) {
liveness.markAllParametersEscaped();
}
liveness.analyze();
UndiGraph interferenceGraph =
computeVariableNamesInterferenceGraph(
t, cfg, (Set) liveness.getEscapedLocals());
GraphColoring coloring =
new GreedyGraphColoring<>(interferenceGraph,
coloringTieBreaker);
coloring.color();
colorings.push(coloring);
}
@Override
public void exitScope(NodeTraversal t) {
if (!shouldOptimizeScope(t.getScope())) {
return;
}
colorings.pop();
}
@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 = t.getScope().getVar(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.name);
compiler.reportCodeChange();
if (parent.isVar()) {
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.
String pseudoName = null;
Set allMergedNames = new TreeSet<>();
for (Iterator i = t.getScope().getVars(); i.hasNext();) {
Var iVar = i.next();
// 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.name);
}
}
// Keep its original name.
if (allMergedNames.size() == 1) {
return;
}
pseudoName = Joiner.on("_").join(allMergedNames);
while (t.getScope().isDeclared(pseudoName, true)) {
pseudoName += "$";
}
n.setString(pseudoName);
compiler.reportCodeChange();
if (!vNode.getValue().equals(coalescedVar) && parent.isVar()) {
removeVarDeclaration(n);
}
}
}
private UndiGraph computeVariableNamesInterferenceGraph(
NodeTraversal t, ControlFlowGraph cfg, Set escaped) {
UndiGraph interferenceGraph =
LinkedUndirectedGraph.create();
Scope scope = t.getScope();
// First create a node for each non-escaped variable.
for (Iterator i = scope.getVars(); i.hasNext();) {
Var v = i.next();
if (!escaped.contains(v)) {
// TODO(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
// 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()) {
interferenceGraph.createNode(v);
}
}
}
// Go through each variable and try to connect them.
for (Iterator i1 = scope.getVars(); i1.hasNext();) {
Var v1 = i1.next();
NEXT_VAR_PAIR:
for (Iterator i2 = scope.getVars(); i2.hasNext();) {
Var v2 = i2.next();
// Skip duplicate pairs.
if (v1.index >= v2.index) {
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.getParentNode().isParamList() &&
v2.getParentNode().isParamList()) {
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.getDirectedGraphNodes()) {
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(v1) && state.getIn().isLive(v2)) ||
(state.getOut().isLive(v1) && state.getOut().isLive(v2))) {
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.getDirectedGraphNodes()) {
if (cfg.isImplicitReturn(cfgNode)) {
continue NEXT_INTRA_CFG_NODE;
}
FlowState state = cfgNode.getAnnotation();
boolean v1OutLive = state.getOut().isLive(v1);
boolean v2OutLive = state.getOut().isLive(v2);
CombinedLiveRangeChecker checker = new CombinedLiveRangeChecker(
new LiveRangeChecker(v1, v2OutLive ? null : v2),
new LiveRangeChecker(v2, v1OutLive ? null : v1));
NodeTraversal.traverse(
compiler,
cfgNode.getValue(),
checker);
if (checker.connectIfCrossed(interferenceGraph)) {
continue NEXT_VAR_PAIR;
}
}
}
}
return interferenceGraph;
}
/**
* A simple wrapper calls to call two AbstractCfgNodeTraversalCallback
* callback during the same traversal. Both traversals must have the same
* "shouldTraverse" conditions.
*/
private static class CombinedLiveRangeChecker
extends AbstractCfgNodeTraversalCallback {
private final LiveRangeChecker callback1;
private final LiveRangeChecker callback2;
CombinedLiveRangeChecker(
LiveRangeChecker callback1,
LiveRangeChecker callback2) {
this.callback1 = callback1;
this.callback2 = callback2;
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (LiveRangeChecker.shouldVisit(n)) {
callback1.visit(t, n, parent);
callback2.visit(t, n, parent);
}
}
boolean connectIfCrossed(UndiGraph interferenceGraph) {
if (callback1.crossed || callback2.crossed) {
Var v1 = callback1.getDef();
Var v2 = callback2.getDef();
interferenceGraph.connectIfNotFound(v1, null, v2);
return true;
}
return false;
}
}
/**
* Tries to remove variable declaration if the variable has been coalesced
* with another variable that has already been declared.
*/
private static void removeVarDeclaration(Node name) {
Node var = name.getParent();
Node parent = var.getParent();
// Special case when we are in FOR-IN loop.
if (NodeUtil.isForIn(parent)) {
var.removeChild(name);
parent.replaceChild(var, name);
} else if (var.hasOneChild()) {
// The removal is easy when there is only one variable in the VAR node.
if (name.hasChildren()) {
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.isFor()) {
assign = NodeUtil.newExpr(assign);
}
parent.replaceChild(var, assign);
} else {
// In a FOR( ; ; ) node, we must replace it with an EMPTY or else it
// becomes a FOR-IN node.
NodeUtil.removeChild(parent, var);
}
} else {
if (!name.hasChildren()) {
var.removeChild(name);
}
// We are going to leave duplicated declaration otherwise.
}
}
private static class LiveRangeChecker
extends AbstractCfgNodeTraversalCallback {
boolean defFound = false;
boolean crossed = false;
private final Var def;
private final Var use;
public LiveRangeChecker(Var def, Var use) {
this.def = def;
this.use = use;
}
Var getDef() {
return def;
}
/**
* @return Whether any LiveRangeChecker would be interested in the node.
*/
public static boolean shouldVisit(Node n) {
return (n.isName()
|| (n.hasChildren() && n.getFirstChild().isName()));
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (!defFound && isAssignTo(def, n, parent)) {
defFound = true;
}
if (defFound && (use == null || isReadFrom(use, n))) {
crossed = true;
}
}
private static boolean isAssignTo(Var var, Node n, Node parent) {
if (n.isName() && var.getName().equals(n.getString()) &&
parent != null) {
if (parent.isParamList()) {
// In a function declaration, the formal parameters are assigned.
return true;
} else if (parent.isVar()) {
// If this is a VAR declaration, if the name node has a child, we are
// assigning to that name.
return n.hasChildren();
}
return false; // Definitely a read.
} else {
// Lastly, any assignmentOP is also an assign.
Node name = n.getFirstChild();
return name != null && name.isName() &&
var.getName().equals(name.getString()) &&
NodeUtil.isAssignmentOp(n);
}
}
private static boolean isReadFrom(Var var, Node name) {
return name != null && name.isName() &&
var.getName().equals(name.getString()) &&
!NodeUtil.isVarOrSimpleAssignLhs(name, name.getParent());
}
}
}