com.google.javascript.jscomp.DataFlowAnalysis 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 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.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.base.format.SimpleFormat;
import com.google.javascript.jscomp.graph.Annotation;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;
import com.google.javascript.jscomp.graph.LatticeElement;
import com.google.javascript.rhino.Node;
import java.util.ArrayDeque;
import java.util.Comparator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import org.jspecify.nullness.Nullable;
/**
* A framework to help writing static program analysis.
*
* A subclass of this framework should specify how a single node changes the state of a program.
* This class finds a safe estimate (a fixed-point) for the whole program. The proven facts about
* the program will be annotated with {@link
* com.google.javascript.jscomp.graph.GraphNode#setAnnotation} to the given control flow graph's
* nodes in form of {@link LatticeElement} after calling {@link #analyze()}.
*
*
As a guideline, the following is a list of behaviors that any analysis can take:
*
*
* - Flow Direction: Is the analysis a forward or backward analysis?
*
- Lattice Elements: How does the analysis represent the state of the program at any given
* point?
*
- Branching: Does the analysis propagate a different lattice element along each branch
* exiting a node?
*
- JOIN Operation: Given two incoming paths and a lattice state value, what can the compiler
* conclude at the join point?
*
- Flow Equations: How does an instruction modify the state of program in terms of lattice
* values?
*
- Initial Entry Value: What can the compiler assume at the beginning of the program?
*
- Initial Estimate: What can the compiler assume at each point of the program? (What is the
* BOTTOM value of the lattice) By definition this lattice JOIN {@code x} for any {@code x}
* must also be {@code x}.
*
- (Optional) Branch Operation: How should the flow branch along edges?
*
*
* To make these behaviors known to the framework, the following steps must be taken.
*
*
* - Flow Direction: Implement {@link #isForward()}.
*
- Lattice Elements: Implement {@link LatticeElement}.
*
- JOIN Operation: Implement {@link JoinOp#apply}.
*
- Flow Equations: Implement {@link #flowThrough(Object, LatticeElement)}.
*
- Initial Entry Value: Implement {@link #createEntryLattice()}.
*
- Initial Estimate: Implement {@link #createInitialEstimateLattice()}.
*
- (Optional) Branch Operation: Return true from {@link #isBranched()} and implement {@link
* #createFlowBrancher}.
*
*
* Upon execution of the {@link #analyze()} method, nodes of the input control flow graph will be
* annotated with a {@link FlowState} object that represents maximum fixed point solution. Any
* previous annotations at the nodes of the control flow graph will be lost.
*
* @param The control flow graph's node value type.
* @param Lattice element type.
*/
abstract class DataFlowAnalysis {
private final ControlFlowGraph cfg;
private final UniqueQueue> workQueue;
/**
* The maximum number of steps per individual CFG node before we assume the analysis is divergent.
*
* TODO(b/196398705): This is way too high. Find traversal ordering heurisitc that reduces it.
*/
public static final int MAX_STEPS_PER_NODE = 20000;
/**
* Constructs a data flow analysis.
*
*
Typical usage
*
*
* DataFlowAnalysis dfa = ...
* dfa.analyze();
*
*
* {@link #analyze()} annotates the result to the control flow graph by means of {@link
* DiGraphNode#setAnnotation} without any modification of the graph itself. Additional calls to
* {@link #analyze()} recomputes the analysis which can be useful if the control flow graph has
* been modified.
*
* @param cfg The control flow graph object that this object performs on. Modification of the
* graph requires a separate call to {@link #analyze()}.
* @see #analyze()
*/
DataFlowAnalysis(ControlFlowGraph cfg) {
this.cfg = cfg;
this.workQueue = new UniqueQueue<>(cfg.getOptionalNodeComparator(isForward()));
if (this.isBranched()) {
checkState(this.isForward());
}
}
/**
* Returns the control flow graph that this analysis was performed on. Modifications can be done
* on this graph, however, the only time that the annotations are correct is after {@link
* #analyze()} is called and before the graph has been modified.
*/
final ControlFlowGraph getCfg() {
return cfg;
}
/**
* Checks whether the analysis is a forward flow analysis or backward flow analysis.
*
* @return {@code true} if it is a forward analysis.
*/
abstract boolean isForward();
/** Whether or not {@link #createFlowBrancher} should be used. */
boolean isBranched() {
return false;
}
final L join(L latticeA, L latticeB) {
FlowJoiner joiner = this.createFlowJoiner();
joiner.joinFlow(latticeA);
joiner.joinFlow(latticeB);
return joiner.finish();
}
/**
* Gets a new joiner for an analysis step.
*
* The joiner is invoked once for each input edge and then the final joined result is
* retrieved. No joiner will be created for a single input.
*/
abstract FlowJoiner createFlowJoiner();
/** A reducer that joins flow states from distinct input states into a single input state. */
interface FlowJoiner {
void joinFlow(L input);
L finish();
}
/**
* Gets a new brancher for an analysis step.
*
* The brancher is invoked for each output edge. The returned states are annotated onto each
* edge and compared against the previous state to determine whether a fixed-point has been
* reached.
*
* @param node The node at which to branch.
* @param output The output state of {@link #flowThrough} at node.
*/
FlowBrancher createFlowBrancher(N node, L output) {
throw new UnsupportedOperationException();
}
/** A callback that branches a flow state into a distinct state for each output edge. */
@FunctionalInterface
interface FlowBrancher {
L branchFlow(Branch branch);
}
/**
* Computes the output state for a given node given its input state.
*
* @param node The node.
* @param input Input lattice that should be read-only.
* @return Output lattice.
*/
abstract L flowThrough(N node, L input);
/**
* Finds a fixed-point solution. The function has the side effect of replacing the existing node
* annotations with the computed solutions using {@link
* com.google.javascript.jscomp.graph.GraphNode#setAnnotation(Annotation)}.
*
* Initially, each node's input and output flow state contains the value given by {@link
* #createInitialEstimateLattice()} (with the exception of the entry node of the graph which takes
* on the {@link #createEntryLattice()} value. Each node will use the output state of its
* predecessor and compute an output state according to the instruction. At that time, any nodes
* that depend on the node's newly modified output value will need to recompute their output state
* again. Each step will perform a computation at one node until no extra computation will modify
* any existing output state anymore.
*/
final void analyze() {
initialize();
while (!this.workQueue.isEmpty()) {
DiGraphNode curNode = this.workQueue.removeFirst();
LinearFlowState curState = curNode.getAnnotation();
if (curState.stepCount++ > MAX_STEPS_PER_NODE) {
throw new IllegalStateException("Dataflow analysis appears to diverge around: " + curNode);
}
joinInputs(curNode);
if (flow(curNode)) {
// If there is a change in the current node, we want to grab the list
// of nodes that this node affects.
List> nextNodes =
isForward() ? cfg.getDirectedSuccNodes(curNode) : cfg.getDirectedPredNodes(curNode);
for (DiGraphNode nextNode : nextNodes) {
if (nextNode != cfg.getImplicitReturn()) {
this.workQueue.add(nextNode);
}
}
}
}
if (isForward()) {
joinInputs(getCfg().getImplicitReturn());
}
}
/**
* Gets the state of the initial estimation at each node.
*
* @return Initial state.
*/
abstract L createInitialEstimateLattice();
/**
* Gets the incoming state of the entry node.
*
* @return Entry state.
*/
abstract L createEntryLattice();
/** Initializes the work list and the control flow graph. */
private void initialize() {
// TODO(user): Calling clear doesn't deallocate the memory in a
// LinkedHashSet. Consider creating a new work set if we plan to repeatedly
// call analyze.
workQueue.clear();
for (DiGraphNode node : cfg.getNodes()) {
node.setAnnotation(
new LinearFlowState<>(createInitialEstimateLattice(), createInitialEstimateLattice()));
if (node != cfg.getImplicitReturn()) {
workQueue.add(node);
}
}
if (this.isBranched()) {
for (DiGraphEdge edge : cfg.getEdges()) {
edge.setAnnotation(this.createInitialEstimateLattice());
}
}
}
/**
* Performs a single flow through a node.
*
* @return {@code true} if the flow state differs from the previous state.
*/
private boolean flow(DiGraphNode node) {
LinearFlowState state = node.getAnnotation();
if (isForward()) {
L outBefore = state.getOut();
state.setOut(flowThrough(node.getValue(), state.getIn()));
boolean changed = !outBefore.equals(state.getOut());
if (this.isBranched()) {
FlowBrancher brancher = this.createFlowBrancher(node.getValue(), state.getOut());
for (DiGraphEdge outEdge : node.getOutEdges()) {
L outBranchBefore = outEdge.getAnnotation();
outEdge.setAnnotation(brancher.branchFlow(outEdge.getValue()));
if (!changed) {
changed = !outBranchBefore.equals(outEdge.getAnnotation());
}
}
}
return changed;
} else {
L inBefore = state.getIn();
state.setIn(flowThrough(node.getValue(), state.getOut()));
return !inBefore.equals(state.getIn());
}
}
/**
* Computes the new flow state at a given node's entry by merging the output (input) lattice of
* the node's predecessor (successor).
*
* @param node Node to compute new join.
*/
private void joinInputs(DiGraphNode node) {
LinearFlowState state = node.getAnnotation();
if (this.isForward() && cfg.getEntry() == node) {
state.setIn(createEntryLattice());
return;
}
List> inEdges =
this.isForward() ? node.getInEdges() : node.getOutEdges();
final L result;
switch (inEdges.size()) {
case 0:
return;
case 1:
result = this.getInputFromEdge(inEdges.get(0));
break;
default:
{
FlowJoiner joiner = this.createFlowJoiner();
for (DiGraphEdge inEdge : inEdges) {
joiner.joinFlow(this.getInputFromEdge(inEdge));
}
result = joiner.finish();
}
}
if (this.isForward()) {
state.setIn(result);
} else {
state.setOut(result);
}
}
private L getInputFromEdge(DiGraphEdge edge) {
if (this.isBranched()) {
return edge.getAnnotation();
} else if (this.isForward()) {
LinearFlowState state = edge.getSource().getAnnotation();
return state.getOut();
} else {
DiGraphNode node = edge.getDestination();
if (node == this.cfg.getImplicitReturn()) {
return this.createEntryLattice();
}
LinearFlowState state = node.getAnnotation();
return state.getIn();
}
}
/** The in and out states of a node. */
static final class LinearFlowState implements Annotation {
private int stepCount = 0;
private L in;
private L out;
private LinearFlowState(L in, L out) {
checkNotNull(in);
checkNotNull(out);
this.in = in;
this.out = out;
}
int getStepCount() {
return this.stepCount;
}
final L getIn() {
return in;
}
private final void setIn(L in) {
checkNotNull(in);
this.in = in;
}
final L getOut() {
return out;
}
private final void setOut(L out) {
checkNotNull(out);
this.out = out;
}
@Override
public final String toString() {
return SimpleFormat.format("IN: %s OUT: %s", in, out);
}
}
/**
* Compute set of escaped variables. When a variable is escaped in a dataflow analysis, it can be
* referenced outside of the code that we are analyzing. A variable is escaped if any of the
* following is true:
*
* 1. Exported variables as they can be needed after the script terminates. 2. Names of named
* functions because in JavaScript, function foo(){} does not kill foo in the dataflow.
*
* @param jsScope Must be a function scope
*/
static void computeEscaped(
final Scope jsScope,
final Set escaped,
AbstractCompiler compiler,
ScopeCreator scopeCreator,
Map allVarsInFn) {
// Optimize for Class Static Blocks.
checkArgument(jsScope.isFunctionScope());
AbstractPostOrderCallback finder =
new AbstractPostOrderCallback() {
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (!n.isName() || parent.isFunction()) {
return;
}
String name = n.getString();
Var var = t.getScope().getVar(name);
if (var == null) {
return;
}
Scope variableCfgScope = var.getScope().getClosestCfgRootScope();
// If the current examined scope and var decleration scope are different then var is
// 'not escaped'.
if (variableCfgScope != jsScope) {
return;
}
Scope referenceCfgScope = t.getScope().getClosestCfgRootScope();
// Variable is referenced outside of decleration scope it is 'escaped'.
if (referenceCfgScope != variableCfgScope) {
escaped.add(var);
}
}
};
NodeTraversal.builder()
.setCompiler(compiler)
.setCallback(finder)
.setScopeCreator(scopeCreator)
.traverseAtScope(jsScope);
// TODO (simranarora) catch variables should not be considered escaped in ES6. Getting rid of
// the catch check is causing breakages however
for (Var var : allVarsInFn.values()) {
if (var.getParentNode().isCatch()
|| compiler.getCodingConvention().isExported(var.getName())) {
escaped.add(var);
}
}
}
private static final class UniqueQueue {
private final LinkedHashSet seenSet = new LinkedHashSet<>();
private final Queue queue;
UniqueQueue(@Nullable Comparator priority) {
this.queue = (priority == null) ? new ArrayDeque<>() : new PriorityQueue<>(priority);
}
boolean isEmpty() {
return this.queue.isEmpty();
}
T removeFirst() {
T t = this.queue.poll();
this.seenSet.remove(t);
return t;
}
void add(T t) {
if (this.seenSet.add(t)) {
this.queue.add(t);
}
}
void clear() {
this.seenSet.clear();
this.queue.clear();
}
}
}