com.google.javascript.jscomp.MaybeReachingVariableUse Maven / Gradle / Ivy
/*
* Copyright 2009 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.collect.HashMultimap;
import com.google.common.collect.Multimap;
import com.google.common.collect.MultimapBuilder;
import com.google.javascript.jscomp.ControlFlowGraph.Branch;
import com.google.javascript.jscomp.MaybeReachingVariableUse.ReachingUses;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.GraphNode;
import com.google.javascript.jscomp.graph.LatticeElement;
import com.google.javascript.rhino.Node;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Set;
/**
* Computes "may be" reaching use for all definitions of each variable.
*
* A use of {@code A} in {@code alert(A)} is a "may be" reaching use of the definition of {@code
* A} at {@code A = foo()} if at least one path from the definition node to the end node reaches
* that use and it is the last definition before the use on that path.
*
*
Example:
*
*
* D1: var A = foo();
* U1: alert(A);
* if(....) {
* D2: A = bar();
* U2: alert(A);
* }
* U3: alert(A);
*
*
*
Here, MaybeReachingUses[D1] = {U1, U3} and MaybeReachingUses[D2]={U2, U3}. The use U3 is not
* guaranteed to use def D1: this is a "may-be" analysis.
*
*
This pass is a backwards-analysis pass, i.e. it traverses the CFG nodes bottom-up, `MAX_STEPS`
* times or till a fixed point solution is reached. At each `cfgNode`, it:
*
*
* - 1. Creates a new output lattice element to store the set of upward exposed variable uses at
* `cfgNode`.
*
- 2. Propagates the set of existing upwards exposed variable uses at `cfgNode` from the input
* lattice to output lattice
*
- 3. Adds new exposed uses of a variable to the upward exposed set in the output lattice
*
- 4. Removes killed(unconditionally redefined) variables from upward exposed set in the
* output lattice
*
*/
class MaybeReachingVariableUse extends DataFlowAnalysis {
// The scope of the function that we are analyzing.
private final Set escaped;
private final Map allVarsInFn;
MaybeReachingVariableUse(
ControlFlowGraph cfg, Set escaped, Map allVarsInFn) {
super(cfg, new ReachingUsesJoinOp());
this.escaped = escaped;
this.allVarsInFn = allVarsInFn;
}
/**
* May use definition lattice representation. It captures a product lattice for each local
* (non-escaped) variable. The sub-lattice is a n + 2 power set element lattice with all the Nodes
* in the program, TOP and BOTTOM. This is better explained with an example:
*
* Consider: A sub-lattice element representing the variable A represented by { N_4, N_5} where
* N_x is a Node in the program. This implies at that particular point in the program the content
* of A is "upward exposed" at point N_4 and N_5.
*
*
Example:
*
*
*
* A = 1;
* ...
* N_3:
* N_4: print(A);
* N_5: y = A;
* N_6: A = 1;
* N_7: print(A);
*
*
*
At N_3, reads of A in {N_4, N_5} are said to be upward exposed.
*/
static final class ReachingUses implements LatticeElement {
// Maps variables to all their uses that are upward exposed at the current cfgNode.
final Multimap mayUseMap;
public ReachingUses() {
mayUseMap = HashMultimap.create();
}
/**
* Copy constructor.
*
* @param other The constructed object is a replicated copy of this element.
*/
public ReachingUses(ReachingUses other) {
mayUseMap = MultimapBuilder.hashKeys().hashSetValues().build(other.mayUseMap);
}
@Override
public boolean equals(Object other) {
return (other instanceof ReachingUses)
&& ((ReachingUses) other).mayUseMap.equals(this.mayUseMap);
}
@Override
public int hashCode() {
return mayUseMap.hashCode();
}
}
/**
* The join is a simple union because of the "may be" nature of the analysis.
*
* Consider: A = 1; if (x) { A = 2 }; alert(A);
*
*
The read of A "may be" exposed to A = 1 in the beginning.
*/
private static class ReachingUsesJoinOp implements JoinOp {
@Override
public ReachingUses apply(List from) {
ReachingUses result = new ReachingUses();
for (ReachingUses uses : from) {
result.mayUseMap.putAll(uses.mayUseMap);
}
return result;
}
}
@Override
boolean isForward() {
return false;
}
@Override
ReachingUses createEntryLattice() {
return new ReachingUses();
}
@Override
ReachingUses createInitialEstimateLattice() {
return new ReachingUses();
}
/**
* Computes the new LatticeElement for a given node given its LatticeElement from previous
* iteration.
*
* @param n node
* @param input - Backward dataflow analyses compute their LatticeElement bottom-up (i.e.
* FlowState.out to FlowState.in). See {@link DataFlowAnalysis#flow(DiGraphNode)}. Here param
* `input` is the readonly input FlowState.out that was constructed as `FlowState.in` in the
* previous iteration, or the initial lattice element if this is the first iteration.
*/
@Override
ReachingUses flowThrough(Node n, ReachingUses input) {
ReachingUses output = new ReachingUses(input);
// If there's an ON_EX edge, this cfgNode may or may not get executed.
// We can express this concisely by just pretending this happens in
// a conditional.
boolean conditional = hasExceptionHandler(n);
computeMayUse(n, n, output, conditional);
return output;
}
private boolean hasExceptionHandler(Node cfgNode) {
List> branchEdges = getCfg().getOutEdges(cfgNode);
for (DiGraphEdge edge : branchEdges) {
if (edge.getValue() == Branch.ON_EX) {
return true;
}
}
return false;
}
/**
* Given a cfgNode, updates the output LatticeElement at that node by finding and storing all
* variables and their uses that are upward exposed at the cfgNode.
*
* @param n The explorer node which searches for variables
* @param cfgNode The CFG node for which the upward exposed variables are being searched.
* @param conditional Whether {@code n} is only conditionally evaluated given that {@code cfgNode}
* is evaluated. Do not remove conditionally redefined variables from the reaching uses set.
*/
private void computeMayUse(Node n, Node cfgNode, ReachingUses output, boolean conditional) {
switch (n.getToken()) {
case BLOCK:
case ROOT:
case FUNCTION:
return;
case NAME:
if (NodeUtil.isLhsByDestructuring(n)) {
if (!conditional) {
removeFromUseIfLocal(n.getString(), output);
}
} else {
addToUseIfLocal(n.getString(), cfgNode, output);
}
return;
case WHILE:
case DO:
case IF:
case FOR:
Node condExpr = NodeUtil.getConditionExpression(n);
computeMayUse(condExpr, cfgNode, output, conditional);
return;
case FOR_IN:
case FOR_OF:
case FOR_AWAIT_OF:
// for(x in y) {...}
Node lhs = n.getFirstChild();
Node rhs = lhs.getNext();
if (NodeUtil.isNameDeclaration(lhs)) {
lhs = lhs.getLastChild(); // for(var x in y) {...}
if (lhs.isDestructuringLhs()) {
lhs = lhs.getFirstChild(); // for (let [x] of obj) {...}
}
}
if (lhs.isName() && !conditional) {
removeFromUseIfLocal(lhs.getString(), output);
} else if (lhs.isDestructuringPattern()) {
computeMayUse(lhs, cfgNode, output, true);
}
computeMayUse(rhs, cfgNode, output, conditional);
return;
case AND:
case OR:
case COALESCE:
case OPTCHAIN_GETPROP:
case OPTCHAIN_GETELEM:
computeMayUse(n.getLastChild(), cfgNode, output, /* conditional= */ true);
computeMayUse(n.getFirstChild(), cfgNode, output, conditional);
return;
case OPTCHAIN_CALL:
// As args are evaluated in AST order, we traverse in reverse AST order for backward
// dataflow analysis.
for (Node c = n.getLastChild(); c != n.getFirstChild(); c = c.getPrevious()) {
computeMayUse(c, cfgNode, output, /* conditional= */ true);
}
computeMayUse(n.getFirstChild(), cfgNode, output, conditional);
return;
case HOOK:
computeMayUse(n.getLastChild(), cfgNode, output, /* conditional= */ true);
computeMayUse(n.getSecondChild(), cfgNode, output, /* conditional= */ true);
computeMayUse(n.getFirstChild(), cfgNode, output, conditional);
return;
case VAR:
case LET:
case CONST:
Node varName = n.getFirstChild();
checkState(n.hasChildren(), "AST should be normalized", n);
if (varName.isDestructuringLhs()) {
// Note: since destructuring is evaluated in reverse AST order, we traverse the first
// child before the second in order to do our backwards data flow analysis.
computeMayUse(varName.getFirstChild(), cfgNode, output, conditional);
computeMayUse(varName.getSecondChild(), cfgNode, output, conditional);
} else if (varName.hasChildren()) {
computeMayUse(varName.getFirstChild(), cfgNode, output, conditional);
if (!conditional) {
removeFromUseIfLocal(varName.getString(), output);
}
} // else var name declaration with no initial value
return;
case DEFAULT_VALUE:
if (n.getFirstChild().isDestructuringPattern()) {
computeMayUse(n.getFirstChild(), cfgNode, output, conditional);
computeMayUse(n.getSecondChild(), cfgNode, output, true);
} else if (n.getFirstChild().isName()) {
// assigning to the name occurs after evaluating the default value
if (!conditional) {
removeFromUseIfLocal(n.getFirstChild().getString(), output);
}
computeMayUse(n.getSecondChild(), cfgNode, output, true);
} else {
computeMayUse(n.getSecondChild(), cfgNode, output, true);
computeMayUse(n.getFirstChild(), cfgNode, output, conditional);
}
break;
default:
if (NodeUtil.isAssignmentOp(n) && n.getFirstChild().isName()) {
Node name = n.getFirstChild();
if (!conditional) {
removeFromUseIfLocal(name.getString(), output);
}
// In case of a += "Hello". There is a read of a.
if (!n.isAssign()) {
addToUseIfLocal(name.getString(), cfgNode, output);
}
computeMayUse(name.getNext(), cfgNode, output, conditional);
} else if (n.isAssign() && n.getFirstChild().isDestructuringPattern()) {
// Note: the rhs of destructuring is evaluated before the lhs
computeMayUse(n.getFirstChild(), cfgNode, output, conditional);
computeMayUse(n.getSecondChild(), cfgNode, output, conditional);
} else {
/*
* We want to traverse in reverse order because we want the LAST
* definition in the sub-tree.
*/
for (Node c = n.getLastChild(); c != null; c = c.getPrevious()) {
computeMayUse(c, cfgNode, output, conditional);
}
}
}
}
/**
* Sets the variable for the given name to the node value in the upward exposed lattice. Do
* nothing if the variable name is one of the escaped variable.
*/
private void addToUseIfLocal(String name, Node node, ReachingUses use) {
Var var = allVarsInFn.get(name);
if (var == null) {
return;
}
if (!escaped.contains(var)) {
use.mayUseMap.put(var, node);
}
}
/**
* Removes the variable for the given name from the node value in the upward exposed lattice. Do
* nothing if the variable name is one of the escaped variable.
*/
private void removeFromUseIfLocal(String name, ReachingUses use) {
Var var = allVarsInFn.get(name);
if (var == null) {
return;
}
if (!escaped.contains(var)) {
use.mayUseMap.removeAll(var);
}
}
/**
* Gets a list of nodes that may be using the value assigned to {@code name} in {@code defNode}.
* {@code defNode} must be one of the control flow graph nodes.
*
* @param name name of the variable. It can only be names of local variable that are not function
* parameters, escaped variables or variables declared in catch.
* @param defNode the control flow graph node that may assign a value to {@code name}
* @return the list of upward exposed uses of the variable {@code name} at defNode.
*/
Collection getUses(String name, Node defNode) {
GraphNode n = getCfg().getNode(defNode);
checkNotNull(n);
FlowState state = n.getAnnotation();
return state.getOut().mayUseMap.get(allVarsInFn.get(name));
}
}