com.google.javascript.jscomp.FlowSensitiveInlineVariables 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 com.google.common.base.Preconditions;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.javascript.jscomp.ControlFlowGraph.AbstractCfgNodeTraversalCallback;
import com.google.javascript.jscomp.ControlFlowGraph.Branch;
import com.google.javascript.jscomp.MustBeReachingVariableDef.Definition;
import com.google.javascript.jscomp.NodeTraversal.AbstractShallowCallback;
import com.google.javascript.jscomp.NodeTraversal.ScopedCallback;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;
import com.google.javascript.rhino.Node;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.Set;
/**
* Inline variables when possible. Using the information from
* {@link MaybeReachingVariableUse} and {@link MustBeReachingVariableDef},
* this pass attempts to inline a variable by placing the value at the
* definition where the variable is used. The basic requirements for inlining
* are the following:
*
*
* - There is exactly one reaching definition at the use of that variable
*
* - There is exactly one use for that definition of the variable
*
*
*
* Other requirements can be found in {@link Candidate#canInline}. Currently
* this pass does not operate on the global scope due to compilation time.
*
*/
class FlowSensitiveInlineVariables implements CompilerPass, ScopedCallback {
/**
* Implementation:
*
* This pass first perform a traversal to gather a list of Candidates that
* could be inlined using {@link GatherCandiates}.
*
* The second step involves verifying that each candidate is actually safe
* to inline with {@link Candidate#canInline(Scope)} and finally perform
* inlining using {@link Candidate#inlineVariable()}.
*
* The reason for the delayed evaluation of the candidates is because we
* need two separate dataflow result.
*/
private final AbstractCompiler compiler;
// These two pieces of data is persistent in the whole execution of enter
// scope.
private ControlFlowGraph cfg;
private Set candidates;
private MustBeReachingVariableDef reachingDef;
private MaybeReachingVariableUse reachingUses;
private static final Predicate SIDE_EFFECT_PREDICATE =
new Predicate() {
@Override
public boolean apply(Node n) {
// When the node is null it means, we reached the implicit return
// where the function returns (possibly without an return statement)
if (n == null) {
return false;
}
// TODO(user): We only care about calls to functions that
// passes one of the dependent variable to a non-side-effect free
// function.
if (n.isCall() && NodeUtil.functionCallHasSideEffects(n)) {
return true;
}
if (n.isNew() && NodeUtil.constructorCallHasSideEffects(n)) {
return true;
}
if (n.isDelProp()) {
return true;
}
for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
if (!ControlFlowGraph.isEnteringNewCfgNode(c) && apply(c)) {
return true;
}
}
return false;
}
};
public FlowSensitiveInlineVariables(AbstractCompiler compiler) {
this.compiler = compiler;
}
@Override
public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
return !n.isScript() || !t.getInput().isExtern();
}
@Override
public void enterScope(NodeTraversal t) {
if (t.inGlobalScope()) {
return; // Don't even brother. All global variables are likely escaped.
}
Preconditions.checkState(t.getScopeRoot().isFunction());
Node scopeRoot = t.getScopeRoot();
if (!isCandidateFunction(scopeRoot)) {
return;
}
if (LiveVariablesAnalysis.MAX_VARIABLES_TO_ANALYZE < t.getScope().getVarCount()) {
return;
}
// Compute the forward reaching definition.
ControlFlowAnalysis cfa = new ControlFlowAnalysis(compiler, false, true);
// Process the body of the function.
cfa.process(null, t.getScopeRoot());
cfg = cfa.getCfg();
reachingDef = new MustBeReachingVariableDef(cfg, t.getScope(), compiler);
reachingDef.analyze();
candidates = new LinkedHashSet<>();
// Using the forward reaching definition search to find all the inline
// candidates
NodeTraversal.traverseEs6(compiler, t.getScopeRoot().getLastChild(), new GatherCandiates());
// Compute the backward reaching use. The CFG can be reused.
reachingUses = new MaybeReachingVariableUse(cfg, t.getScope(), compiler);
reachingUses.analyze();
while (!candidates.isEmpty()) {
Candidate c = candidates.iterator().next();
if (c.canInline(t.getScope())) {
c.inlineVariable();
candidates.remove(c);
// If candidate "c" has dependencies, then inlining it may have introduced new dependencies
// for our other inlining candidates. MustBeReachingVariableDef uses a dependency graph in
// its analysis. Generating a new dependency graph will need another CFG computation.
// Ideally we should iterate to a fixed point, but that can be costly. Therefore, we use
// a conservative heuristic here: For each candidate "other", we back off if its set of
// dependencies cannot contain all of "c"'s dependencies.
if (!c.defMetadata.depends.isEmpty()) {
for (Iterator it = candidates.iterator(); it.hasNext();) {
Candidate other = it.next();
if (other.defMetadata.depends.contains(t.getScope().getVar(c.varName))
&& !other.defMetadata.depends.containsAll(c.defMetadata.depends)) {
it.remove();
}
}
}
} else {
candidates.remove(c);
}
}
}
private boolean isCandidateFunction(Node fn) {
Node fnBody = fn.getLastChild();
return containsCandidateExpressions(fnBody);
}
private static boolean containsCandidateExpressions(Node n) {
if (n.isFunction()) {
// don't recurse into inner functions or into expressions the can't contain declarations.
return false;
}
if (NodeUtil.isNameDeclaration(n) || isAssignmentToName(n)) {
// if it is a simple assignment
if (n.getFirstChild().isName()) {
return true;
}
}
for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
if (containsCandidateExpressions(c)) {
return true;
}
}
return false;
}
private static boolean isAssignmentToName(Node n) {
if (NodeUtil.isAssignmentOp(n) || n.isDec() || n.isInc()) {
// if it is a simple assignment
return (n.getFirstChild().isName());
}
return false;
}
@Override
public void exitScope(NodeTraversal t) {}
@Override
public void process(Node externs, Node root) {
(new NodeTraversal(compiler, this)).traverseRoots(externs, root);
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
// TODO(user): While the helpers do a subtree traversal on the AST, the
// compiler pass itself only traverse the AST to look for function
// declarations to perform dataflow analysis on. We could combine
// the traversal in DataFlowAnalysis's computeEscaped later to save some
// time.
}
private class GatherCandiatesCfgNodeCallback extends AbstractCfgNodeTraversalCallback {
Node cfgNode = null;
public void setCfgNode(Node cfgNode) {
this.cfgNode = cfgNode;
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (n.isName()) {
// n.getParent() isn't null. This just the case where n is the root
// node that gatherCb started at.
if (parent == null) {
return;
}
// Make sure that the name node is purely a read.
if ((NodeUtil.isAssignmentOp(parent) && parent.getFirstChild() == n)
|| parent.isVar() || parent.isInc() || parent.isDec()
|| parent.isParamList() || parent.isCatch()) {
return;
}
String name = n.getString();
if (compiler.getCodingConvention().isExported(name)) {
return;
}
Definition def = reachingDef.getDef(name, cfgNode);
// TODO(nicksantos): We need to add some notion of @const outer
// scope vars. We can inline those just fine.
if (def != null && !reachingDef.dependsOnOuterScopeVars(def)) {
candidates.add(new Candidate(name, def, n, cfgNode));
}
}
}
};
/**
* Gathers a list of possible candidates for inlining based only on
* information from {@link MustBeReachingVariableDef}. The list will be stored
* in {@code candidates} and the validity of each inlining Candidate should
* be later verified with {@link Candidate#canInline(Scope)} when
* {@link MaybeReachingVariableUse} has been performed.
*/
private class GatherCandiates extends AbstractShallowCallback {
final GatherCandiatesCfgNodeCallback gatherCb = new GatherCandiatesCfgNodeCallback();
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
DiGraphNode graphNode = cfg.getDirectedGraphNode(n);
if (graphNode == null) {
// Not a CFG node.
return;
}
final Node cfgNode = n;
gatherCb.setCfgNode(cfgNode);
NodeTraversal.traverseEs6(compiler, cfgNode, gatherCb);
}
}
/**
* Models the connection between a definition and a use of that definition.
*/
private class Candidate {
// Name of the variable.
private final String varName;
// Nodes related to the definition.
private Node def;
private final Definition defMetadata;
// Nodes related to the use.
private final Node use;
private final Node useCfgNode;
// Number of uses of the variable within the current CFG node.
private int numUsesWithinCfgNode;
Candidate(String varName, Definition defMetadata,
Node use, Node useCfgNode) {
Preconditions.checkArgument(use.isName());
this.varName = varName;
this.defMetadata = defMetadata;
this.use = use;
this.useCfgNode = useCfgNode;
}
private Node getDefCfgNode() {
return defMetadata.node;
}
private boolean canInline(final Scope scope) {
// Cannot inline a parameter.
if (getDefCfgNode().isFunction()) {
return false;
}
getDefinition(getDefCfgNode());
getNumUseInUseCfgNode(useCfgNode);
// Definition was not found.
if (def == null) {
return false;
}
// Check that the assignment isn't used as a R-Value.
// TODO(user): Certain cases we can still inline.
if (def.isAssign() && !NodeUtil.isExprAssign(def.getParent())) {
return false;
}
// The right of the definition has side effect:
// Example, for x:
// x = readProp(b), modifyProp(b); print(x);
if (checkRightOf(def, getDefCfgNode(), SIDE_EFFECT_PREDICATE)) {
return false;
}
// Similar check as the above but this time, all the sub-expressions
// left of the use of the variable.
// x = readProp(b); modifyProp(b), print(x);
if (checkLeftOf(use, useCfgNode, SIDE_EFFECT_PREDICATE)) {
return false;
}
// TODO(user): Side-effect is OK sometimes. As long as there are no
// side-effect function down all paths to the use. Once we have all the
// side-effect analysis tool.
if (NodeUtil.mayHaveSideEffects(def.getLastChild(), compiler)) {
return false;
}
// TODO(user): We could inline all the uses if the expression is short.
// Finally we have to make sure that there are no more than one use
// in the program and in the CFG node. Even when it is semantically
// correctly inlining twice increases code size.
if (numUsesWithinCfgNode != 1) {
return false;
}
// Make sure that the name is not within a loop
if (NodeUtil.isWithinLoop(use)) {
return false;
}
Collection uses = reachingUses.getUses(varName, getDefCfgNode());
if (uses.size() != 1) {
return false;
}
// We give up inlining stuff with R-Value that has:
// 1) GETPROP, GETELEM,
// 2) anything that creates a new object.
// 3) a direct reference to a catch expression.
// Example:
// var x = a.b.c; j.c = 1; print(x);
// Inlining print(a.b.c) is not safe consider j and be alias to a.b.
// TODO(user): We could get more accuracy by looking more in-detail
// what j is and what x is trying to into to.
// TODO(johnlenz): rework catch expression handling when we
// have lexical scope support so catch expressions don't
// need to be special cased.
if (NodeUtil.has(
def.getLastChild(),
new Predicate() {
@Override
public boolean apply(Node input) {
switch (input.getToken()) {
case GETELEM:
case GETPROP:
case ARRAYLIT:
case OBJECTLIT:
case REGEXP:
case NEW:
return true;
case NAME:
Var var = scope.getOwnSlot(input.getString());
if (var != null && var.getParentNode().isCatch()) {
return true;
}
default:
break;
}
return false;
}
},
new Predicate() {
@Override
public boolean apply(Node input) {
// Recurse if the node is not a function.
return !input.isFunction();
}
})) {
return false;
}
// We can skip the side effect check along the paths of two nodes if
// they are just next to each other.
if (NodeUtil.isStatementBlock(getDefCfgNode().getParent()) &&
getDefCfgNode().getNext() != useCfgNode) {
// Similar side effect check as above but this time the side effect is
// else where along the path.
// x = readProp(b); while(modifyProp(b)) {}; print(x);
CheckPathsBetweenNodes
pathCheck = new CheckPathsBetweenNodes<>(
cfg,
cfg.getDirectedGraphNode(getDefCfgNode()),
cfg.getDirectedGraphNode(useCfgNode),
SIDE_EFFECT_PREDICATE,
Predicates.
>alwaysTrue(),
false);
if (pathCheck.somePathsSatisfyPredicate()) {
return false;
}
}
return true;
}
/**
* Actual transformation.
*/
private void inlineVariable() {
Node defParent = def.getParent();
Node useParent = use.getParent();
if (def.isAssign()) {
Node rhs = def.getLastChild();
rhs.detach();
// Oh yes! I have grandparent to remove this.
Preconditions.checkState(defParent.isExprResult());
while (defParent.getParent().isLabel()) {
defParent = defParent.getParent();
}
defParent.detach();
useParent.replaceChild(use, rhs);
} else if (defParent.isVar()) {
Node rhs = def.getLastChild();
def.removeChild(rhs);
useParent.replaceChild(use, rhs);
} else {
throw new IllegalStateException("No other definitions can be inlined.");
}
compiler.reportCodeChange();
}
/**
* Set the def node
*
* @param n A node that has a corresponding CFG node in the CFG.
*/
private void getDefinition(Node n) {
AbstractCfgNodeTraversalCallback gatherCb =
new AbstractCfgNodeTraversalCallback() {
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
switch (n.getToken()) {
case NAME:
if (n.getString().equals(varName) && n.hasChildren()) {
def = n;
}
return;
case ASSIGN:
Node lhs = n.getFirstChild();
if (lhs.isName() && lhs.getString().equals(varName)) {
def = n;
}
return;
default:
break;
}
}
};
NodeTraversal.traverseEs6(compiler, n, gatherCb);
}
/**
* Computes the number of uses of the variable varName and store it in
* numUseWithinUseCfgNode.
*/
private void getNumUseInUseCfgNode(final Node cfgNode) {
numUsesWithinCfgNode = 0;
AbstractCfgNodeTraversalCallback gatherCb =
new AbstractCfgNodeTraversalCallback() {
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (n.isName() && n.getString().equals(varName)) {
// We make a special exception when the entire cfgNode is a chain
// of assignments, since in that case the assignment statements
// will happen after the inlining of the right hand side.
// TODO(blickly): Make the SIDE_EFFECT_PREDICATE check more exact
// and remove this special case.
if (parent.isAssign() && (parent.getFirstChild() == n)
&& isAssignChain(parent, cfgNode)) {
// Don't count lhs of top-level assignment chain
return;
} else {
numUsesWithinCfgNode++;
}
}
}
private boolean isAssignChain(Node child, Node ancestor) {
for (Node n = child; n != ancestor; n = n.getParent()) {
if (!n.isAssign()) {
return false;
}
}
return true;
}
};
NodeTraversal.traverseEs6(compiler, cfgNode, gatherCb);
}
}
/**
* Given an expression by its root and sub-expression n, return true if there
* the predicate is true for some expression on the right of n.
*
* Example:
*
* NotChecked(), NotChecked(), n, Checked(), Checked();
*/
private static boolean checkRightOf(
Node n, Node expressionRoot, Predicate predicate) {
for (Node p = n; p != expressionRoot; p = p.getParent()) {
for (Node cur = p.getNext(); cur != null; cur = cur.getNext()) {
if (predicate.apply(cur)) {
return true;
}
}
}
return false;
}
/**
* Given an expression by its root and sub-expression n, return true if there
* the predicate is true for some expression on the left of n.
*
* Example:
*
* Checked(), Checked(), n, NotChecked(), NotChecked();
*/
private static boolean checkLeftOf(
Node n, Node expressionRoot, Predicate predicate) {
for (Node p = n; p != expressionRoot; p = p.getParent()) {
for (Node cur = p.getParent().getFirstChild(); cur != p;
cur = cur.getNext()) {
if (predicate.apply(cur)) {
return true;
}
}
}
return false;
}
}