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Closure Compiler is a JavaScript optimizing compiler. It parses your
JavaScript, analyzes it, removes dead code and rewrites and minimizes
what's left. It also checks syntax, variable references, and types, and
warns about common JavaScript pitfalls. It is used in many of Google's
JavaScript apps, including Gmail, Google Web Search, Google Maps, and
Google Docs.
This binary checks for style issues such as incorrect or missing JSDoc
usage, and missing goog.require() statements. It does not do more advanced
checks such as typechecking.
/*
* 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.MoreObjects.toStringHelper;
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 static com.google.javascript.jscomp.Es6ToEs3Util.withType;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.common.base.Supplier;
import com.google.javascript.jscomp.MakeDeclaredNamesUnique.ContextualRenamer;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import com.google.javascript.rhino.jstype.JSType;
import com.google.javascript.rhino.jstype.JSTypeNative;
import java.util.Set;
import javax.annotation.Nullable;
/**
* Partially or fully decomposes an expression with respect to some sub-expresison. Initially this
* is intended to expand the locations where inlining can occur, but has other uses as well.
*
* For example: `var x = y() + z();` becomes `var a = y(); var b = z(); var x = a + b;`.
*
*
Decomposing, in this context does not mean full decomposition to "atomic" expressions. While
* it is possible to iteratively apply docomposition to get statements with at most one side-effect,
* that isn't the intended purpose of this class. The focus is on decomposing "just enough" to
* "free" a particular subexpression. For example:
*
*
* - Given: `return (alert() + alert()) + z();`
*
- Exposing: `z()`
*
- Sufficent decomposition: `var temp = alert() + alert(); return temp + z();`
*
*/
class ExpressionDecomposer {
/** @see {@link #canExposeExpression} */
enum DecompositionType {
UNDECOMPOSABLE,
MOVABLE,
DECOMPOSABLE
}
private final AbstractCompiler compiler;
private final AstAnalyzer astAnalyzer;
private final AstFactory astFactory;
private final Supplier safeNameIdSupplier;
private final Set knownConstants;
private final Scope scope;
private final JSType unknownType;
private final JSType voidType;
private final JSType stringType;
/**
* Whether to allow decomposing foo.bar to "var fn = foo.bar; fn.call(foo);" Should be false if
* targeting IE8 or IE9.
*/
private final boolean allowMethodCallDecomposing;
ExpressionDecomposer(
AbstractCompiler compiler,
Supplier safeNameIdSupplier,
Set constNames,
Scope scope,
boolean allowMethodCallDecomposing) {
checkNotNull(compiler);
checkNotNull(safeNameIdSupplier);
checkNotNull(constNames);
this.compiler = compiler;
this.astAnalyzer = compiler.getAstAnalyzer();
this.astFactory = compiler.createAstFactory();
this.safeNameIdSupplier = safeNameIdSupplier;
this.knownConstants = constNames;
this.scope = scope;
this.allowMethodCallDecomposing = allowMethodCallDecomposing;
this.unknownType = compiler.getTypeRegistry().getNativeType(JSTypeNative.UNKNOWN_TYPE);
this.voidType = compiler.getTypeRegistry().getNativeType(JSTypeNative.VOID_TYPE);
this.stringType = compiler.getTypeRegistry().getNativeType(JSTypeNative.STRING_TYPE);
}
// An arbitrary limit to prevent catch infinite recursion.
private static final int MAX_ITERATIONS = 100;
/**
* If required, rewrite the statement containing the expression.
*
* @param expression The expression to be exposed.
* @see #canExposeExpression
*/
void maybeExposeExpression(Node expression) {
// If the expression needs to exposed.
int i = 0;
while (DecompositionType.DECOMPOSABLE == canExposeExpression(expression)) {
exposeExpression(expression);
i++;
if (i > MAX_ITERATIONS) {
throw new IllegalStateException(
"DecomposeExpression depth exceeded on:\n" + expression.toStringTree());
}
}
}
/**
* Perform any rewriting necessary so that the specified expression is {@code MOVABLE}.
*
* This method is a primary entrypoint into this class. It performs a partial expression
* decomposition such that {@code expression} can be moved to a preceding statement without
* changing behaviour.
*
*
Exposing {@code expression} generally doesn't mean that {@code expression} itself will
* moved. An expression is exposed within a larger statement if no preceding expression would
* interact with it.
*
* @see {@link #canExposeExpression}
*/
void exposeExpression(Node expression) {
Node expressionRoot = findExpressionRoot(expression);
checkNotNull(expressionRoot);
checkState(NodeUtil.isStatement(expressionRoot), expressionRoot);
exposeExpression(expressionRoot, expression);
}
/**
* Rewrite {@code expressionRoot} such that {@code subExpression} is a {@code MOVABLE} while
* maintaining evaluation order.
*
*
Two types of subexpressions are extracted from the source expression:
*
*
* - subexpressions with side-effects
*
- conditional expressions that contain {@code subExpression}, which are transformed into IF
* statements.
*
*
* The following terms are used:
*
*
* - expressionRoot: The top-level node, before which the any extracted expressions should be
* placed.
*
- nonconditionalExpr: The node that will be extracted either from expression.
*
*
* @param expressionRoot The root of the subtree within which to expose {@code subExpression}.
* @param subExpression A descendent of {@code expressionRoot} to be exposed.
*/
private void exposeExpression(Node expressionRoot, Node subExpression) {
Node nonconditionalExpr = findNonconditionalParent(subExpression, expressionRoot);
// Before extraction, record whether there are side-effect
boolean hasFollowingSideEffects = astAnalyzer.mayHaveSideEffects(nonconditionalExpr);
Node exprInjectionPoint = findInjectionPoint(nonconditionalExpr);
DecompositionState state = new DecompositionState();
state.sideEffects = hasFollowingSideEffects;
state.extractBeforeStatement = exprInjectionPoint;
// Extract expressions in the reverse order of their evaluation. This is roughly, traverse up
// the AST extracting any preceding expressions that may have side-effects or be side-effected.
Node lastExposedSubexpression = null;
Node expressionToExpose = nonconditionalExpr;
Node expressionParent = expressionToExpose.getParent();
while (expressionParent != expressionRoot) {
checkState(
!isConditionalOp(expressionParent) || expressionToExpose.isFirstChildOf(expressionParent),
expressionParent);
if (expressionParent.isAssign()) {
if (isSafeAssign(expressionParent, state.sideEffects)) {
// It is always safe to inline "foo()" for expressions such as
// "a = b = c = foo();"
// As the assignment is unaffected by side effect of "foo()"
// and the names assigned-to can not influence the state before
// the call to foo.
//
// This is not true of more complex LHS values, such as
// a.x = foo();
// next().x = foo();
// in these cases the checks below are necessary.
} else if (!expressionToExpose.isFirstChildOf(expressionParent)) {
// Alias "next()" in "next().foo"
Node left = expressionParent.getFirstChild();
switch (left.getToken()) {
case GETELEM:
decomposeSubExpressions(left.getLastChild(), null, state);
// Fall through.
case GETPROP:
decomposeSubExpressions(left.getFirstChild(), null, state);
break;
default:
throw new IllegalStateException("Expected a property access: " + left.toStringTree());
}
}
} else if (expressionParent.isCall() && NodeUtil.isGet(expressionParent.getFirstChild())) {
Node callee = expressionParent.getFirstChild();
decomposeSubExpressions(callee.getNext(), expressionToExpose, state);
// Now handle the call expression. We only have to do this if we arrived at decomposing this
// call through one of the arguments, rather than the callee; otherwise the callee would
// already be safe.
if (isExpressionTreeUnsafe(callee, state.sideEffects)
&& lastExposedSubexpression != callee.getFirstChild()) {
checkState(allowMethodCallDecomposing, "Object method calls can not be decomposed.");
// Either there were preexisting side-effects, or this node has side-effects.
state.sideEffects = true;
// Rewrite the call so "this" is preserved and continue walking up from there.
expressionParent = rewriteCallExpression(expressionParent, state);
}
} else {
decomposeSubExpressions(expressionParent.getFirstChild(), expressionToExpose, state);
}
lastExposedSubexpression = expressionToExpose;
expressionToExpose = expressionParent;
expressionParent = expressionToExpose.getParent();
}
// Now extract the expression that the decomposition is being performed to
// to allow to be moved. All expressions that need to be evaluated before
// this have been extracted, so add the expression statement after the
// other extracted expressions and the original statement (or replace
// the original statement.
if (nonconditionalExpr == subExpression) {
// Don't extract the call, as that introduces an extra constant VAR
// that will simply need to be inlined back. It will be handled as
// an EXPRESSION call site type.
// Node extractedCall = extractExpression(decomposition, expressionRoot);
} else {
Node parent = nonconditionalExpr.getParent();
boolean needResult = !parent.isExprResult();
extractConditional(nonconditionalExpr, exprInjectionPoint, needResult);
}
}
/**
* Extract the specified expression from its parent expression.
*
* @see #canExposeExpression
*/
void moveExpression(Node expression) {
// TODO(johnlenz): This is not currently used by the function inliner,
// as moving the call out of the expression before the actual function call
// causes additional variables to be introduced. As the variable
// inliner is improved, this might be a viable option.
String resultName = getResultValueName();
Node injectionPoint = findInjectionPoint(expression);
checkNotNull(injectionPoint);
Node injectionPointParent = injectionPoint.getParent();
checkNotNull(injectionPointParent);
checkState(NodeUtil.isStatementBlock(injectionPointParent));
// Replace the expression with a reference to the new name.
Node expressionParent = expression.getParent();
expressionParent.replaceChild(
expression, withType(IR.name(resultName), expression.getJSType()));
// Re-add the expression at the appropriate place.
Node newExpressionRoot = NodeUtil.newVarNode(resultName, expression);
newExpressionRoot.getFirstChild().setJSType(expression.getJSType());
injectionPointParent.addChildBefore(newExpressionRoot, injectionPoint);
compiler.reportChangeToEnclosingScope(injectionPointParent);
}
/**
* @return "expression" or the node closest to "expression", that does not have a conditional
* ancestor.
*/
private static Node findNonconditionalParent(Node subExpression, Node expressionRoot) {
Node result = subExpression;
for (Node child = subExpression, parent = child.getParent();
parent != expressionRoot;
child = parent, parent = child.getParent()) {
if (isConditionalOp(parent) && !child.isFirstChildOf(parent)) {
// Only the first child is always executed, if the function may never
// be called, don't inline it.
result = parent;
}
}
return result;
}
/**
* A simple class to track two things: - whether side effects have been seen. - the last statement
* inserted
*/
private static class DecompositionState {
boolean sideEffects;
Node extractBeforeStatement;
@Override
public String toString() {
return toStringHelper(this)
.add("sideEffects", sideEffects)
.add("extractBeforeStatement", extractBeforeStatement)
.toString();
}
}
/**
* @param n The node with which to start iterating.
* @param stopNode A node after which to stop iterating.
*/
private void decomposeSubExpressions(Node n, Node stopNode, DecompositionState state) {
if (n == null || n == stopNode) {
return;
}
// Decompose the children in reverse evaluation order. This simplifies determining if any of
// the children following have side-effects. If they do we need to be more aggressive about
// removing values from the expression. Reverse order also maintains evaluation order as each
// extracted statemented is inserted on top of the others.
decomposeSubExpressions(n.getNext(), stopNode, state);
// Now this node.
if (NodeUtil.mayBeObjectLitKey(n)
// TODO(b/111621528): Delete when fixed.
|| n.isComputedProp()) {
if (n.isComputedProp()) {
// If the prop is computed we have to fork the decomposition between the key and value. This
// is because we can't move the property assignment itself; COMPUTED_PROP must remain a
// child of OBJECTLIT for example.
//
// We decompose the value of the prop first because decomposition is in reverse order of
// evaluation.
decomposeSubExpressions(n.getSecondChild(), stopNode, state);
}
// Decompose the children of the prop rather than the prop itself. In the computed case this
// will be the key, otherwise it will be the value.
n = n.getFirstChild();
} else if (n.isTemplateLitSub()) {
// A template literal substitution expression like ${f()} is represented in the AST as
// TEMPLATELIT_SUB
// CALL
// NAME f
// The TEMPLATELIT_SUB node is not actually an expression and can't be extracted, but we may
// need to extract the expression inside of it.
n = n.getFirstChild();
} else if (n.isSpread()) {
// SPREADs aren't expression but they can still be extracted using temp variables.
//
// Because object-spread can trigger getters we assume all spreads have side-effects.
// TODO(nickreid): Use `assumeGettersArePure` here. It would have been a pain to pipe it down
// here and write all the tests. Since there are very few cases, and it doesn't affect code
// removal, we didn't bother initially. Everything always works one way.
} else if (!IR.mayBeExpression(n)) {
// If n is not an expression then it can't be extracted. For example if n is the destructuring
// pattern on the left side of a VAR statement:
// var {pattern} = rhs();
// See test case: testExposeExpression18
return;
}
// TODO(johnlenz): Move "safety" code to a shared class.
if (isExpressionTreeUnsafe(n, state.sideEffects)) {
// Either there were preexisting side-effects, or this node has side-effects.
state.sideEffects = true;
state.extractBeforeStatement = extractExpression(n, state.extractBeforeStatement);
}
}
/**
* @param expr The conditional expression to extract.
* @param injectionPoint The before which extracted expression, would be injected.
* @param needResult Whether the result of the expression is required.
* @return The node that contains the logic of the expression after extraction.
*/
private Node extractConditional(Node expr, Node injectionPoint, boolean needResult) {
Node parent = expr.getParent();
String tempName = getTempValueName();
// Break down the conditional.
Node first = expr.getFirstChild();
Node second = first.getNext();
Node last = expr.getLastChild();
// Isolate the children nodes.
expr.detachChildren();
// Transform the conditional to an IF statement.
Node cond = null;
Node trueExpr = astFactory.createBlock().srcref(expr);
Node falseExpr = astFactory.createBlock().srcref(expr);
switch (expr.getToken()) {
case HOOK:
// a = x?y:z --> if (x) {a=y} else {a=z}
cond = first;
trueExpr.addChildToFront(
astFactory.exprResult(buildResultExpression(second, needResult, tempName)));
falseExpr.addChildToFront(
astFactory.exprResult(buildResultExpression(last, needResult, tempName)));
break;
case AND:
// a = x&&y --> if (a=x) {a=y} else {}
cond = buildResultExpression(first, needResult, tempName);
trueExpr.addChildToFront(
astFactory.exprResult(buildResultExpression(last, needResult, tempName)));
break;
case OR:
// a = x||y --> if (a=x) {} else {a=y}
cond = buildResultExpression(first, needResult, tempName);
falseExpr.addChildToFront(
astFactory.exprResult(buildResultExpression(last, needResult, tempName)));
break;
case COALESCE:
// a = x ?? y --> if ((temp=x)!=null) {a=temp} else {a=y}
String tempNameAssign = getTempValueName();
Node tempVarNodeAssign =
astFactory
.createSingleVarNameDeclaration(tempNameAssign)
.useSourceInfoIfMissingFromForTree(expr);
Node injectionPointParent = injectionPoint.getParent();
injectionPointParent.addChildBefore(tempVarNodeAssign, injectionPoint);
Node assignLhs = buildResultExpression(first, true, tempNameAssign);
Node nullNode = astFactory.createNull().useSourceInfoFrom(expr);
cond = astFactory.createNe(assignLhs, nullNode).useSourceInfoFrom(expr);
trueExpr.addChildToFront(
astFactory.exprResult(
buildResultExpression(
astFactory
.createName(tempNameAssign, first.getJSType())
.useSourceInfoFrom(expr),
needResult,
tempName)));
falseExpr.addChildToFront(
astFactory.exprResult(buildResultExpression(last, needResult, tempName)));
break;
default:
// With a valid tree we should never get here.
throw new IllegalStateException("Unexpected expression: " + expr);
}
Node ifNode;
if (falseExpr.hasChildren()) {
ifNode = astFactory.createIf(cond, trueExpr, falseExpr);
} else {
ifNode = astFactory.createIf(cond, trueExpr);
}
ifNode.useSourceInfoIfMissingFrom(expr);
if (needResult) {
Node tempVarNode =
astFactory
.createSingleVarNameDeclaration(tempName)
.useSourceInfoIfMissingFromForTree(expr);
Node injectionPointParent = injectionPoint.getParent();
injectionPointParent.addChildBefore(tempVarNode, injectionPoint);
injectionPointParent.addChildAfter(ifNode, tempVarNode);
// Replace the expression with the temporary name.
Node replacementValueNode = withType(IR.name(tempName), expr.getJSType());
parent.replaceChild(expr, replacementValueNode);
} else {
// Only conditionals that are the direct child of an expression statement
// don't need results, for those simply replace the expression statement.
checkArgument(parent.isExprResult());
Node grandparent = parent.getParent();
grandparent.replaceChild(parent, ifNode);
}
return ifNode;
}
/**
* Create an expression tree for an expression.
*
* If the result of the expression is needed, then:
*
*
* ASSIGN
* tempName
* expr
*
*
* otherwise, simply: `expr`
*/
private static Node buildResultExpression(Node expr, boolean needResult, String tempName) {
if (needResult) {
JSType type = expr.getJSType();
return withType(IR.assign(withType(IR.name(tempName), type), expr), type).srcrefTree(expr);
} else {
return expr;
}
}
private boolean isConstantNameNode(Node n) {
// Non-constant names values may have been changed.
return n.isName()
&& (NodeUtil.isConstantVar(n, scope) || knownConstants.contains(n.getString()));
}
/**
* @param expr The expression to extract.
* @param injectionPoint The node before which to added the extracted expression.
* @return The extracted statement node.
*/
private Node extractExpression(Node expr, Node injectionPoint) {
Node parent = expr.getParent();
boolean isLhsOfAssignOp =
NodeUtil.isAssignmentOp(parent) && !parent.isAssign() && expr.isFirstChildOf(parent);
Node firstExtractedNode = null;
// Expressions on the LHS of an assignment-op must have any possible
// side-effects extracted as the value must be duplicated:
// next().foo += 2;
// becomes:
// var t1 = next();
// t1.foo = t1.foo + 2;
if (isLhsOfAssignOp && NodeUtil.isGet(expr)) {
for (Node n : expr.children()) {
if (!n.isString() && !isConstantNameNode(n)) {
Node extractedNode = extractExpression(n, injectionPoint);
if (firstExtractedNode == null) {
firstExtractedNode = extractedNode;
}
}
}
}
// The temp is known to be constant.
String tempName = getTempConstantValueName();
Node replacementValueNode = IR.name(tempName).setJSType(expr.getJSType()).srcref(expr);
Node tempNameValue;
// If it is ASSIGN_XXX, keep the assignment in place and extract the
// original value of the LHS operand.
if (isLhsOfAssignOp) {
checkState(expr.isName() || NodeUtil.isGet(expr), expr);
// Transform "x += 2" into "x = temp + 2"
Node opNode =
withType(new Node(NodeUtil.getOpFromAssignmentOp(parent)), parent.getJSType())
.useSourceInfoIfMissingFrom(parent);
Node rightOperand = parent.getLastChild();
parent.setToken(Token.ASSIGN);
parent.replaceChild(rightOperand, opNode);
opNode.addChildToFront(replacementValueNode);
opNode.addChildToBack(rightOperand);
// The original expression is still being used, so make a clone.
tempNameValue = expr.cloneTree();
} else if (expr.isSpread()) {
// We need to treat spreads differently because unlike other expressions, they can't be
// directly assigned to new variables. Instead we wrap them in a literal.
//
// We make sure to do `var tmp = [...fn()];` rather than `var tmp = fn()` because the
// execution of a spread on an arbitrary iterable/object can both have side-effects and be
// side-effected. However, once done we are then sure that spreading `tmp` is isolated.
// Replace the expression with the spread for the temporary name.
Node spreadCopy = expr.cloneNode();
spreadCopy.addChildToBack(replacementValueNode);
expr.replaceWith(spreadCopy);
// Move the original node into a legal context.
switch (parent.getToken()) {
case ARRAYLIT:
case CALL:
case NEW:
tempNameValue = astFactory.createArraylit(expr).useSourceInfoFrom(expr.getOnlyChild());
break;
case OBJECTLIT:
tempNameValue = astFactory.createObjectLit(expr).useSourceInfoFrom(expr.getOnlyChild());
break;
default:
throw new IllegalStateException("Unexpected parent of SPREAD:" + parent.toStringTree());
}
} else {
// Replace the expression with the temporary name.
parent.replaceChild(expr, replacementValueNode);
// Keep the original node so that CALL expressions can still be found
// and inlined properly.
tempNameValue = expr;
}
// Re-add the expression in the declaration of the temporary name.
Node tempVarNode = NodeUtil.newVarNode(tempName, tempNameValue);
tempVarNode.getFirstChild().setJSType(tempNameValue.getJSType());
Node injectionPointParent = injectionPoint.getParent();
injectionPointParent.addChildBefore(tempVarNode, injectionPoint);
if (firstExtractedNode == null) {
firstExtractedNode = tempVarNode;
}
checkState(firstExtractedNode.isVar());
return firstExtractedNode;
}
/**
* Rewrite the call so "this" is preserved.
*
* a.b(c);
*
* becomes:
*
*
* var temp1 = a; var temp0 = temp1.b;
* temp0.call(temp1,c);
*
*
* @return The replacement node.
*/
private Node rewriteCallExpression(Node call, DecompositionState state) {
checkArgument(call.isCall(), call);
Node first = call.getFirstChild();
checkArgument(NodeUtil.isGet(first), first);
// Find the type of (fn expression).call
JSType fnType = first.getJSType();
JSType fnCallType = null;
if (fnType != null) {
fnCallType =
fnType.isFunctionType()
? fnType.toMaybeFunctionType().getPropertyType("call")
: unknownType;
}
// Extracts the expression representing the function to call. For example:
// "a['b'].c" from "a['b'].c()"
Node getVarNode = extractExpression(first, state.extractBeforeStatement);
state.extractBeforeStatement = getVarNode;
// Extracts the object reference to be used as "this". For example:
// "a['b']" from "a['b'].c"
Node getExprNode = getVarNode.getFirstFirstChild();
checkArgument(NodeUtil.isGet(getExprNode), getExprNode);
Node thisVarNode = extractExpression(getExprNode.getFirstChild(), state.extractBeforeStatement);
state.extractBeforeStatement = thisVarNode;
// Rewrite the CALL expression.
Node thisNameNode = thisVarNode.getFirstChild();
Node functionNameNode = getVarNode.getFirstChild();
// CALL
// GETPROP
// functionName
// "call"
// thisName
// original-parameter1
// original-parameter2
// ...
Node newCall =
IR.call(
withType(
IR.getprop(
functionNameNode.cloneNode(), withType(IR.string("call"), stringType)),
fnCallType),
thisNameNode.cloneNode())
.setJSType(call.getJSType())
.useSourceInfoIfMissingFromForTree(call);
// Throw away the call name
call.removeFirstChild();
if (call.hasChildren()) {
// Add the call parameters to the new call.
newCall.addChildrenToBack(call.removeChildren());
}
call.replaceWith(newCall);
return newCall;
}
private String tempNamePrefix = "JSCompiler_temp";
private String resultNamePrefix = "JSCompiler_inline_result";
/** Allow the temp name to be overridden to make tests more readable. */
@VisibleForTesting
public void setTempNamePrefix(String prefix) {
this.tempNamePrefix = prefix;
}
/** Create a unique temp name. */
private String getTempValueName() {
return tempNamePrefix + ContextualRenamer.UNIQUE_ID_SEPARATOR + safeNameIdSupplier.get();
}
/** Allow the temp name to be overridden to make tests more readable. */
@VisibleForTesting
public void setResultNamePrefix(String prefix) {
this.resultNamePrefix = prefix;
}
/** Create a unique name for call results. */
private String getResultValueName() {
return resultNamePrefix + ContextualRenamer.UNIQUE_ID_SEPARATOR + safeNameIdSupplier.get();
}
/** Create a constant unique temp name. */
private String getTempConstantValueName() {
String name =
tempNamePrefix
+ "_const"
+ ContextualRenamer.UNIQUE_ID_SEPARATOR
+ safeNameIdSupplier.get();
this.knownConstants.add(name);
return name;
}
private boolean isTempConstantValueName(Node name) {
return name.isName()
&& name.getString()
.startsWith(tempNamePrefix + "_const" + ContextualRenamer.UNIQUE_ID_SEPARATOR);
}
/**
* @return For the subExpression, find the nearest statement Node before which it can be inlined.
* Null if no such location can be found.
*/
@Nullable
static Node findInjectionPoint(Node subExpression) {
Node expressionRoot = findExpressionRoot(subExpression);
checkNotNull(expressionRoot);
Node injectionPoint = expressionRoot;
Node parent = injectionPoint.getParent();
while (parent.isLabel()) {
injectionPoint = parent;
parent = injectionPoint.getParent();
}
checkState(NodeUtil.isStatementBlock(parent), parent);
return injectionPoint;
}
/** @return Whether the node is a conditional op. */
private static boolean isConditionalOp(Node n) {
switch (n.getToken()) {
case HOOK:
case AND:
case OR:
case COALESCE:
return true;
default:
return false;
}
}
/**
* Finds the statement containing {@code subExpression}.
*
* If {@code subExpression} is not contained by a statement where inlining is known to be
* possible, {@code null} is returned. For example, the condition expression of a WHILE loop.
*/
@Nullable
private static Node findExpressionRoot(Node subExpression) {
Node child = subExpression;
for (Node current : child.getAncestors()) {
Node parent = current.getParent();
switch (current.getToken()) {
// Supported expression roots:
// SWITCH and IF can have multiple children, but the CASE, DEFAULT,
// or BLOCK will be encountered first for any of the children other
// than the condition.
case EXPR_RESULT:
case IF:
case SWITCH:
case RETURN:
case THROW:
Preconditions.checkState(child.isFirstChildOf(current));
return current;
case VAR:
// Normalization will remove LABELs from VARs.
case LET:
case CONST:
if (NodeUtil.isAnyFor(parent)) {
break; // Name declarations may not be roots if they're for-loop initializers.
}
return current;
// Any of these indicate an unsupported expression:
case FOR:
if (child.isFirstChildOf(current)) {
// Only the initializer of a for-loop could possibly be decomposed since the other
// statements need to execute each iteration.
return current;
}
// fall through
case FOR_IN:
case FOR_OF:
case FOR_AWAIT_OF:
case DO:
case WHILE:
case SCRIPT:
case BLOCK:
case LABEL:
case CASE:
case DEFAULT_CASE:
case DEFAULT_VALUE:
case PARAM_LIST:
return null;
default:
break;
}
child = current;
}
throw new IllegalStateException("Unexpected AST structure.");
}
/**
* Determines if {@code subExpression} can be moved before {@code expressionRoot} without changing
* the behaviour of the code, or if there is a rewriting that would make such motion possible.
*
*
Walks the AST from {@code subExpression} to {@code expressionRoot} and verifies that the
* portions of the {@code expressionRoot} subtree that are evaluated before {@code subExpression}:
*
*
* - are unaffected by the side-effects, if any, of the {@code subExpression}.
*
- have no side-effects that may influence the {@code subExpression}.
*
- have a syntactically legal rewriting.
*
*
* Examples:
*
*
*
* - {@code expressionRoot} = `a = 1 + x();`
*
- {@code subExpression} = `x()`, has side-effects
*
- {@code MOVABLE} because the final value of `a` can not be influenced by `x()`.
*
*
* - {@code expressionRoot} = `a = b + x();`
*
- {@code subExpression} = `x()`, has side-effects
*
- {@code DECOMPOSABLE} because `b` may be modified by `x()`, but `b` can be cached.
*
*
* - {@code expressionRoot} = `a = b + x();`
*
- {@code subExpression} = `x()`, no side-effects
*
- {@code MOVABLE} because `x()` can be computed before or after `b` is resolved.
*
*
* - {@code expressionRoot} = `a = (b = c) + x();`
*
- {@code subExpression} = `x()`, no side-effects, is side-effected
*
- {@code DECOMPOSABLE} because `x()` may read `b`.
*
*
*
* @return
*
* - {@code MOVABLE} if {@code subExpression} can already be moved.
*
- {@code DECOMPOSABLE} if the {@code expressionRoot} subtree could be rewritten such
* that {@code subExpression} would be made movable.
*
- {@code UNDECOMPOSABLE} otherwise.
*
*/
DecompositionType canExposeExpression(Node subExpression) {
Node expressionRoot = findExpressionRoot(subExpression);
if (expressionRoot != null) {
return isSubexpressionMovable(expressionRoot, subExpression);
}
return DecompositionType.UNDECOMPOSABLE;
}
/** @see {@link #canDecomposeExpression} */
private DecompositionType isSubexpressionMovable(Node expressionRoot, Node subExpression) {
boolean requiresDecomposition = false;
boolean seenSideEffects = astAnalyzer.mayHaveSideEffects(subExpression);
Node child = subExpression;
for (Node parent : child.getAncestors()) {
if (NodeUtil.isNameDeclaration(parent) && !child.isFirstChildOf(parent)) {
// Case: `let x = 5, y = 2 * x;` where `child = y`.
// Compound declarations cannot generally be decomposed. Later declarations might reference
// earlier ones and if it were possible to separate them, `Normalize` would already have
// done so. Therefore, we only support decomposing the first declaration.
// TODO(b/121157467): FOR initializers are probably the only source of these cases.
return DecompositionType.UNDECOMPOSABLE;
}
if (parent == expressionRoot) {
// Done. The walk back to the root of the expression is complete, and
// nothing was encountered that blocks the call from being moved.
return requiresDecomposition ? DecompositionType.DECOMPOSABLE : DecompositionType.MOVABLE;
}
if (isConditionalOp(parent)) {
// Only the first child is always executed, otherwise it must be
// decomposed.
if (child != parent.getFirstChild()) {
requiresDecomposition = true;
}
} else {
// Only inline the call if none of the preceding siblings in the
// expression have side-effects, and are unaffected by the side-effects,
// if any, of the call in question.
// NOTE: The siblings are not always in the order in which they are evaluated, so we call
// getEvaluationDirection to see in which order to traverse the siblings.
// SPECIAL CASE: Assignment to a simple name
if (isSafeAssign(parent, seenSideEffects)) {
// It is always safe to inline "foo()" for expressions such as
// "a = b = c = foo();"
// As the assignment is unaffected by side effect of "foo()"
// and the names assigned-to can not influence the state before
// the call to foo.
//
// This is not true of more complex LHS values, such as
// a.x = foo();
// next().x = foo();
// in these cases the checks below are necessary.
} else {
// Everything else.
EvaluationDirection direction = getEvaluationDirection(parent);
for (Node n = getFirstEvaluatedChild(parent, direction);
n != null;
n = getNextEvaluatedSibling(n, direction)) {
if (n == child) {
// None of the preceding siblings have side-effects.
// This is OK.
break;
}
if (isExpressionTreeUnsafe(n, seenSideEffects)) {
seenSideEffects = true;
requiresDecomposition = true;
}
}
// In Internet Explorer, DOM objects and other external objects
// methods can not be called indirectly, as is required when the
// object or its property can be side-effected. For example,
// when exposing expression f() (with side-effects) in: x.m(f())
// either the value of x or its property m might have changed, so
// both the 'this' value ('x') and the function to be called ('x.m')
// need to be preserved. Like so:
// var t1 = x, t2 = x.m, t3 = f();
// t2.call(t1, t3);
// As IE doesn't support the call to these non-JavaScript objects
// methods in this way. We can't do this.
// We don't currently distinguish between these types of objects
// in the extern definitions and if we did we would need accurate
// type information.
//
Node first = parent.getFirstChild();
if (requiresDecomposition && parent.isCall() && NodeUtil.isGet(first)) {
if (allowMethodCallDecomposing) {
return DecompositionType.DECOMPOSABLE;
} else {
return DecompositionType.UNDECOMPOSABLE;
}
}
}
}
// Continue looking up the expression tree.
child = parent;
}
// With a valid tree we should never get here.
throw new IllegalStateException("Unexpected.");
}
private enum EvaluationDirection {
FORWARD,
REVERSE
}
/**
* Returns the order in which the given node's children should be evaluated.
*
* In most cases, this is EvaluationDirection.FORWARD because the AST order matches the actual
* evaluation order. A few nodes require reversed evaluation instead.
*/
private static EvaluationDirection getEvaluationDirection(Node node) {
switch (node.getToken()) {
case DESTRUCTURING_LHS:
case ASSIGN:
case DEFAULT_VALUE:
if (node.getFirstChild().isDestructuringPattern()) {
// The lhs of a destructuring assignment is evaluated AFTER the rhs. This is only true for
// destructuring, though, not assignments like "first().x = second()" where "first()" is
// evaluated first.
return EvaluationDirection.REVERSE;
}
// fall through
default:
return EvaluationDirection.FORWARD;
}
}
private Node getFirstEvaluatedChild(Node parent, EvaluationDirection direction) {
return direction == EvaluationDirection.FORWARD
? parent.getFirstChild()
: parent.getLastChild();
}
private Node getNextEvaluatedSibling(Node node, EvaluationDirection direction) {
return direction == EvaluationDirection.FORWARD ? node.getNext() : node.getPrevious();
}
/**
* It is always safe to inline "foo()" for expressions such as "a = b = c = foo();" As the
* assignment is unaffected by side effect of "foo()" and the names assigned-to can not influence
* the state before the call to foo.
*
*
It is also safe in cases where the object is constant:
*
*
* CONST_NAME.a = foo()
* CONST_NAME[CONST_VALUE] = foo();
*
*
* This is not true of more complex LHS values, such as
*
*
* a.x = foo();
* next().x = foo();
*
*
* in these cases the checks below are necessary.
*
* @param seenSideEffects If true, check to see if node-tree maybe affected by side-effects,
* otherwise if the tree has side-effects. @see isExpressionTreeUnsafe
* @return Whether the assignment is safe from side-effects.
*/
private boolean isSafeAssign(Node n, boolean seenSideEffects) {
if (n.isAssign()) {
Node lhs = n.getFirstChild();
switch (lhs.getToken()) {
case NAME:
return true;
case GETPROP:
return !isExpressionTreeUnsafe(lhs.getFirstChild(), seenSideEffects);
case GETELEM:
return !isExpressionTreeUnsafe(lhs.getFirstChild(), seenSideEffects)
&& !isExpressionTreeUnsafe(lhs.getLastChild(), seenSideEffects);
default:
break;
}
}
return false;
}
/**
* Determines if there is any subexpression below {@code tree} that would make it incorrect for
* some expression that follows {@code tree}, {@code E}, to be executed before {@code tree}.
*
* @param followingSideEffectsExist whether {@code E} causes side-effects.
* @return {@code true} if {@code tree} contains any subexpressions that would make movement
* incorrect.
*/
private boolean isExpressionTreeUnsafe(Node tree, boolean followingSideEffectsExist) {
if (tree.isSpread()) {
// Spread expressions would cause recursive rewriting if not special cased here.
// When extracted, spreads can't be assigned to a single variable and instead are put into
// a literal. However, that literal must be spread again at the original site. This
// check is what prevents the original spread from triggering recursion.
if (isTempConstantValueName(tree.getOnlyChild())) {
return false;
}
}
if (followingSideEffectsExist) {
// If the call to be inlined has side-effects, check to see if this
// expression tree can be affected by any side-effects.
// Assume that "tmp1.call(...)" is safe (where tmp1 is a const temp variable created by
// ExpressionDecomposer) otherwise we end up trying to decompose the same tree
// an infinite number of times.
Node parent = tree.getParent();
if (NodeUtil.isObjectCallMethod(parent, "call")
&& tree.isFirstChildOf(parent)
&& isTempConstantValueName(tree.getFirstChild())) {
return false;
}
// This is a superset of "NodeUtil.mayHaveSideEffects".
return NodeUtil.canBeSideEffected(tree, this.knownConstants, scope);
} else {
// The function called doesn't have side-effects but check to see if there
// are side-effects that that may affect it.
return astAnalyzer.mayHaveSideEffects(tree);
}
}
}