emit.js
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Created: Mon, Feb 3, 16:15

emit.js
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	/**
	* Copyright (c) 2014-present, Facebook, Inc.
	*
	* This source code is licensed under the MIT license found in the
	* LICENSE file in the root directory of this source tree.
	*/

	import assert from "assert";
	import * as leap from "./leap";
	import * as meta from "./meta";
	import * as util from "./util";

	let hasOwn = Object.prototype.hasOwnProperty;

	function Emitter(contextId) {
	assert.ok(this instanceof Emitter);

	util.getTypes().assertIdentifier(contextId);

	// Used to generate unique temporary names.
	this.nextTempId = 0;

	// In order to make sure the context object does not collide with
	// anything in the local scope, we might have to rename it, so we
	// refer to it symbolically instead of just assuming that it will be
	// called "context".
	this.contextId = contextId;

	// An append-only list of Statements that grows each time this.emit is
	// called.
	this.listing = [];

	// A sparse array whose keys correspond to locations in this.listing
	// that have been marked as branch/jump targets.
	this.marked = [true];

	this.insertedLocs = new Set();

	// The last location will be marked when this.getDispatchLoop is
	// called.
	this.finalLoc = this.loc();

	// A list of all leap.TryEntry statements emitted.
	this.tryEntries = [];

	// Each time we evaluate the body of a loop, we tell this.leapManager
	// to enter a nested loop context that determines the meaning of break
	// and continue statements therein.
	this.leapManager = new leap.LeapManager(this);
	}

	let Ep = Emitter.prototype;
	exports.Emitter = Emitter;

	// Offsets into this.listing that could be used as targets for branches or
	// jumps are represented as numeric Literal nodes. This representation has
	// the amazingly convenient benefit of allowing the exact value of the
	// location to be determined at any time, even after generating code that
	// refers to the location.
	Ep.loc = function() {
	const l = util.getTypes().numericLiteral(-1)
	this.insertedLocs.add(l);
	return l;
	}

	Ep.getInsertedLocs = function() {
	return this.insertedLocs;
	}

	Ep.getContextId = function() {
	return util.getTypes().clone(this.contextId);
	}

	// Sets the exact value of the given location to the offset of the next
	// Statement emitted.
	Ep.mark = function(loc) {
	util.getTypes().assertLiteral(loc);
	let index = this.listing.length;
	if (loc.value === -1) {
	loc.value = index;
	} else {
	// Locations can be marked redundantly, but their values cannot change
	// once set the first time.
	assert.strictEqual(loc.value, index);
	}
	this.marked[index] = true;
	return loc;
	};

	Ep.emit = function(node) {
	const t = util.getTypes();

	if (t.isExpression(node)) {
	node = t.expressionStatement(node);
	}

	t.assertStatement(node);
	this.listing.push(node);
	};

	// Shorthand for emitting assignment statements. This will come in handy
	// for assignments to temporary variables.
	Ep.emitAssign = function(lhs, rhs) {
	this.emit(this.assign(lhs, rhs));
	return lhs;
	};

	// Shorthand for an assignment statement.
	Ep.assign = function(lhs, rhs) {
	const t = util.getTypes();
	return t.expressionStatement(
	t.assignmentExpression("=", t.cloneDeep(lhs), rhs));
	};

	// Convenience function for generating expressions like context.next,
	// context.sent, and context.rval.
	Ep.contextProperty = function(name, computed) {
	const t = util.getTypes();
	return t.memberExpression(
	this.getContextId(),
	computed ? t.stringLiteral(name) : t.identifier(name),
	!!computed
	);
	};

	// Shorthand for setting context.rval and jumping to `context.stop()`.
	Ep.stop = function(rval) {
	if (rval) {
	this.setReturnValue(rval);
	}

	this.jump(this.finalLoc);
	};

	Ep.setReturnValue = function(valuePath) {
	util.getTypes().assertExpression(valuePath.value);

	this.emitAssign(
	this.contextProperty("rval"),
	this.explodeExpression(valuePath)
	);
	};

	Ep.clearPendingException = function(tryLoc, assignee) {
	const t = util.getTypes();

	t.assertLiteral(tryLoc);

	let catchCall = t.callExpression(
	this.contextProperty("catch", true),
	[t.clone(tryLoc)]
	);

	if (assignee) {
	this.emitAssign(assignee, catchCall);
	} else {
	this.emit(catchCall);
	}
	};

	// Emits code for an unconditional jump to the given location, even if the
	// exact value of the location is not yet known.
	Ep.jump = function(toLoc) {
	this.emitAssign(this.contextProperty("next"), toLoc);
	this.emit(util.getTypes().breakStatement());
	};

	// Conditional jump.
	Ep.jumpIf = function(test, toLoc) {
	const t = util.getTypes();

	t.assertExpression(test);
	t.assertLiteral(toLoc);

	this.emit(t.ifStatement(
	test,
	t.blockStatement([
	this.assign(this.contextProperty("next"), toLoc),
	t.breakStatement()
	])
	));
	};

	// Conditional jump, with the condition negated.
	Ep.jumpIfNot = function(test, toLoc) {
	const t = util.getTypes();

	t.assertExpression(test);
	t.assertLiteral(toLoc);

	let negatedTest;
	if (t.isUnaryExpression(test) &&
	test.operator === "!") {
	// Avoid double negation.
	negatedTest = test.argument;
	} else {
	negatedTest = t.unaryExpression("!", test);
	}

	this.emit(t.ifStatement(
	negatedTest,
	t.blockStatement([
	this.assign(this.contextProperty("next"), toLoc),
	t.breakStatement()
	])
	));
	};

	// Returns a unique MemberExpression that can be used to store and
	// retrieve temporary values. Since the object of the member expression is
	// the context object, which is presumed to coexist peacefully with all
	// other local variables, and since we just increment `nextTempId`
	// monotonically, uniqueness is assured.
	Ep.makeTempVar = function() {
	return this.contextProperty("t" + this.nextTempId++);
	};

	Ep.getContextFunction = function(id) {
	const t = util.getTypes();

	return t.functionExpression(
	id \|\| null/Anonymous/,
	[this.getContextId()],
	t.blockStatement([this.getDispatchLoop()]),
	false, // Not a generator anymore!
	false // Nor an expression.
	);
	};

	// Turns this.listing into a loop of the form
	//
	// while (1) switch (context.next) {
	// case 0:
	// ...
	// case n:
	// return context.stop();
	// }
	//
	// Each marked location in this.listing will correspond to one generated
	// case statement.
	Ep.getDispatchLoop = function() {
	const self = this;
	const t = util.getTypes();
	let cases = [];
	let current;

	// If we encounter a break, continue, or return statement in a switch
	// case, we can skip the rest of the statements until the next case.
	let alreadyEnded = false;

	self.listing.forEach(function(stmt, i) {
	if (self.marked.hasOwnProperty(i)) {
	cases.push(t.switchCase(
	t.numericLiteral(i),
	current = []));
	alreadyEnded = false;
	}

	if (!alreadyEnded) {
	current.push(stmt);
	if (t.isCompletionStatement(stmt))
	alreadyEnded = true;
	}
	});

	// Now that we know how many statements there will be in this.listing,
	// we can finally resolve this.finalLoc.value.
	this.finalLoc.value = this.listing.length;

	cases.push(
	t.switchCase(this.finalLoc, [
	// Intentionally fall through to the "end" case...
	]),

	// So that the runtime can jump to the final location without having
	// to know its offset, we provide the "end" case as a synonym.
	t.switchCase(t.stringLiteral("end"), [
	// This will check/clear both context.thrown and context.rval.
	t.returnStatement(
	t.callExpression(this.contextProperty("stop"), [])
	)
	])
	);

	return t.whileStatement(
	t.numericLiteral(1),
	t.switchStatement(
	t.assignmentExpression(
	"=",
	this.contextProperty("prev"),
	this.contextProperty("next")
	),
	cases
	)
	);
	};

	Ep.getTryLocsList = function() {
	if (this.tryEntries.length === 0) {
	// To avoid adding a needless [] to the majority of runtime.wrap
	// argument lists, force the caller to handle this case specially.
	return null;
	}

	const t = util.getTypes();
	let lastLocValue = 0;

	return t.arrayExpression(
	this.tryEntries.map(function(tryEntry) {
	let thisLocValue = tryEntry.firstLoc.value;
	assert.ok(thisLocValue >= lastLocValue, "try entries out of order");
	lastLocValue = thisLocValue;

	let ce = tryEntry.catchEntry;
	let fe = tryEntry.finallyEntry;

	let locs = [
	tryEntry.firstLoc,
	// The null here makes a hole in the array.
	ce ? ce.firstLoc : null
	];

	if (fe) {
	locs[2] = fe.firstLoc;
	locs[3] = fe.afterLoc;
	}

	return t.arrayExpression(locs.map(loc => loc && t.clone(loc)));
	})
	);
	};

	// All side effects must be realized in order.

	// If any subexpression harbors a leap, all subexpressions must be
	// neutered of side effects.

	// No destructive modification of AST nodes.

	Ep.explode = function(path, ignoreResult) {
	const t = util.getTypes();
	let node = path.node;
	let self = this;

	t.assertNode(node);

	if (t.isDeclaration(node))
	throw getDeclError(node);

	if (t.isStatement(node))
	return self.explodeStatement(path);

	if (t.isExpression(node))
	return self.explodeExpression(path, ignoreResult);

	switch (node.type) {
	case "Program":
	return path.get("body").map(
	self.explodeStatement,
	self
	);

	case "VariableDeclarator":
	throw getDeclError(node);

	// These node types should be handled by their parent nodes
	// (ObjectExpression, SwitchStatement, and TryStatement, respectively).
	case "Property":
	case "SwitchCase":
	case "CatchClause":
	throw new Error(
	node.type + " nodes should be handled by their parents");

	default:
	throw new Error(
	"unknown Node of type " +
	JSON.stringify(node.type));
	}
	};

	function getDeclError(node) {
	return new Error(
	"all declarations should have been transformed into " +
	"assignments before the Exploder began its work: " +
	JSON.stringify(node));
	}

	Ep.explodeStatement = function(path, labelId) {
	const t = util.getTypes();
	let stmt = path.node;
	let self = this;
	let before, after, head;

	t.assertStatement(stmt);

	if (labelId) {
	t.assertIdentifier(labelId);
	} else {
	labelId = null;
	}

	// Explode BlockStatement nodes even if they do not contain a yield,
	// because we don't want or need the curly braces.
	if (t.isBlockStatement(stmt)) {
	path.get("body").forEach(function (path) {
	self.explodeStatement(path);
	});
	return;
	}

	if (!meta.containsLeap(stmt)) {
	// Technically we should be able to avoid emitting the statement
	// altogether if !meta.hasSideEffects(stmt), but that leads to
	// confusing generated code (for instance, `while (true) {}` just
	// disappears) and is probably a more appropriate job for a dedicated
	// dead code elimination pass.
	self.emit(stmt);
	return;
	}

	switch (stmt.type) {
	case "ExpressionStatement":
	self.explodeExpression(path.get("expression"), true);
	break;

	case "LabeledStatement":
	after = this.loc();

	// Did you know you can break from any labeled block statement or
	// control structure? Well, you can! Note: when a labeled loop is
	// encountered, the leap.LabeledEntry created here will immediately
	// enclose a leap.LoopEntry on the leap manager's stack, and both
	// entries will have the same label. Though this works just fine, it
	// may seem a bit redundant. In theory, we could check here to
	// determine if stmt knows how to handle its own label; for example,
	// stmt happens to be a WhileStatement and so we know it's going to
	// establish its own LoopEntry when we explode it (below). Then this
	// LabeledEntry would be unnecessary. Alternatively, we might be
	// tempted not to pass stmt.label down into self.explodeStatement,
	// because we've handled the label here, but that's a mistake because
	// labeled loops may contain labeled continue statements, which is not
	// something we can handle in this generic case. All in all, I think a
	// little redundancy greatly simplifies the logic of this case, since
	// it's clear that we handle all possible LabeledStatements correctly
	// here, regardless of whether they interact with the leap manager
	// themselves. Also remember that labels and break/continue-to-label
	// statements are rare, and all of this logic happens at transform
	// time, so it has no additional runtime cost.
	self.leapManager.withEntry(
	new leap.LabeledEntry(after, stmt.label),
	function() {
	self.explodeStatement(path.get("body"), stmt.label);
	}
	);

	self.mark(after);

	break;

	case "WhileStatement":
	before = this.loc();
	after = this.loc();

	self.mark(before);
	self.jumpIfNot(self.explodeExpression(path.get("test")), after);
	self.leapManager.withEntry(
	new leap.LoopEntry(after, before, labelId),
	function() { self.explodeStatement(path.get("body")); }
	);
	self.jump(before);
	self.mark(after);

	break;

	case "DoWhileStatement":
	let first = this.loc();
	let test = this.loc();
	after = this.loc();

	self.mark(first);
	self.leapManager.withEntry(
	new leap.LoopEntry(after, test, labelId),
	function() { self.explode(path.get("body")); }
	);
	self.mark(test);
	self.jumpIf(self.explodeExpression(path.get("test")), first);
	self.mark(after);

	break;

	case "ForStatement":
	head = this.loc();
	let update = this.loc();
	after = this.loc();

	if (stmt.init) {
	// We pass true here to indicate that if stmt.init is an expression
	// then we do not care about its result.
	self.explode(path.get("init"), true);
	}

	self.mark(head);

	if (stmt.test) {
	self.jumpIfNot(self.explodeExpression(path.get("test")), after);
	} else {
	// No test means continue unconditionally.
	}

	self.leapManager.withEntry(
	new leap.LoopEntry(after, update, labelId),
	function() { self.explodeStatement(path.get("body")); }
	);

	self.mark(update);

	if (stmt.update) {
	// We pass true here to indicate that if stmt.update is an
	// expression then we do not care about its result.
	self.explode(path.get("update"), true);
	}

	self.jump(head);

	self.mark(after);

	break;

	case "TypeCastExpression":
	return self.explodeExpression(path.get("expression"));

	case "ForInStatement":
	head = this.loc();
	after = this.loc();

	let keyIterNextFn = self.makeTempVar();
	self.emitAssign(
	keyIterNextFn,
	t.callExpression(
	util.runtimeProperty("keys"),
	[self.explodeExpression(path.get("right"))]
	)
	);

	self.mark(head);

	let keyInfoTmpVar = self.makeTempVar();
	self.jumpIf(
	t.memberExpression(
	t.assignmentExpression(
	"=",
	keyInfoTmpVar,
	t.callExpression(t.cloneDeep(keyIterNextFn), [])
	),
	t.identifier("done"),
	false
	),
	after
	);

	self.emitAssign(
	stmt.left,
	t.memberExpression(
	t.cloneDeep(keyInfoTmpVar),
	t.identifier("value"),
	false
	)
	);

	self.leapManager.withEntry(
	new leap.LoopEntry(after, head, labelId),
	function() { self.explodeStatement(path.get("body")); }
	);

	self.jump(head);

	self.mark(after);

	break;

	case "BreakStatement":
	self.emitAbruptCompletion({
	type: "break",
	target: self.leapManager.getBreakLoc(stmt.label)
	});

	break;

	case "ContinueStatement":
	self.emitAbruptCompletion({
	type: "continue",
	target: self.leapManager.getContinueLoc(stmt.label)
	});

	break;

	case "SwitchStatement":
	// Always save the discriminant into a temporary variable in case the
	// test expressions overwrite values like context.sent.
	let disc = self.emitAssign(
	self.makeTempVar(),
	self.explodeExpression(path.get("discriminant"))
	);

	after = this.loc();
	let defaultLoc = this.loc();
	let condition = defaultLoc;
	let caseLocs = [];

	// If there are no cases, .cases might be undefined.
	let cases = stmt.cases \|\| [];

	for (let i = cases.length - 1; i >= 0; --i) {
	let c = cases[i];
	t.assertSwitchCase(c);

	if (c.test) {
	condition = t.conditionalExpression(
	t.binaryExpression("===", t.cloneDeep(disc), c.test),
	caseLocs[i] = this.loc(),
	condition
	);
	} else {
	caseLocs[i] = defaultLoc;
	}
	}

	let discriminant = path.get("discriminant");
	util.replaceWithOrRemove(discriminant, condition);
	self.jump(self.explodeExpression(discriminant));

	self.leapManager.withEntry(
	new leap.SwitchEntry(after),
	function() {
	path.get("cases").forEach(function(casePath) {
	let i = casePath.key;
	self.mark(caseLocs[i]);

	casePath.get("consequent").forEach(function (path) {
	self.explodeStatement(path);
	});
	});
	}
	);

	self.mark(after);
	if (defaultLoc.value === -1) {
	self.mark(defaultLoc);
	assert.strictEqual(after.value, defaultLoc.value);
	}

	break;

	case "IfStatement":
	let elseLoc = stmt.alternate && this.loc();
	after = this.loc();

	self.jumpIfNot(
	self.explodeExpression(path.get("test")),
	elseLoc \|\| after
	);

	self.explodeStatement(path.get("consequent"));

	if (elseLoc) {
	self.jump(after);
	self.mark(elseLoc);
	self.explodeStatement(path.get("alternate"));
	}

	self.mark(after);

	break;

	case "ReturnStatement":
	self.emitAbruptCompletion({
	type: "return",
	value: self.explodeExpression(path.get("argument"))
	});

	break;

	case "WithStatement":
	throw new Error("WithStatement not supported in generator functions.");

	case "TryStatement":
	after = this.loc();

	let handler = stmt.handler;

	let catchLoc = handler && this.loc();
	let catchEntry = catchLoc && new leap.CatchEntry(
	catchLoc,
	handler.param
	);

	let finallyLoc = stmt.finalizer && this.loc();
	let finallyEntry = finallyLoc &&
	new leap.FinallyEntry(finallyLoc, after);

	let tryEntry = new leap.TryEntry(
	self.getUnmarkedCurrentLoc(),
	catchEntry,
	finallyEntry
	);

	self.tryEntries.push(tryEntry);
	self.updateContextPrevLoc(tryEntry.firstLoc);

	self.leapManager.withEntry(tryEntry, function() {
	self.explodeStatement(path.get("block"));

	if (catchLoc) {
	if (finallyLoc) {
	// If we have both a catch block and a finally block, then
	// because we emit the catch block first, we need to jump over
	// it to the finally block.
	self.jump(finallyLoc);

	} else {
	// If there is no finally block, then we need to jump over the
	// catch block to the fall-through location.
	self.jump(after);
	}

	self.updateContextPrevLoc(self.mark(catchLoc));

	let bodyPath = path.get("handler.body");
	let safeParam = self.makeTempVar();
	self.clearPendingException(tryEntry.firstLoc, safeParam);

	bodyPath.traverse(catchParamVisitor, {
	getSafeParam: () => t.cloneDeep(safeParam),
	catchParamName: handler.param.name
	});

	self.leapManager.withEntry(catchEntry, function() {
	self.explodeStatement(bodyPath);
	});
	}

	if (finallyLoc) {
	self.updateContextPrevLoc(self.mark(finallyLoc));

	self.leapManager.withEntry(finallyEntry, function() {
	self.explodeStatement(path.get("finalizer"));
	});

	self.emit(t.returnStatement(t.callExpression(
	self.contextProperty("finish"),
	[finallyEntry.firstLoc]
	)));
	}
	});

	self.mark(after);

	break;

	case "ThrowStatement":
	self.emit(t.throwStatement(
	self.explodeExpression(path.get("argument"))
	));

	break;

	default:
	throw new Error(
	"unknown Statement of type " +
	JSON.stringify(stmt.type));
	}
	};

	let catchParamVisitor = {
	Identifier: function(path, state) {
	if (path.node.name === state.catchParamName && util.isReference(path)) {
	util.replaceWithOrRemove(path, state.getSafeParam());
	}
	},

	Scope: function(path, state) {
	if (path.scope.hasOwnBinding(state.catchParamName)) {
	// Don't descend into nested scopes that shadow the catch
	// parameter with their own declarations.
	path.skip();
	}
	}
	};

	Ep.emitAbruptCompletion = function(record) {
	if (!isValidCompletion(record)) {
	assert.ok(
	false,
	"invalid completion record: " +
	JSON.stringify(record)
	);
	}

	assert.notStrictEqual(
	record.type, "normal",
	"normal completions are not abrupt"
	);

	const t = util.getTypes();
	let abruptArgs = [t.stringLiteral(record.type)];

	if (record.type === "break" \|\|
	record.type === "continue") {
	t.assertLiteral(record.target);
	abruptArgs[1] = this.insertedLocs.has(record.target)
	? record.target
	: t.cloneDeep(record.target);
	} else if (record.type === "return" \|\|
	record.type === "throw") {
	if (record.value) {
	t.assertExpression(record.value);
	abruptArgs[1] = this.insertedLocs.has(record.value)
	? record.value
	: t.cloneDeep(record.value);
	}
	}

	this.emit(
	t.returnStatement(
	t.callExpression(
	this.contextProperty("abrupt"),
	abruptArgs
	)
	)
	);
	};

	function isValidCompletion(record) {
	let type = record.type;

	if (type === "normal") {
	return !hasOwn.call(record, "target");
	}

	if (type === "break" \|\|
	type === "continue") {
	return !hasOwn.call(record, "value")
	&& util.getTypes().isLiteral(record.target);
	}

	if (type === "return" \|\|
	type === "throw") {
	return hasOwn.call(record, "value")
	&& !hasOwn.call(record, "target");
	}

	return false;
	}


	// Not all offsets into emitter.listing are potential jump targets. For
	// example, execution typically falls into the beginning of a try block
	// without jumping directly there. This method returns the current offset
	// without marking it, so that a switch case will not necessarily be
	// generated for this offset (I say "not necessarily" because the same
	// location might end up being marked in the process of emitting other
	// statements). There's no logical harm in marking such locations as jump
	// targets, but minimizing the number of switch cases keeps the generated
	// code shorter.
	Ep.getUnmarkedCurrentLoc = function() {
	return util.getTypes().numericLiteral(this.listing.length);
	};

	// The context.prev property takes the value of context.next whenever we
	// evaluate the switch statement discriminant, which is generally good
	// enough for tracking the last location we jumped to, but sometimes
	// context.prev needs to be more precise, such as when we fall
	// successfully out of a try block and into a finally block without
	// jumping. This method exists to update context.prev to the freshest
	// available location. If we were implementing a full interpreter, we
	// would know the location of the current instruction with complete
	// precision at all times, but we don't have that luxury here, as it would
	// be costly and verbose to set context.prev before every statement.
	Ep.updateContextPrevLoc = function(loc) {
	const t = util.getTypes();
	if (loc) {
	t.assertLiteral(loc);

	if (loc.value === -1) {
	// If an uninitialized location literal was passed in, set its value
	// to the current this.listing.length.
	loc.value = this.listing.length;
	} else {
	// Otherwise assert that the location matches the current offset.
	assert.strictEqual(loc.value, this.listing.length);
	}

	} else {
	loc = this.getUnmarkedCurrentLoc();
	}

	// Make sure context.prev is up to date in case we fell into this try
	// statement without jumping to it. TODO Consider avoiding this
	// assignment when we know control must have jumped here.
	this.emitAssign(this.contextProperty("prev"), loc);
	};

	Ep.explodeExpression = function(path, ignoreResult) {
	const t = util.getTypes();
	let expr = path.node;
	if (expr) {
	t.assertExpression(expr);
	} else {
	return expr;
	}

	let self = this;
	let result; // Used optionally by several cases below.
	let after;

	function finish(expr) {
	t.assertExpression(expr);
	if (ignoreResult) {
	self.emit(expr);
	} else {
	return expr;
	}
	}

	// If the expression does not contain a leap, then we either emit the
	// expression as a standalone statement or return it whole.
	if (!meta.containsLeap(expr)) {
	return finish(expr);
	}

	// If any child contains a leap (such as a yield or labeled continue or
	// break statement), then any sibling subexpressions will almost
	// certainly have to be exploded in order to maintain the order of their
	// side effects relative to the leaping child(ren).
	let hasLeapingChildren = meta.containsLeap.onlyChildren(expr);

	// In order to save the rest of explodeExpression from a combinatorial
	// trainwreck of special cases, explodeViaTempVar is responsible for
	// deciding when a subexpression needs to be "exploded," which is my
	// very technical term for emitting the subexpression as an assignment
	// to a temporary variable and the substituting the temporary variable
	// for the original subexpression. Think of exploded view diagrams, not
	// Michael Bay movies. The point of exploding subexpressions is to
	// control the precise order in which the generated code realizes the
	// side effects of those subexpressions.
	function explodeViaTempVar(tempVar, childPath, ignoreChildResult) {
	assert.ok(
	!ignoreChildResult \|\| !tempVar,
	"Ignoring the result of a child expression but forcing it to " +
	"be assigned to a temporary variable?"
	);

	let result = self.explodeExpression(childPath, ignoreChildResult);

	if (ignoreChildResult) {
	// Side effects already emitted above.

	} else if (tempVar \|\| (hasLeapingChildren &&
	!t.isLiteral(result))) {
	// If tempVar was provided, then the result will always be assigned
	// to it, even if the result does not otherwise need to be assigned
	// to a temporary variable. When no tempVar is provided, we have
	// the flexibility to decide whether a temporary variable is really
	// necessary. Unfortunately, in general, a temporary variable is
	// required whenever any child contains a yield expression, since it
	// is difficult to prove (at all, let alone efficiently) whether
	// this result would evaluate to the same value before and after the
	// yield (see #206). One narrow case where we can prove it doesn't
	// matter (and thus we do not need a temporary variable) is when the
	// result in question is a Literal value.
	result = self.emitAssign(
	tempVar \|\| self.makeTempVar(),
	result
	);
	}
	return result;
	}

	// If ignoreResult is true, then we must take full responsibility for
	// emitting the expression with all its side effects, and we should not
	// return a result.

	switch (expr.type) {
	case "MemberExpression":
	return finish(t.memberExpression(
	self.explodeExpression(path.get("object")),
	expr.computed
	? explodeViaTempVar(null, path.get("property"))
	: expr.property,
	expr.computed
	));

	case "CallExpression":
	let calleePath = path.get("callee");
	let argsPath = path.get("arguments");

	let newCallee;
	let newArgs;

	let hasLeapingArgs = argsPath.some(
	argPath => meta.containsLeap(argPath.node)
	);

	let injectFirstArg = null;

	if (t.isMemberExpression(calleePath.node)) {
	if (hasLeapingArgs) {
	// If the arguments of the CallExpression contained any yield
	// expressions, then we need to be sure to evaluate the callee
	// before evaluating the arguments, but if the callee was a member
	// expression, then we must be careful that the object of the
	// member expression still gets bound to `this` for the call.

	let newObject = explodeViaTempVar(
	// Assign the exploded callee.object expression to a temporary
	// variable so that we can use it twice without reevaluating it.
	self.makeTempVar(),
	calleePath.get("object")
	);

	let newProperty = calleePath.node.computed
	? explodeViaTempVar(null, calleePath.get("property"))
	: calleePath.node.property;

	injectFirstArg = newObject;

	newCallee = t.memberExpression(
	t.memberExpression(
	t.cloneDeep(newObject),
	newProperty,
	calleePath.node.computed
	),
	t.identifier("call"),
	false
	);

	} else {
	newCallee = self.explodeExpression(calleePath);
	}

	} else {
	newCallee = explodeViaTempVar(null, calleePath);

	if (t.isMemberExpression(newCallee)) {
	// If the callee was not previously a MemberExpression, then the
	// CallExpression was "unqualified," meaning its `this` object
	// should be the global object. If the exploded expression has
	// become a MemberExpression (e.g. a context property, probably a
	// temporary variable), then we need to force it to be unqualified
	// by using the (0, object.property)(...) trick; otherwise, it
	// will receive the object of the MemberExpression as its `this`
	// object.
	newCallee = t.sequenceExpression([
	t.numericLiteral(0),
	t.cloneDeep(newCallee)
	]);
	}
	}

	if (hasLeapingArgs) {
	newArgs = argsPath.map(argPath => explodeViaTempVar(null, argPath));
	if (injectFirstArg) newArgs.unshift(injectFirstArg);

	newArgs = newArgs.map(arg => t.cloneDeep(arg));
	} else {
	newArgs = path.node.arguments;
	}

	return finish(t.callExpression(newCallee, newArgs));

	case "NewExpression":
	return finish(t.newExpression(
	explodeViaTempVar(null, path.get("callee")),
	path.get("arguments").map(function(argPath) {
	return explodeViaTempVar(null, argPath);
	})
	));

	case "ObjectExpression":
	return finish(t.objectExpression(
	path.get("properties").map(function(propPath) {
	if (propPath.isObjectProperty()) {
	return t.objectProperty(
	propPath.node.key,
	explodeViaTempVar(null, propPath.get("value")),
	propPath.node.computed
	);
	} else {
	return propPath.node;
	}
	})
	));

	case "ArrayExpression":
	return finish(t.arrayExpression(
	path.get("elements").map(function(elemPath) {
	if (elemPath.isSpreadElement()) {
	return t.spreadElement(
	explodeViaTempVar(null, elemPath.get("argument"))
	);
	} else {
	return explodeViaTempVar(null, elemPath);
	}
	})
	));

	case "SequenceExpression":
	let lastIndex = expr.expressions.length - 1;

	path.get("expressions").forEach(function(exprPath) {
	if (exprPath.key === lastIndex) {
	result = self.explodeExpression(exprPath, ignoreResult);
	} else {
	self.explodeExpression(exprPath, true);
	}
	});

	return result;

	case "LogicalExpression":
	after = this.loc();

	if (!ignoreResult) {
	result = self.makeTempVar();
	}

	let left = explodeViaTempVar(result, path.get("left"));

	if (expr.operator === "&&") {
	self.jumpIfNot(left, after);
	} else {
	assert.strictEqual(expr.operator, "\|\|");
	self.jumpIf(left, after);
	}

	explodeViaTempVar(result, path.get("right"), ignoreResult);

	self.mark(after);

	return result;

	case "ConditionalExpression":
	let elseLoc = this.loc();
	after = this.loc();
	let test = self.explodeExpression(path.get("test"));

	self.jumpIfNot(test, elseLoc);

	if (!ignoreResult) {
	result = self.makeTempVar();
	}

	explodeViaTempVar(result, path.get("consequent"), ignoreResult);
	self.jump(after);

	self.mark(elseLoc);
	explodeViaTempVar(result, path.get("alternate"), ignoreResult);

	self.mark(after);

	return result;

	case "UnaryExpression":
	return finish(t.unaryExpression(
	expr.operator,
	// Can't (and don't need to) break up the syntax of the argument.
	// Think about delete a[b].
	self.explodeExpression(path.get("argument")),
	!!expr.prefix
	));

	case "BinaryExpression":
	return finish(t.binaryExpression(
	expr.operator,
	explodeViaTempVar(null, path.get("left")),
	explodeViaTempVar(null, path.get("right"))
	));

	case "AssignmentExpression":
	if (expr.operator === "=") {
	// If this is a simple assignment, the left hand side does not need
	// to be read before the right hand side is evaluated, so we can
	// avoid the more complicated logic below.
	return finish(t.assignmentExpression(
	expr.operator,
	self.explodeExpression(path.get("left")),
	self.explodeExpression(path.get("right"))
	));
	}

	const lhs = self.explodeExpression(path.get("left"));
	const temp = self.emitAssign(self.makeTempVar(), lhs);

	// For example,
	//
	// x += yield y
	//
	// becomes
	//
	// context.t0 = x
	// x = context.t0 += yield y
	//
	// so that the left-hand side expression is read before the yield.
	// Fixes https://github.com/facebook/regenerator/issues/345.

	return finish(t.assignmentExpression(
	"=",
	t.cloneDeep(lhs),
	t.assignmentExpression(
	expr.operator,
	t.cloneDeep(temp),
	self.explodeExpression(path.get("right"))
	)
	));

	case "UpdateExpression":
	return finish(t.updateExpression(
	expr.operator,
	self.explodeExpression(path.get("argument")),
	expr.prefix
	));

	case "YieldExpression":
	after = this.loc();
	let arg = expr.argument && self.explodeExpression(path.get("argument"));

	if (arg && expr.delegate) {
	let result = self.makeTempVar();

	let ret = t.returnStatement(t.callExpression(
	self.contextProperty("delegateYield"),
	[
	arg,
	t.stringLiteral(result.property.name),
	after
	]
	));
	ret.loc = expr.loc;

	self.emit(ret);
	self.mark(after);

	return result;
	}

	self.emitAssign(self.contextProperty("next"), after);

	let ret = t.returnStatement(t.cloneDeep(arg) \|\| null);
	// Preserve the `yield` location so that source mappings for the statements
	// link back to the yield properly.
	ret.loc = expr.loc;
	self.emit(ret);
	self.mark(after);

	return self.contextProperty("sent");

	default:
	throw new Error(
	"unknown Expression of type " +
	JSON.stringify(expr.type));
	}
	};

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