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Further general optimizations. #389

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Further general optimizations. #389

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859f415
Avoid accessing arguments array without length check.
cscott Dec 16, 2015
b9c8385
Ensure that ES.* abstract operations are able to be inlined.
cscott Dec 16, 2015
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20 changes: 12 additions & 8 deletions es6-shim.js
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Original file line number Diff line number Diff line change
Expand Up @@ -267,7 +267,9 @@

var $String = String;

var ES = {
// Assign these functions to a prototype to give v8 a hint that it
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how does this impact other engines? v8-specific optimizations have hurt other engines' performance in the past, with bluebird for example.

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I can't imagine it would hurt performance, since it's just a throwaway assignment. I'll try to double-check, but I don't know of any equivalent low-level IR tools on the firefox side. (jsperf can be highly misleading.)

// should optimize them as constant functions (and inline them).
var ES = (function () {}).prototype = {
// https://people.mozilla.org/~jorendorff/es6-draft.html#sec-call-f-v-args
Call: function Call(F, V) {
var args = arguments.length > 2 ? arguments[2] : [];
Expand Down Expand Up @@ -1096,7 +1098,9 @@

var ArrayPrototypeShims = {
copyWithin: function copyWithin(target, start) {
var end = arguments[2]; // copyWithin.length must be 2
// copyWithin.length must be 2, so we can't add `end` to the arguments
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this comment isn't necessary, we should definitely be consistently checking arguments.length before accessing an index - which we already do in many places.

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Well, I was just repeating and expanding the comment which was there originally, which was trying to explain why this couldn't be simply declared as copyWithin(target, start, end). But in theory our test cases should actually be verifying the length property of the function, so the comment should be unnecessary...

// directly.
var end = arguments.length > 2 ? arguments[2] : void 0;
var o = ES.ToObject(this);
var len = ES.ToLength(o.length);
var relativeTarget = ES.ToInteger(target);
Expand Down Expand Up @@ -1243,7 +1247,7 @@
if (!arrayFromHandlesUndefinedMapFunction) {
var origArrayFrom = Array.from;
overrideNative(Array, 'from', function from(items) {
if (arguments.length > 0 && typeof arguments[1] !== 'undefined') {
if (arguments.length > 1 && typeof arguments[1] !== 'undefined') {
return ES.Call(origArrayFrom, this, arguments);
} else {
return _call(origArrayFrom, this, items);
Expand Down Expand Up @@ -2013,7 +2017,7 @@
// some environments don't have setTimeout - no way to shim here.
if (typeof setTimeout !== 'function' && typeof setTimeout !== 'object') { return; }

ES.IsPromise = function (promise) {
var IsPromise = function (promise) {
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why wouldn't we want this on the ES object?

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Because mutating an object after you've defined it ensures that IsPromise won't be optimized. The number of slots is fixed at definition time. But even if you add IsPromise: null to the definition of ES to hold the space, this ends up being initialized as a property reference stored in the ES object, not a "constant function reference" stored in the object's map (which can then be inlined after a hoisted type check).

This is described a bit by http://mrale.ph/blog/2012/09/23/grokking-v8-closures-for-fun.html

if (!ES.TypeIsObject(promise)) {
return false;
}
Expand Down Expand Up @@ -2421,7 +2425,7 @@
if (!ES.TypeIsObject(C)) {
throw new TypeError('Bad promise constructor');
}
if (ES.IsPromise(v)) {
if (IsPromise(v)) {
var constructor = v.constructor;
if (constructor === C) { return v; }
}
Expand All @@ -2439,10 +2443,10 @@

then: function then(onFulfilled, onRejected) {
var promise = this;
if (!ES.IsPromise(promise)) { throw new TypeError('not a promise'); }
if (!IsPromise(promise)) { throw new TypeError('not a promise'); }
var C = ES.SpeciesConstructor(promise, Promise);
var resultCapability;
var returnValueIsIgnored = (arguments[2] === PROMISE_FAKE_CAPABILITY);
var returnValueIsIgnored = (arguments.length > 2 && arguments[2] === PROMISE_FAKE_CAPABILITY);
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might as well remove these unnecessary parens also

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Hm, I guess---although I find having two different equals signs on a line without explicit grouping impairs readability.

if (returnValueIsIgnored && C === Promise) {
resultCapability = PROMISE_FAKE_CAPABILITY;
} else {
Expand Down Expand Up @@ -3302,7 +3306,7 @@
if (!ES.IsConstructor(constructor)) {
throw new TypeError('First argument must be a constructor.');
}
var newTarget = arguments.length < 3 ? constructor : arguments[2];
var newTarget = arguments.length > 2 ? arguments[2] : constructor;
if (!ES.IsConstructor(newTarget)) {
throw new TypeError('new.target must be a constructor.');
}
Expand Down