1696178 - Optimize Object.assign more

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P3)

Description

[Jan de Mooij [:jandem]]

Object.assign({}, ...) is very hot on Reddit, Slack and other modern websites.

This is related to bug 1648546, but there's more we can do in the C++ implementation before adding IC stubs for this.

Comment 1

[Jan de Mooij [:jandem]]

I have some patches for this. For the micro-benchmark below, I get (best of 3 runs):

before:        2351 ms
+ bug 1697143: 1732 ms
+ parts 1-4:   1653 ms
+ parts 5-6:    885 ms
+ part 7:       756 ms

function f() {
    var from1 = {a: 1, b: 2, c: 3};
    var from2 = {};
    var from3 = {b: 4};
    for (var i = 0; i < 10_000_000; i++) {
        Object.assign({}, from1, from2, from3);
    }
}
var t = dateNow();
f();
print(dateNow() - t);

Comment 2

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 1 - Simplify/optimize shape check in TryAssignNative and TryEnumerableOwnPropertiesNative. r?anba!

When we had unboxed objects, not every object had a shape, but now we can just
do a shape check without checking it's a NativeObject first.

Comment 3

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 2 - Don't handle proxies in Shape::slotSpan. r?jonco!

Checking for proxies was slowing us down for no good reason. Non-native objects
already manage their own layout; slotSpan only applies to native objects.

Depends on D108403

Comment 4

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 3 - Remove Shape::slotSpan overload with JSClass argument. r?jonco!

One of the advantages of the Shape/BaseShape overhaul is that non-dictionary
shapes never change their BaseShape, because the BaseShape no longer contains
the ShapeTable and "owned" BaseShapes don't exist anymore.

This lets us simplify some comments and code.

Depends on D108404

Comment 5

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 4 - Add NativeObject::setLastPropertyForNewDataProperty. r?jonco!

NativeObject::addEnumerableDataProperty is very hot and this is a measurable speedup
because it's not as branchy as setLastProperty.

Depends on D108405

Comment 6

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 5 - Add ObjectFlag::HasNonWritableOrAccessorExcludingProto. r?jonco!

This will be used by the next patch to optimize Object.assign.

Depends on D108406

Comment 7

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 6 - Add a fast path for Object.assign with plain objects. r?anba!

Websites like Reddit and Slack call Object.assign thousands of times. The vast majority
of these calls (more than 95%) are with plain objects. This patch adds a fast path for
this case.

This lets us bypass the generic SetProperty code and some of the JSClass checks
that aren't relevant for PlainObject.

Depends on D108407

Comment 8

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 7 - Add a faster path for when the shape can be reused directly. r?jonco!

Depends on D108408

Comment 9

[Jan de Mooij [:jandem]]

Attachment: Bug 1696178 part 8 - Use initSlot instead of setSlot in a few places. r?jonco!

These are all setting newly added slots so we can avoid the pre-barrier.

Comment 10

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/6b91b2e87f18
part 1 - Simplify/optimize shape check in TryAssignNative and TryEnumerableOwnPropertiesNative. r=anba
https://hg.mozilla.org/integration/autoland/rev/4ca97ae7f9ce
part 2 - Don't handle proxies in Shape::slotSpan. r=jonco
https://hg.mozilla.org/integration/autoland/rev/aa7ea7bda43e
part 3 - Remove Shape::slotSpan overload with JSClass argument. r=jonco
https://hg.mozilla.org/integration/autoland/rev/12574c8b209d
part 4 - Add NativeObject::setLastPropertyForNewDataProperty. r=jonco
https://hg.mozilla.org/integration/autoland/rev/bad57461dcc4
part 5 - Add ObjectFlag::HasNonWritableOrAccessorExcludingProto. r=jonco
https://hg.mozilla.org/integration/autoland/rev/f16ed78f5471
part 6 - Add a fast path for Object.assign with plain objects. r=anba
https://hg.mozilla.org/integration/autoland/rev/c9bb3660abd3
part 7 - Add a faster path for when the shape can be reused directly. r=jonco
https://hg.mozilla.org/integration/autoland/rev/ecf43b0add73
part 8 - Use initSlot instead of setSlot in a few places. r=jonco

Comment 11

[Andreea Pavel [:apavel]]

https://hg.mozilla.org/mozilla-central/rev/6b91b2e87f18
https://hg.mozilla.org/mozilla-central/rev/4ca97ae7f9ce
https://hg.mozilla.org/mozilla-central/rev/aa7ea7bda43e
https://hg.mozilla.org/mozilla-central/rev/12574c8b209d
https://hg.mozilla.org/mozilla-central/rev/bad57461dcc4
https://hg.mozilla.org/mozilla-central/rev/f16ed78f5471
https://hg.mozilla.org/mozilla-central/rev/c9bb3660abd3
https://hg.mozilla.org/mozilla-central/rev/ecf43b0add73