1697143 - Use NewObjectGCKind for empty object literals

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P3)

Description

[Jan de Mooij [:jandem]]

While working on optimizing Object.assign (bug 1696178) I noticed empty object literals result in an object with no fixed slots. This means we usually have to allocate dynamic slots for Object.assign({}, from).

For new (and the Object constructor), we use NewObjectGCKind which gives the object 4 fixed slots. This is sufficient for the majority of assign calls on Reddit and Slack.

Looking at AWSY data this doesn't affect our memory usage numbers much.

Comment 1

[Jan de Mooij [:jandem]]

Attachment: Bug 1697143 part 1 - Fix JSObject::sizeOfIncludingThisInNursery to not include fixed slots twice. r?jonco!

This fixes a test issue with the next patch.

Comment 2

[Jan de Mooij [:jandem]]

Attachment: Bug 1697143 part 2 - Use NewObjectGCKind for empty object literals. r?jonco!

This helps avoid a lot of dynamic slot allocations on popular websites which
improves performance. AWSY seems unaffected.

Depends on D107632

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/bf705899fc86
part 1 - Fix JSObject::sizeOfIncludingThisInNursery to not include fixed slots twice. r=jonco
https://hg.mozilla.org/integration/autoland/rev/a527a3ec7f54
part 2 - Use NewObjectGCKind for empty object literals. r=jonco

Comment 4

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/bf705899fc86
https://hg.mozilla.org/mozilla-central/rev/a527a3ec7f54