1364345 - Use shifted-elements optimization for splice

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement)

Description

[Jan de Mooij [:jandem]]

The shifted-elements optimization added for bug 1348772 can be used for splice too, when deleting elements from the start of the array: arr.splice(0, x). This is pretty common, some grepping shows a few benchmarks like Octane-pdfjs do this.

Comment 1

[Jan de Mooij [:jandem]]

This should be straight-forward, but I'll wait for bug 1362753 to land to avoid merge conflicts for one of us :)

Comment 2

[Jan de Mooij [:jandem]]

Attachment: Patch

We now call tryShiftDenseElements when splice is shrinking the array.

When calling arr.splice(0, 2) on an array with 50,000 elements, until the array is empty, I get the following:

Chrome:        5 ms
Safari:      163 ms
Nightly old: 484 ms
Nightly new:   5 ms

Like with shift(), Chrome has a perf cliff around 50k elements. For instance when there are 80,000 elements:

Chrome:      1264 ms
Safari:       447 ms
Nightly old: 1275 ms
Nightly new:    9 ms

I added some logging and this optimization hits pretty often on the web, and it seems useful to have in case people are relying on V8's fast path.

Comment 3

[André Bargull [:anba]]

Comment on attachment 8869706 [details] [diff] [review]
Patch

Review of attachment 8869706 [details] [diff] [review]:
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Great results!

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/fe4dbfcf9328
Optimize splice to shift the elements header when deleting from the start of the array. r=anba

Comment 5

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/fe4dbfcf9328