1500052 - Iterator cache should check initialized length instead of empty elements singleton

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement)

Description

[Jan de Mooij [:jandem]]

Instead of checking hasEmptyElements() we should check getDenseInitializedLength() == 0. This eliminates thousands of iterator objects on Google Docs (I noticed this while looking into bug 505818 comment 18).

The patch improves the micro-benchmark below a lot:

before: 13575 ms
after:    170 ms

function f() {
    var a = new Array(10);
    var t = new Date;
    for (var i = 0; i < 10000000; i++) {
        for (var x in a) {}
    }
    print(new Date - t);
}
f();

Comment 1

[Jan de Mooij [:jandem]]

Attachment: Bug 1500052 - Fix iterator cache to check getDenseInitializedLength() == 0 instead of hasEmptyElements(). r?evilpie

hasEmptyElements() is too pessimistic: arrays often have no elements but aren't
using the empty elements singleton.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/609976e5c798
Fix iterator cache to check getDenseInitializedLength() == 0 instead of hasEmptyElements(). r=evilpie

Comment 3

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/609976e5c798