1927405 - Map/Set clear function is slow

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P1)

Description

[Jan de Mooij [:jandem]]

If the Map or Set is not empty, clear() always does a malloc + free. We should change this to instead destroy all elements and then only shrink the table if needed, similar to how remove works.

This also avoids calling init() after the table has been initialized and this simplifies bug 1851662.

Changing this improves the micro-benchmark below from ~105 ms to ~35 ms.

function f() {
    var s = new Set;
    var t = new Date;
    for (var i = 0; i < 1_000_000; i++) {
        s.add(1);
        s.clear();
    }
    print(new Date - t);
    return s;
}
f();

Comment 1

[Jan de Mooij [:jandem]]

Attachment: Bug 1927405 - Simplify and optimize OrderedHashTable::clear. r?jonco!

Change clear to destroy all elements and then shrink the table only if needed,
similar to how remove is implemented.

This eliminates a free and malloc if there are only a few elements but it's also
simpler because it avoids calling init more than once which is a footgun.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/a07d438ea22a
Simplify and optimize OrderedHashTable::clear. r=jonco

Comment 3

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/a07d438ea22a