1659472 - Tracing a JS Map/Set key unnecessarily triggers a pre-barrier

Status: RESOLVED FIXED

(Core :: JavaScript: GC, task, P3)

Description

[Jon Coppeard (:jonco)]

The TraceKey function constructs a HashableValue on the stack, and that contains a PreBarriered<Value> so we run a pre-barrier in its destructor:

https://searchfox.org/mozilla-central/source/js/src/builtin/MapObject.cpp#462

Comment 1

[Jon Coppeard (:jonco)]

Attachment: Bug 1659472 - Avoid pre-barrier when marking JS Map/Set keys r?sfink

Ideally this would be reworked to separate out the barrier from HashableValue
but that ended up being more work than expected. This is a small fix to clear
the value and avoid the barrier in this situation.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/cf98d258454a
Avoid pre-barrier when marking JS Map/Set keys r=sfink

Comment 3

[Narcis Beleuzu [:NarcisB]]

https://hg.mozilla.org/mozilla-central/rev/cf98d258454a