1195585 - AutoGCRooter roots get traced multiple times

Status: RESOLVED WORKSFORME

(Core :: JavaScript: GC, defect)

Description

[Nick Fitzgerald [:fitzgen] [⏰PST; UTC-8]]

I noticed this while I was debugging a pointer that wasn't being updated properly. I was logging every time my AutoGCRooter subclass's trace method was called and it was called tens of times for a single gc (I was using trc->runtime()->gc.gcNumber() to determine that it was the same gc).

The test case I was debugging was js/src/jit-test/tests/debug/Memory-takeCensus-09.js and I was using zeal 14. The AutoGCRooter subclass was https://dxr.mozilla.org/mozilla-central/rev/0876695d1abdeb363a780bda8b6cc84f20ba51c9/js/src/vm/DebuggerMemory.cpp#656

I know AutoGCRooter is on its way out, being replaced with JS::Traceable and Rooted, but if we are unnecessarily tracing the same things multiple times, I figured y'all would want to investigate.

Comment 1

[Jon Coppeard (:jonco)]

Thanks for the report!

For a compacting GC we mark the roots once per zone we compact because there's no way of marking the roots in a particular zone only.

This is not ideal, but also it's not the most costly part of compacting either.