705128 - investigate caching C++ -> JS -> C++ cycle collector reachability

Status: RESOLVED WONTFIX

(Core :: XPCOM, defect)

Description

[Andrew McCreight (out until 2026) [:mccr8]]

If an object is gray, that means that it hasn't been modified since the last GC.  Thus, if the CC runs twice in a row without a GC in between, in theory we could cache the results of the CC's traversal of the GC heap.  These results take the form of a mapping from C++ objects that hold JS objects, to sets of C++ objects that are pointed to by JS objects.  When you reach one of those C++ objects, you just look up in the cache what other C++ objects it points to.

The drawback here is that we'd have to compute and store this extra information.  Having multiple CCs in a row isn't that rare, but it isn't that common either.  Looking at my console, it looks like it happens maybe one out of every 5 or 6 times.  Certain kinds of workloads could cause more CCs without GCs, I suppose.

Comment 1

[Andrew McCreight (out until 2026) [:mccr8]]

I'm not really sure how to do this, and using a more standard GC marking path for the CC is probably good enough, so I'm going to close this for now.