The stencil project (bug 1544117) will let us parse JS without allocating GC things. This will allow us to make a number of simplifications to the GC.

Off the top of my head, we can do the following:

• use a single atoms table and remove the partitioning
• remove merging of realms/zones used by off-thread parsing
• remove suppression of atoms GC while off-thread parsing
• remove blocking off-thread parsing while atoms GC is active
• make HeapThreshold::bytes_ MainThreadData<>
• remove anywhere we wait for parse tasks to finish to avoid them creating GC things (e.g. runtime stats collection)

Bug 1662102 will add a pref to change the runtime behaviour. Once we are happy with that we can strip out all the machinery.

This removes allocation paths that are no longer used.

This was added to improve performance when helper threads could alloate atoms
concurrently with the main thread.

Depends on D131779

This was used to allow concurrent allocation of atoms from more than one
thread.

Depends on D131780

Depends on D131781

Depends on D131782

Depends on D131783

https://hg.mozilla.org/mozilla-central/rev/3c8c8b37b18d
https://hg.mozilla.org/mozilla-central/rev/62fbe8c0f683
https://hg.mozilla.org/mozilla-central/rev/e7b0599c0b81
https://hg.mozilla.org/mozilla-central/rev/c0f1f59e9690
https://hg.mozilla.org/mozilla-central/rev/0c714249d686
https://hg.mozilla.org/mozilla-central/rev/6a70ca3a2229