I think VM.disableGC() exists only to deal with the possibility of the GC moving stacks from under one's feet while working on a stack.
Since I believe we no longer support gc-movable stacks, it should be deprecated.
Is it true that gc-movable stacks are no longer supported? If so, we should expunge code that depends on or is due to this. VM.disableGC() seems to be an example of this.