At least on 32-bit x86 this isn't true. CMPXCHG8B can atomically write 64 bits of data and has been supported by CPUs for decades. The x86-64 equivalent, CMPXCHG16B, for atomic 128-bit writes is more problematic, as early Athlon64/Opteron CPUs did not support it. But even on 64-bit systems, a 64-bit encoding should be good enough. Technically, the address space can comprise 48 bits, but even if you steal a couple more than 16 for the NaN coding you should have more than enough for a few decades.
PowerPC, ARM, etc. have different atomicity guarantees though, which make this sort of thing more problematic. However, as far as I know, even Web Worker threads don't share JS context state with the main thread, so this is probably a non-issue.
I don't really see the practical benefit.
Probably to avoid cache misses.
On 32-bit architectures. 64-bits architectures have atomic XCHG and CMPXCHG operations.
That being said, the plan for Jaegermonkey is to upgrade jsvals from 64 to 128 bits so that's not the final solution. However...
> Is their JS engine inherently single-threaded only?
Yes, for certain values of 'yes'. Each script runs in its own context. There can be many scripts and many contexts but each context is only executed by a single thread.
So JS won't need fine-grained locking in the foreseeable future.
IIRC, they ran into some very serious problems with threading. If you use a two-word representation, you can't atomically set a value - there's always the chance that the thread will be pre-empted in between setting the value and the tag and some other thread will be able to run. This of course is disaster, because you'll get a value interpreted as a type that it's not.
I wonder how Mozilla plans to handle this. Is their JS engine inherently single-threaded only? Fine-grained locking seems impractical, because you'd need locks around every assignment. Would they do something like Python's GIL?
What's the reason behind this representation anyway? It was an interesting experiment when Gwydion did it, but I don't really see the practical benefit.