User model stacking: current status

[Brian Goetz]

> And with Rational.val requiring atomic access, we can only flatten it
> if the underlying HW supports it (in this case, 2 ints fits nicely in
> 64bits so we're good).  Larger .val's can only be flattened if marked
> as "non-atomic" (the B3n case).  And because there's no tearing,
> handing out the flattened Rational.val[] is safe.
>
> Do I have that right?

Correct.  If Rational were non-atomic, the values could tear, but the 
array would still be null/init-safe.

I'd add there is one more thing going on that makes handing out the 
Rational.val[] as a Rational; the user can't put zeroes in it, because 
they can't construct the zero.  (And they can't put nulls in it, because 
the array store check will prevent that.)