Objects vs. values, the continuation

[Brian Goetz]

The fact that these are "small" (at most 64 bits) is incidental, not essential; introducing a new quadruple type would not destabilize our concept of a primitive value.

If we can tip the user's mental model so that they believe "small is
good" for B3 values, then we aid them in hitting the sweet space of
the design and help them avoid tearing issues.  It doesn't change the
model but the more we can encourage the belief that B3 values should
be <= 64it the happier users will be with the results.

I think its reasonable to say that “we can flatten 64 bits better than we can flatten 256, but go ahead and write the code you want, and we’ll do what we can.”  Recent data suggests that we can get to 128 more quickly than we had initially expected, and (especially if we can drop the null footprint tax, as B3 does), you can do a lot in 128.  Presumably in some future hardware generation this number will go up again, whether that is 5 or 10 years from now, we don’t know right now.

The tangible things I think we want permission from the user to do are:

 - drop identity
 - drop nullity
 - drop atomicity (non-tearing)

B2, as currently sketched, drops the first; B3.val further drops nullity and atomicity together.  Whether this is the right stacking is a good discussion to be having now, but ultimately we need permission for each of these.  While “small” objects may sidestep the atomicity constraint, we’d like this to remain an implementation detail, not an in-your-face aspect of the programming model.