User model stacking: current status

[Brian Goetz]

Maybe :)  But I don't want to prune this exploration just yet.

On 5/5/2022 6:00 PM, Dan Smith wrote:
>> On May 5, 2022, at 1:21 PM, Brian Goetz<brian.goetz at oracle.com>  wrote:
>>
>> Let's write this out more explicitly.  Suppose that T1 writes a non-null value (d, t, true), and T2 writes null as (0, 0, false).  Then it would be possible to observe (0, 0, true), which means that we would be conceivably exposing the zero value to the user, even though a B2 class might want to hide its zero.
>>
>> So, suppose instead that we implemented writing a null as simply storing false to the synthetic boolean field.  Then, in the event of a race between reader and writer, we could only see values for date and time that were previously put there by some thread.  This satisfies the OOTA (out of thin air) safety requirements of the JMM.
> (0, 0, false) is the initial value of a field/array, even if the VM implements a "narrow write" strategy. That is, if I write (1, 1, true) at the moment of reading from a fresh field, I could easily get (0, 0, true).
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> This is significant because the primary reason to declare a B2 rather than a B3 is to guarantee that the all-zeros value cannot be created. (A secondary reason, valid but one I'm less sympathetic to, is that the all-zeros value is okay but inconvenient, and it would be nice to reduce how much it pops up. A third reason is reference-defaultness, important for migration if we don't offer it in B3.)
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> This leads me to conclude that if you're declaring a non-atomic B2, you might as well just declare a non-atomic B3.
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> Said differently: a B2 author usually wants to associate a cross-field invariant with the null flag (zero-value fields iff null). But in declaring the class non-atomic, they've sworn off cross-field invariants.
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> This was a useful discovery for me yesterday: that, in fact, nullability and atomicity are closely related. There's a strong theoretical defense for the idea that opting out of identity and supporting a non-null type (i.e., B3) are prerequisites to non-atomic flattening.
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