User model stacking: current status

[Brian Goetz]

The current stacking discussion is motivated by several factors:

  - experiences prototyping both B2 and B3
  - recently discovered hardware improvements in atomic operations 
(e.g., Intel's recent specification strengthening around 128-bit vector 
loads and stores)
  - further thought on the consequences of the B2/B3 model, particularly 
with regard to tearing

The B2/B3 split was a useful proxy during prototyping, with each being 
built around a known use case: B2 around value-based classes, and B3 
around numeric abstractions.  My main objection is twofold: there are 
gratuitous-seeming differences in performance model (B3s flatten much 
better currently), which puts users to bad choices between semantics and 
performance, and the degree to which tearing is hidden behind some other 
proxy ("primitive-ness", non-nullity, etc), which is likely to surprise 
users when invariants are checked in the constructor but not necessarily 
obeyed at runtime.  I want the observed behavioral distinctions between 
buckets to be clearly related to their semantic differences, and we're 
not there yet.

The differences in flattening and performance between the current B2/B3 
derives directly from the possibility of tearing. When tearing is 
unacceptable, we are likely to fall back on using indirections to make 
loads and stores of references atomic (the "non-flat" option); even 
where we are able to gain some flattening through compiler heroics (the 
"low flat" option), these hit the ceiling pretty fast (we're unlikely to 
get above 128 bits any time soon, and may need at least one bit for 
null) and these also have other costs (wider loads and stores means more 
data movement and more register shuffling, in addition to the complexity 
of the required compiler heroics.)  Full-flat requires tearing.  But I 
don't see an intrinsic reason (yet) while we can't have full-flat for 
VBCs like Optional.

The most encouraging direction is to factor atomicity out of the bucket 
model.  We can make both buckets (VBC and primitive-like) atomic by 
default; this still gets us all the calling convention optimizations, 
and for very small values (such as single field ones, like Optional), we 
can probably achieve full flattening in the heap, and more flattening 
for small-ish values with low-flat heroics.  We can allow both buckets 
to opt into non-atomicity, which unlocks full-flat layout in the heap, 
with the only difference being whether we have to perturb the 
representation to make null representable.

This gets us to something like:

     [ atomic | non-atomic ] __value class B2 { }
     [ atomic | non-atomic ] __primitive class B3 { }

There are many bikesheds here, including the spelling of all these 
things, and whether or not we say "class" or "struct" or "primitive" or 
nothing at all, or whether these work with records, but painting can 
come later.   There are also many other decisions to make, but I'll 
observe several properties we've already gained by this stacking:

  - non-atomicity is explicit, rather than hiding it behind "primitive" 
or "non-nullable" or "zero-happy"
  - non-atomicity is orthogonal, which means that the performance 
difference between B2 and B3 (or B3.val and B3.ref), for either polarity 
of atomicity, is exclusively that imposed by the null-encoding requirement
  - safe by default, can opt into more performance by opting out of some 
safety
  - non-atomic sounds "just scary enough" to make people think twice, or 
at least learn what non-atomic means

Atomicity is only needed when a class has cross-field invariants (or 
when it's construction API varies significantly from its 
representation.)  Numeric classes like Complex have no invariants, and 
Rational has only single-field invariants, but classes like IntRange 
would have cross-field invariants.  In cases where the VM can provide 
atomicity for free (e.g., single-field classes), it wouldn't make a 
difference.

If we further opt for Kevin's "ref is default" proposal, then we add 
another:

  - All unadorned type names are reference types

Separately, I think we can reconsider where we spend the "value" 
keyword.  Previously "value" meant "non-identity", but I think it is 
better spent meaning "has a value projection", which leads us to the 
minor reshuffling presented yesterday:

     class B1 { }                 // ref only, == based on identity
     value-based class B2 { }     // ref only, == based on state
value class B3 { }           // Has ref and val projections

This affirms B2 as "value-lite", connects to the term we colonized in 
Java 8 for "classes that have value-like semantics", and moves away from 
"primitive".

Let's work through Kevin's examples here:

  - Rational.  Here, the default value is particularly bad (denominator 
should not be zero).  This leads to an uncomfortable choice; choose B2, 
or choose B3 and deal with the DBZE as "user error" when it happens.  
Internal methods (e.g., multiply two rationals) can treat the default 
value as "0/1" instead and produce a valid rational, but any code that 
pulls out the denominator and operate on it externally will confront the 
zero anyway.  Whichever way one chooses, people will complain "but 
that's bad".  Rational is interesting because it _has_ a sensible 
default, it is just not the zero representation.
  - EmployeeId.  Similar, but maybe more tolerable to treat as a B2, and 
doesn't require atomicity.
  - Instant.  Seems this is a (probably non-atomic) B2.
  - Complex.  Solid non-atomic B3.
  - Optional, OptionalInt, etc.  In a world where B3 is ref-default, 
these can be B3; otherwise B2.
  - IntRange: atomic B3 (cross-field invariant.)

There are lots of other things to discuss here, including a discussion 
of what does non-atomic B2 really mean, and whether there are additional 
risks that come from tearing _between the null and the fields_.  I'll 
address that in a separate mail, but I think that factoring out atomic 
into its own explicit thing is a pure win, and that in turn exposes some 
sensible terminology shuffling in the other buckets.

Also, bikeshed topics to cover (please, let's not let this drown the 
discussion):
  - How to spell atomic / non-atomic
  - How to spell B2 and B3
  - How to spell .ref and .val
  - ref-default vs val-default for B3
    - if we go ref-default, reconciling this with universal generics
    - reconciling this with nullable types

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