128bits value type and VarHande.compareAndSet() Was: Objects vs. values, the continuation

[Remi Forax]

> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "Dan Heidinga" <heidinga at redhat.com>
> Cc: "Kevin Bourrillion" <kevinb at google.com>, "valhalla-spec-experts"
> <valhalla-spec-experts at openjdk.java.net>
> Sent: Monday, April 25, 2022 4:59:14 PM
> Subject: Re: Objects vs. values, the continuation

>>> 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.

This is tangential but i write it here because i will forget again. 
There is an issue with representing B2 as a 128 bits value, while Intel and ARM both provides 128 atomic read/write if vectorized registers are used, they do not provide a CAS (or the equivament on ARM) on 128 bits. 

This is an issue for VarHandle because 
- one can create a VarHandle on any field (array cell), it does not have to be volatile (there is no way to declare the content of an array volatile) 
- VarHandle.compareAndSet() has to work. 

If we keep the exact same semantics for VarHandle, we can not use 128 bits for fields declared as B2 because a VarHandle may be constructed on it later. 

Rémi