RFR: 8330465: Stable Values and Collections (Internal)

[Per Minborg]

On Wed, 17 Apr 2024 15:17:52 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> Also, I want to mention a few important differences between `@Stable` and Stable Values:
>> 
>> Patterns:
>> 1. Benign race (does not exist in StableValue API): multiple threads can create an instance and upload, any non-null instance is functionally equivalent so race is ok (seen in most of JDK)
>> 2. compareAndSet (setIfUnset): multiple threads can create instance, only one will succeed in uploading; usually for when the instance computation is cheap but we want single witness.
>> 3. atomic computation (computeIfUnset): only one thread can create instance which will be witnessed by other threads; this pattern ensures correctness and prevents wasteful computation by other threads at the cost of locking and lambda creation.
>> 
>> Allocation in objects: `@Stable` field is local to an object but `StableValue` is another object; thus sharing strategy may differ, as stable fields are copied over but StableValue uses a shared cache (which is even better for avoiding redundant computation)
>> 
>> Question:
>> 1. Will we ever try to expose the stable benign race model to users?
>> 2. Will we ever try to inline the stable values in object layout like a stable field?
>
>> Question:
>> 
>> 1. Will we ever try to expose the stable benign race model to users?
>> 2. Will we ever try to inline the stable values in object layout like a stable field?
> 
> 1. I think there is little or no upside in exposing the benign race. It would also be difficult to assert the invariant, competing objects are functionally equivalent. So, I think no.
> 
> 2. In a static context, the stable value will be inlined (or rather constant-folded). So we are partly already there. For pure instance contexts, I have some ideas about this but it is non-trivial.

> @minborg Just curious, why are you adding holder-in-holder benchmark cases?

I'd like to test the transitive constant folding capabilities.

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PR Comment: https://git.openjdk.org/jdk/pull/18794#issuecomment-2075119450