RFR: 8330465: Stable Values and Collections (Internal) [v19]

[Per Minborg]

> # Stable Values & Collections (Internal)
> 
> ## Summary
> This PR proposes to introduce an internal _Stable Values & Collections_ API, which provides immutable value holders where elements are initialized _at most once_. Stable Values & Collections offer the performance and safety benefits of final fields while offering greater flexibility as to the timing of initialization.
> 
> ## Goals
>  * Provide an easy and intuitive API to describe value holders that can change at most once.
>  * Decouple declaration from initialization without significant footprint or performance penalties.
>  * Reduce the amount of static initializer and/or field initialization code.
>  * Uphold integrity and consistency, even in a multi-threaded environment.
>  
> For more details, see the draft JEP: https://openjdk.org/jeps/8312611
> 
> ## Performance 
> Performance compared to instance variables using two `AtomicReference` and two protected by double-checked locking under concurrent access by all threads:
> 
> 
> Benchmark                                      Mode  Cnt      Score      Error   Units
> StableBenchmark.atomic                        thrpt   10    259.478 ?   36.809  ops/us
> StableBenchmark.dcl                           thrpt   10    225.710 ?   26.638  ops/us
> StableBenchmark.stable                        thrpt   10   4382.478 ? 1151.472  ops/us <- StableValue significantly faster
> 
> 
> Performance compared to static variables protected by `AtomicReference`, class-holder idiom holder, and double-checked locking (all threads):
> 
> 
> Benchmark                                      Mode  Cnt      Score      Error   Units
> StableStaticBenchmark.atomic                  thrpt   10   6487.835 ?  385.639  ops/us
> StableStaticBenchmark.dcl                     thrpt   10   6605.239 ?  210.610  ops/us
> StableStaticBenchmark.stable                  thrpt   10  14338.239 ? 1426.874  ops/us
> StableStaticBenchmark.staticCHI               thrpt   10  13780.341 ? 1839.651  ops/us
> 
> 
> Performance for stable lists (thread safe) in both instance and static contexts whereby we access a single value compared to `ArrayList` instances (which are not thread-safe) (all threads):
> 
> 
> Benchmark                                      Mode  Cnt      Score      Error   Units
> StableListElementBenchmark.instanceArrayList  thrpt   10   5812.992 ? 1169.730  ops/us
> StableListElementBenchmark.instanceList       thrpt   10   4818.643 ?  704.893  ops/us
> StableListElementBenchmark.staticArrayList    thrpt   10   7614.741 ?  564.777  ops/us
> StableListElementBe...

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Update ofList and ofMap docs

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18794/files
  - new: https://git.openjdk.org/jdk/pull/18794/files/0f798a70..dd0ceaf0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18794&range=18
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18794&range=17-18

  Stats: 12 lines in 1 file changed: 0 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/18794.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18794/head:pull/18794

PR: https://git.openjdk.org/jdk/pull/18794