RFR: 8330465: Stable Values and Collections (Internal) [v20]

[Jens Lidestrom]

On Fri, 17 May 2024 09:31:33 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> # Stable Values & Collections (Internal)
>> 
>> <img src="https://github.com/openjdk/jdk/assets/7457876/db4b22a1-af87-4914-adac-b05a87e7cb42" width=20% height=20%>
>> 
>> ## 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...
>
> Per Minborg has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Add benchmarks for memoized IntFunction and Function
>  - Add benchmark for memoized supplier

src/java.base/share/classes/jdk/internal/lang/StableArray.java line 66:

> 64:      * @throws IllegalArgumentException if the provided {@code length} is {@code < 0}
> 65:      */
> 66:     static <V> StableArray<V> of(int length) {

I interpret the method name `of` as a method that creates an object that contains the argument as some kind of member, in the way that `List.of` and friends work.

My intuitive interpretation of `StableArray.of(10)` is that it returns an array with the single element 10.

I think a method like this should be named `empty`, or `emptyOfLength` or something like that.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1606529054