RFR: 8327858: Improve spliterator and forEach for single-element immutable collections

[Chen Liang]

On Tue, 12 Mar 2024 10:12:18 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> Please review this patch that:
>> 1. Implemented `forEach` to optimize for 1 or 2 element collections.
>> 2. Implemented `spliterator` to optimize for a single element.
>> 
>> The default implementations for multiple-element immutable collections are fine as-is, specializing implementation doesn't provide much benefit.
>
> src/java.base/share/classes/java/util/ImmutableCollections.java line 926:
> 
>> 924:             if (!REVERSE && e1 != EMPTY) {
>> 925:                 action.accept((E) e1);
>> 926:             }
> 
> I'm curious to know how the following alternative would fare:
> 
> Suggestion:
> 
>             if (e1 != EMPTY) {
>                 action.accept(REVERSE ? (E)e1 : (E)e0); // implicit null check
>                 action.accept(REVERSE ? (E)e0 : (E)e1);
>             } else {
>                 action.accept(e0); // Implicit null check
>             }

I used this:

            if (e1 == EMPTY) {
                action.accept(e0); // Implicit null check
            } else {
                action.accept(REVERSE ? (E)e1 : e0); // implicit null check
                action.accept(REVERSE ? e0 : (E)e1);
            }


The results are about the same as the current patch.
With the alternative:

Benchmark                        Mode  Cnt    Score    Error   Units
ImmutableColls.forEachOverList  thrpt   15  367.651 ± 10.730  ops/us
ImmutableColls.forEachOverSet   thrpt   15   84.397 ± 12.235  ops/us
ImmutableColls.getOrDefault     thrpt   15  243.874 ±  1.891  ops/us
ImmutableColls.iterateOverList  thrpt   15  149.525 ±  3.652  ops/us
ImmutableColls.iterateOverSet   thrpt   15   60.202 ±  5.053  ops/us

Current patch:

Benchmark                        Mode  Cnt    Score   Error   Units
ImmutableColls.forEachOverList  thrpt   15  364.515 ± 8.076  ops/us
ImmutableColls.forEachOverSet   thrpt   15   86.012 ± 2.420  ops/us
ImmutableColls.getOrDefault     thrpt   15  243.723 ± 1.071  ops/us
ImmutableColls.iterateOverList  thrpt   15  149.392 ± 5.328  ops/us
ImmutableColls.iterateOverSet   thrpt   15   63.579 ± 4.671  ops/us

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PR Review Comment: https://git.openjdk.org/jdk/pull/15834#discussion_r1522201572