RFR: 8265029: Preserve SIZED characteristics on slice operations (skip, limit) [v7]

[Tagir F.Valeev]

On Mon, 24 May 2021 19:51:04 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Tagir F. Valeev has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Trailing whitespace removed
>
> src/java.base/share/classes/java/util/stream/AbstractPipeline.java line 471:
> 
>> 469:         int flags = getStreamAndOpFlags();
>> 470:         long size = StreamOpFlag.SIZED.isKnown(flags) ? spliterator.getExactSizeIfKnown() : -1;
>> 471:         if (size != -1 && StreamOpFlag.SIZE_ADJUSTING.isKnown(flags) && !isParallel()) {
> 
> Very nice. It's a good compromise to support only for sequential streams, since we have no size adjusting intermediate stateless op. If that was the case we would need to step back through the pipeline until the depth was zero, then step forward. I think it worth a comment here to inform our future selves if we ever add such an operation. 
> 
> Strictly speaking we only need to call `exactOutputSize` if the stage is size adjusting. Not sure it really matters perf-wise. If we leave as is maybe add a comment.

It's hard to imagine SIZE_ADJUSTING stateless intermediate op (probably the op that duplicates every stream element like `flatMap(x -> Stream.of(x, x))` as a single op?). Nevertheless, I added the comment. Please check if it's clear enough. Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/3427