Minor optimizations / questions about stream implementation

[Daniel Avery]

Hi,

I recently finished a deep-dive through the code in java.util.stream, and
came across a few corners that stuck out to me as being odd, in a "small
missed opportunity" kind of way. I figured I'd raise them here just in case
any are legitimate, or I could get some better insight. I am looking at JDK
24 [build 30] (
https://github.com/openjdk/jdk/tree/jdk-24%2B30/src/java.base/share/classes/java
).

1. AbstractPipeline.wrap() is passed isParallel(), which ultimately (only)
plays into determining if StreamSpliterators.AbstractWrappingSpliterator is
allowed to split. It looks like the code could be more lenient, and instead
pass (isParallel() || !hasAnyStateful()), i.e. allow splitting the
spliterator() from sequential streams that do not have stateful ops.

2. It looks like StreamSpliterators.UnorderedSliceSpliterator.trySplit()
should disable splitting if (permitStatus() == NO_MORE), instead of
(permits.get() == 0). As is, it would appear to unnecessarily disable
splitting after skipping, in the skip-only case.

3. It looks like SortedOps could override opEvaluateParallelLazy to no-op
in the already-sorted case, similar to DistinctOps in the already-distinct
case.

4. It looks like SliceOps.makeRef() could fuse adjacent skip()/limit() ops
(though I could see that being an overly niche optimization, especially if
it makes linked/consumed handling painful).

5. It looks like there is some dead code in StreamOpFlag? I don't see a
path where isPreserved() would return true in practice (it appears to only
be called in exactly 2 'minor optimization' places: SliceTask.doLeaf() and
DropWhileTask.doLeaf()). With the way StreamOpFlag.combineOpFlags() works,
it looks like in practice 0b00 is used to preserve, rather than 0b11. I
also don't see Type.OP, Type.TERMINAL_OP, or Type.UPSTREAM_TERMINAL_OP
being used anywhere. I assume some of the methods here are intended for
tests only (isStreamFlag(), canSet())... But given how much of this class
I'm not seeing being used, I'm wondering if I'm just missing something.

Thank you,
Daniel
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