RFR: JDK-8302666: Replace CHM with VarHandle in ForeachOrderedTask

[Viktor Klang]

On Thu, 16 Feb 2023 17:10:17 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> I noticed when looking at the code that there was no real need to use a CHM to perform the tracking of activation in an ordered fashion on ForEachOrderedTask, but instead a VarHandle can be used, reducing allocations and indirection.
>
> src/java.base/share/classes/java/util/stream/ForEachOps.java line 431:
> 
>> 429:                 // leftChild and rightChild were just created and not fork():ed
>> 430:                 // yet so no need for a volatile write
>> 431:                 NEXT.set(leftChild, rightChild);
> 
> Make `next` a plain field and shuffle up the assignment (`leftChild.next = rightChild`) to occur immediately after construction of the right child task? (FWIW `addToPendingCount` operates on a volatile field of `CountedCompleter`).

@PaulSandoz I'm usually a bit weary of piggybacking if it is not done on the same object, as future reorderings of the code might break that assumption. I wouldn't want to break anything silently so I made a rather conservative change. On which side should I err? :)

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PR: https://git.openjdk.org/jdk/pull/12320