RFR: 8261954: Dependencies: Improve iteration over class hierarchy under context class

[Vladimir Ivanov]

On Thu, 18 Feb 2021 22:41:22 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Simplify `ClassHierarchyWalker::find_witness_anywhere()` which iterates over class hierarchy under context class searching for witnesses.
>> 
>> Current implementation traverses the hierarchy in a breadth-first manner and keeps a stack-allocated array to keep a worklist.
>> But all the subclasses are already part of a singly linked list formed by `Klass::subklass()`/`next_sibling()`/`superklass()`.
>> 
>> Proposed refactoring gets rid of the explicit worklist and switches to the traversal over the linked list (encapsulated into `ClassHierarchyIterator`). It performs depth-first pre-order hierarchy traversal. 
>> 
>> (There are some other minor refactorings applied in `ClassHierarchyWalker` along the way.) 
>> 
>> Testing:
>> - [x] hs-tier1 - hs-tier8
>> - [x] additional verification that CHA decisions aren't affected
>
> This seems like a nice change.  Some questions.

Thanks for the reviews, Vladimir and Coleen.

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PR: https://git.openjdk.java.net/jdk/pull/2630