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

[Erik Österlund]

On Thu, 18 Feb 2021 17:05:08 GMT, Vladimir Ivanov <vlivanov 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

Looks good! (move next() to cpp file if you want to)

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Marked as reviewed by eosterlund (Reviewer).

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