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

[Vladimir Ivanov]

On Thu, 18 Feb 2021 22:40:03 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
>
> src/hotspot/share/code/dependencies.cpp line 1345:
> 
>> 1343:     } else if (nof_impls == 1) { // unique implementor
>> 1344:       assert(context_type != context_type->implementor(), "not unique");
>> 1345:       context_type = InstanceKlass::cast(context_type->implementor());
> 
> There's no reason that implementor() should return Klass* rather than InstanceKlass* ? We can clean that up also later to reduce casts. (I thought I already tried to do this once).

Yes, I don't see a compelling reason for `implementor()` to return `Klass*` instead of `InstanceKlass*`. Would be nice to clean it up. Do you want me to file an RFE?

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