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

[Coleen Phillimore]

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

This seems like a nice change.  Some questions.

src/hotspot/share/oops/instanceKlass.hpp line 1486:

> 1484:   }
> 1485: };
> 1486: 

The _subklass and _next_sibling fields and implementation are in Klass (klass.hpp/cpp) but I always wonder why they are not in InstanceKlass (instanceKlass.hpp/cpp).  If this new class is in instanceKlass.hpp, these fields should be in the same.  If you agree, we should file an RFE and I will move them.  If the fields truly belong in klass.hpp, then this should be there too. ie. they should match.

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).

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Marked as reviewed by coleenp (Reviewer).

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