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

[Vladimir Ivanov]

On Thu, 18 Feb 2021 17:53:41 GMT, Vladimir Kozlov <kvn 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 1329:
> 
>> 1327:   assert_locked_or_safepoint(Compile_lock);
>> 1328: 
>> 1329:   bool do_counts = count_find_witness_calls();
> 
> Does count_find_witness_calls() have side effects and required to be called here in product? If not,
> `do_counts` is used only for not_product code, so consider enclose it into  NOT_PRODUCT() here and where it is used.

`count_find_witness_calls()` is a no-op in product binaries:
#ifndef PRODUCT
...
static bool count_find_witness_calls() {
...
#else
#define count_find_witness_calls() (0)
#endif //PRODUCT

-------------

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