RFR: 8302108: Clean up placeholder supername code [v2]

[Ioi Lam]

On Fri, 10 Feb 2023 19:11:17 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Please review change to make PlaceholderEntry::_supername a SymbolHandle to correctly refcount the symbol rather than have ad-hoc code.  It also moves the private functions to private and asserts invariant that find_and_remove must always find a matching entry.  Finally, this also more eagerly removes the supername symbol from the placeholder entry, so I have to fix the test in https://github.com/openjdk/jdk/pull/12491 before pushing this.
>> Tested with tier1-4.
>
> Coleen Phillimore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - Fix test for cleanup for nulling out supername when no longer used.
>  - Merge branch 'master' into placeholder-cleanup
>  - 8302108: Clean up placeholder supername code

LGTM. Just a small nit.

src/hotspot/share/classfile/placeholders.hpp line 102:

> 100:   bool remove_seen_thread(JavaThread* thread, PlaceholderTable::classloadAction action);
> 101: 
> 102:   SeenThread*        superThreadQ()        const { return _superThreadQ; }

If you just want to change these function to private, instead of moving them, I would suggest adding `private:` and `public:` keywords around them. That way we can limit the delta.

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Marked as reviewed by iklam (Reviewer).

PR: https://git.openjdk.org/jdk/pull/12495