RFR: 8292286: Convert PlaceholderTable to ResourceHashtable [v2]

[Coleen Phillimore]

On Mon, 15 Aug 2022 15:59:31 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> This change converts the PlaceholderTable to ResourceHashtable.  It's much like the ResolutionErrorTable conversion.  This change has been performance tested with the previous set of testing with no regression.  It's also been tested tier1-3 and 4-7 with the other tables in my patch queue.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use Symbol::maybe increment or decrement refcount.

src/hotspot/share/classfile/placeholders.cpp line 222:

> 220:   // Decrement refcount in key.
> 221:   class_name->decrement_refcount();
> 222:   _placeholders.remove(key);

This was in the wrong order.  We should decrement the refcount after removing the entry.

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PR: https://git.openjdk.org/jdk/pull/9854