RFR: 8292286: Convert PlaceholderTable to ResourceHashtable [v2]

[Ioi Lam]

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.

Changes requested by iklam (Reviewer).

src/hotspot/share/classfile/placeholders.cpp line 36:

> 34: #include "utilities/resourceHash.hpp"
> 35: 
> 36: class PlaceholderKey : public StackObj {

Since the PlaceholderKey is stored inside the _placeholders table, I think it shouldn't be declared as StackObj.

src/hotspot/share/classfile/placeholders.cpp line 46:

> 44:   }
> 45:   static unsigned hash(PlaceholderKey const& k) {
> 46:     return (unsigned) k._name->identity_hash();

Should this be XORed with _loader_data?

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