RFR: 8263834: Work around gdb <incomplete type> for HashtableEntry

[Ioi Lam]

On Fri, 19 Mar 2021 07:06:28 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Please review this one liner to work around a gdb bug in printing subtypes of `HashtableEntry`:
>> 
>> (gdb) p this
>> $1 = (const PlaceholderEntry * const) 0x7ffff0242a30
>> (gdb) p *this
>> $2 = <incomplete type>
>> (gdb) ptype this
>> type = const class PlaceholderEntry {
>>     <incomplete type>
>> } * const
>> 
>> The fix is to no longer subclass `HashtableEntry` from `CHeapObj`. Apparently that makes gdb happy. None of our code requires this subclass relationship.
>> 
>> Tested with tiers 1-4.
>
> Makes sense.

Thanks @tstuefe @tschatzl @coleenp @dholmes-ora for the review!

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PR: https://git.openjdk.java.net/jdk/pull/3084