RFR: 8358339: Handle MethodCounters::_method backlinks after JDK-8355003

[Aleksey Shipilev]

On Tue, 3 Jun 2025 12:00:03 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Found this when reading mainline-vs-premain webrev. [JDK-8355003](https://bugs.openjdk.org/browse/JDK-8355003) introduced a backlink to `Method*` in `MethodCounters`. I believe we need to handle that backlink at least in `CodeBuffer::finalize_oop_references()`. premain does this, while mainline does not. Also, amusingly, we have `MethodCounters::is_methodCounters`, but not the super-class `Metadata::is_methodCounters`. 
>> 
>> I pulled in the hunks that use `is_methodCounters()` and `MethodCounters::method()` from premain into this PR.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `runtime/cds`
>>  - [x] Linux x86_64 server fastdebug, `tier1`
>>  - [x] Linux x86_64 server fastdebug, `all`
>
> And MethodCounters shouldn't be inhertited from Metadata, they're inherited from MetaspaceObj in mainline.  We want to avoid virtual function pointers in this type.

Before I proceed anywhere with this, I need to understand what @coleenp saw in all this :)

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PR Comment: https://git.openjdk.org/jdk/pull/25599#issuecomment-2939215148