RFR: 8316694: Implement relocation of nmethod within CodeCache [v37]

[Chad Rakoczy]

On Mon, 14 Jul 2025 20:34:42 GMT, Chad Rakoczy <duke at openjdk.org> wrote:

>> This PR introduces a new function to replace nmethods, addressing [JDK-8316694](https://bugs.openjdk.org/browse/JDK-8316694). It enables the creation of new nmethods from existing ones, allowing method relocation in the code heap and supporting [JDK-8328186](https://bugs.openjdk.org/browse/JDK-8328186).
>> 
>> When an nmethod is replaced, a deep copy is performed. The corresponding Java method is updated to reference the new nmethod, while the old one is marked as unused. The garbage collector handles final cleanup and deallocation.
>> 
>> This does not modify existing code paths and therefore does not benefit much from existing tests. New tests were created to test the new functionality
>> 
>> Additional Testing:
>> - [ ] Linux x64 fastdebug all
>> - [ ] Linux aarch64 fastdebug all
>> - [ ] ...
>
> Chad Rakoczy has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert is_always_within_branch_range changes

I also don't think the `CodeCache_lock` can be acquired in the `relocate` function. This should be the responsibility of the caller. There is nothing preventing `relocate` from blocking on `CodeCache_lock` and the nmethod in which is waiting for the lock gets purged from the CodeCache

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PR Comment: https://git.openjdk.org/jdk/pull/23573#issuecomment-3076195035