RFR: 8316694: Implement relocation of nmethod within CodeCache [v39]

[Vladimir Kozlov]

On Tue, 22 Jul 2025 01:05:53 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 five additional commits since the last revision:
> 
>  - Fix spacing
>  - Update NMethod.java with immutable data changes
>  - Rename method to nm
>  - Add assert before freeing immutable data
>  - Reorder is_relocatable checks

Looks good to me now.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23573#pullrequestreview-3045062588