RFR: 8316694: Implement relocation of nmethod within CodeCache [v19]

[Evgeny Astigeevich]

On Thu, 29 May 2025 23:18:43 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 change does not modify existing code paths and therefore does not benefit much from existing tests. New tests were created and confirmed to pass on x64/aarch64 for slowdebug/fastdebug/release.
>
> Chad Rakoczy has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - Add requires GC to tests
>  - Add type for immutable_data_references
>  - Fix incorrect destination set if no trampoline available
>  - Update assert note in nmethod::clear_inline_caches

src/hotspot/share/code/nmethod.hpp line 502:

> 500:   // Relocate the nmethod to the code heap identified by code_blob_type.
> 501:   // Returns nullptr if the code heap does not have enough space or the
> 502:   // nmethod is unrelocatable, otherwise the relocated nmethod.

... nmethod is unrelocatable, or nmethod is invalidated during relocation, otherwise ...

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2116601704