RFR: 8316694: Implement relocation of nmethod within CodeCache [v7]

[Evgeny Astigeevich]

On Mon, 24 Mar 2025 23:00:36 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 two additional commits since the last revision:
> 
>  - Relocate nmethod at safepoint
>  - Fix windows build

src/hotspot/share/code/nmethod.cpp line 1492:

> 1490:   // Relocate nmethod at safepoint
> 1491:   VM_RelocateNMethod relocate_nmethod(nm, code_blob_type);
> 1492:   VMThread::execute(&relocate_nmethod);

You should not do this here. It will be a responsibility of a caller to ensure relocation is done at a safepoint.  A caller will get to a safepoint and relocate a bunch of nmethods.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2014063631