RFR: 8316694: Implement relocation of nmethod within CodeCache

[Evgeny Astigeevich]

On Wed, 12 Mar 2025 18:54:10 GMT, Vladimir Kozlov <kvn 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.
>
> src/hotspot/share/code/nmethod.cpp line 1396:
> 
>> 1394: }
>> 1395: 
>> 1396: nmethod::nmethod(nmethod& nm) : CodeBlob(nm.name(), CodeBlobKind::Nmethod, nm.size(), nm.header_size())
> 
> Should this be `clone()` method instead of constructor. Then you will not need `new()`.

+1

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r1993581509