RFR: 8316694: Implement relocation of nmethod within CodeCache [v21]

[Evgeny Astigeevich]

On Fri, 30 May 2025 22:49:42 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 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 one additional commit since the last revision:
> 
>   Change to ImmutableDataReferences

src/hotspot/share/code/nmethod.cpp line 1572:

> 1570: 
> 1571:   // Verify the nm we copied from is still valid
> 1572:   if (method() != nullptr && method()->code() == this && !is_marked_for_deoptimization() && is_in_use()) {

We can turn `method() != nullptr && method()->code() == this` into an assert. If `is_in_use()` returns true they should be true as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2117669694