RFR: 8316694: Implement relocation of nmethod within CodeCache

[Chad Rakoczy]

On Thu, 13 Feb 2025 18:18:24 GMT, Cesar Soares Lucas <cslucas 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.
>
> test/hotspot/jtreg/compiler/whitebox/ReplaceNMethod.java line 65:
> 
>> 63:         compile();
>> 64: 
>> 65:         checkCompiled();
> 
> Looks like the test will fail if the method is currently only queued for compilation, right? Being queued for compilation isn't an error in this situation AFAIU.

The test is run with `-Xbatch` which disables background compilation so I think it should never get to the compilation check before the method is through the queue

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r1955104064