RFR: 8316694: Implement relocation of nmethod within CodeCache [v3]

[Dean Long]

On Fri, 14 Mar 2025 20:49:42 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:
> 
>   Share immutable data between copied nmethods

src/hotspot/share/code/nmethod.hpp line 587:

> 585:   address immutable_data_references_begin      () const { return           _immutable_data + _immutable_data_references_offset   ; }
> 586:   address immutable_data_references_end        () const { return            immutable_data_end(); }
> 587: 

If we are going to add typed fields to this data, maybe we should put it in a struct/class header at the beginning so we can access the field directly?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r1996341389