RFR: 8316694: Implement relocation of nmethod within CodeCache [v31]

[Dean Long]

On Thu, 19 Jun 2025 22:06:31 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 only slightly modifies existing code paths and therefore does not benefit much from existing tests. New tests were created to test the new functionality
>> 
>> Additional Testing:
>> - [ ] Linux x64 fastdebug all
>> - [ ] Linux aarch64 fastdebug all
>> - [ ] ...
>
> Chad Rakoczy has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 88 commits:
> 
>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>  - Fix pointer printing
>  - Use set_destination_mt_safe
>  - Print address as pointer
>  - Use new _metadata_size instead of _jvmci_data_size
>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>  - Only check branch distance for aarch64 and riscv
>  - Move far branch fix to fix_relocation_after_move
>  - Move far branch fix to fix_relocation_after_move
>  - Add test to verify JVMTI events during nmethod relocation
>  - ... and 78 more: https://git.openjdk.org/jdk/compare/0dd50dbb...e51a1a09

The extra work that CallRelocation::fix_relocation_after_move and Relocation::pd_set_call_destination are now doing is only needed for nmethod relocation, right?  We could avoid doing extra work for the old CodeBuffer::relocate_code_to() path by adding a flag to CallRelocation::fix_relocation_after_move() (or creating a new function) and then pass that flag on to pd_set_call_destination.  In particular, it would be nice to avoid unnecessary calls to CodeCache::find_blob().

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

PR Comment: https://git.openjdk.org/jdk/pull/23573#issuecomment-2998261601