RFR: 8316694: Implement relocation of nmethod within CodeCache [v35]

[Andrew Haley]

On Tue, 8 Jul 2025 20:03:17 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 95 commits:
> 
>  - Typo
>  - Merge branch 'master' into JDK-8316694-Final
>  - Update justification for skipping CallRelocation
>  - Enclose ImmutableDataReferencesCounterSize in parentheses
>  - Let trampolines fix their owners
>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>  - Update how call sites are fixed
>  - Merge remote-tracking branch 'origin/master' into JDK-8316694-Final
>  - Fix pointer printing
>  - Use set_destination_mt_safe
>  - ... and 85 more: https://git.openjdk.org/jdk/compare/117f0b40...66d73c16

src/hotspot/share/code/nmethod.cpp line 1392:

> 1390: 
> 1391: 
> 1392: nmethod::nmethod(nmethod* nm) : CodeBlob(nm->_name, nm->_kind, nm->_size, nm->_header_size)

Should this be a copy constructor? 

nmethod::nmethod(const nmethod &nm) : CodeBlob(nm._name, nm._kind, nm._size, nm._header_size)

Even if we don't make it a copy constructor, it looks like its nmethod argument should be `const`, but I haven't checked very deeply.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2203266153