RFR: 8316694: Implement relocation of nmethod within CodeCache [v15]

[Erik Österlund]

On Thu, 8 May 2025 19:31:43 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 54 additional commits since the last revision:
> 
>  - Fix null check
>  - Remove unnecessary include
>  - Add nullptr check to relocate
>  - Fix JVMCI nmethod data
>  - Unexclude JVMCI methods
>  - Add relocate_nmethod_mirror
>  - Only hold NMethodState_lock when needed
>  - Exclude JVMCI nmethods
>  - Remove StressNMethodRelocation
>  - Fix branch_range revert
>  - ... and 44 more: https://git.openjdk.org/jdk/compare/a077da0e...9ca3563a

src/hotspot/share/jvmci/jvmciRuntime.cpp line 852:

> 850: 
> 851: void JVMCINMethodData::relocate_nmethod_mirror(nmethod* nm) {
> 852:   oop nmethod_mirror = get_nmethod_mirror(nm, /* phantom_ref */ false);

Why is phantom false?

src/hotspot/share/jvmci/jvmciRuntime.cpp line 858:

> 856: 
> 857:   JVMCIEnv* jvmciEnv = nullptr;
> 858:   HotSpotJVMCI::InstalledCode::set_address(jvmciEnv, nmethod_mirror, (jlong)(nm));

What's the sync story here? Any lock protecting this? If not, I wonder if readers are okay with inconsistencies. I haven't checked.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2080483634
PR Review Comment: https://git.openjdk.org/jdk/pull/23573#discussion_r2080485969