RFR: 8316694: Implement relocation of nmethod within CodeCache [v17]

[Tom Rodriguez]

On Tue, 27 May 2025 18:48:15 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:
> 
>   Only exclude JVMCI methods that contain a mirror

We'll address this flag in https://bugs.openjdk.org/browse/JDK-8357619 as I think it no longer serves a purpose other than confusion.  Passing false should be correct, but I think we'll probably end up adding this:

diff --git a/src/hotspot/share/jvmci/jvmciRuntime.hpp b/src/hotspot/share/jvmci/jvmciRuntime.hpp
index 884d11f792e..0efc957aa88 100644
--- a/src/hotspot/share/jvmci/jvmciRuntime.hpp
+++ b/src/hotspot/share/jvmci/jvmciRuntime.hpp
@@ -117,6 +117,11 @@ class JVMCINMethodData : public ResourceObj {
   // Gets the JVMCI name of the nmethod (which may be null).
   const char* name() { return _has_name ? (char*)(((address) this) + sizeof(JVMCINMethodData)) : nullptr; }
 
+  // Returns true if this nmethod has a mirror
+  bool has_mirror() const {
+    return _nmethod_mirror_index != -1;
+  }
+
   // Clears the HotSpotNmethod.address field in the  mirror. If nm
   // is dead, the HotSpotNmethod.entryPoint field is also cleared.
   void invalidate_nmethod_mirror(nmethod* nm);

which you could adopt and use for your change.

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PR Comment: https://git.openjdk.org/jdk/pull/23573#issuecomment-2913746463