8378838: Fix issues with "dead" code elimination and serviceability agent in libjvm.so on Linux

[MBaesken]

When using "dead" code linktime elimination (linker flag -Wl,--gc-sections) / LTGC we see a number of hotspot jtreg tests fail.
They are related to the serviceability agent.

Example serviceability/sa/CDSJMapClstats

[2026-02-26T15:48:28.867332189Z] Gathering output for process 2349301
[2026-02-26T15:48:28.867983029Z] Waiting for completion for process 2349301
[2026-02-26T15:48:29.341213744Z] Waiting for completion finished for process 2349301
Output and diagnostic info for process 2349301 was saved into 'pid-2349301-output.log'
Attaching to process ID 2341040, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 27.0.0.1-internal-adhoc.jdk
finding class loader instances ..java.lang.InternalError: Metadata does not appear to be polymorphic
at jdk.hotspot.agent/sun.jvm.hotspot.types.basic.BasicTypeDataBase.findDynamicTypeForAddress(BasicTypeDataBase.java:223)
at jdk.hotspot.agent/sun.jvm.hotspot.runtime.VirtualBaseConstructor.instantiateWrapperFor(VirtualBaseConstructor.java:104)
at jdk.hotspot.agent/sun.jvm.hotspot.oops.Metadata.instantiateWrapperFor(Metadata.java:77)
at jdk.hotspot.agent/sun.jvm.hotspot.memory.SystemDictionary.getClassLoaderKlass(SystemDictionary.java:102)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.ClassLoaderStats.printClassLoaderStatistics(ClassLoaderStats.java:93)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.ClassLoaderStats.run(ClassLoaderStats.java:78)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.run(JMap.java:121)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:278)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.start(Tool.java:241)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.Tool.execute(Tool.java:134)
at jdk.hotspot.agent/sun.jvm.hotspot.tools.JMap.main(JMap.java:202)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.runJMAP(SALauncher.java:344)
at jdk.hotspot.agent/sun.jvm.hotspot.SALauncher.main(SALauncher.java:507)

Seems this happens because LTGC eliminates the vtable of class Metadata. This can be shown
when adding --print-gc-sections to the linker steps, then with LTGC I see this section gets removed ( the _ZTV prefix seems to be about vtable )

removing unused section from '.data.rel.ro._ZTV8Metadata' in file '/mybuild/hotspot/variant-server/libjvm/objs/metadata.o'

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Issue

• JDK-8378838: Fix issues with "dead" code elimination and serviceability agent in libjvm.so on Linux (Bug - P4)

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[MBaesken]

There might also be some C++ 'tricks' / hacks to modify metadata.hpp / cpp and enforce keeping the vtable.
But some use typeid and we get errors when using this because of no-rtti .

[mlbridge]

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• 00: Full (0895076c)