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PojavLauncherTeam
GitHub Repository: PojavLauncherTeam/openj9
 Path: blob/master/doc/compiler/runtime/JITHooks.md
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Background

The VM has a mechansim to register callbacks for specific runtime event. The JIT makes use of these events to perform tasks either for functional correctness, performance, or housekeeping.

Hooks

This section outlines various events or Hooks the JIT registers a callback against. The callbacks associated with these Hooks can be found in HookedByTheJit.cpp.

HookDescriptionCommentLocks Held1
J9HOOK_VM_PROFILING_BYTECODE_BUFFER_FULLNotifies when the bytecode profiling buffer (consumed by the IProfiler) for a given thread is full.VM Access, IProfiler Buffer Arrival Monitor
J9HOOK_VM_LOOKUP_JNI_IDNotifies when a method or field ID is looked up via JNI.The JIT uses this to get notified when main in the Java code is about to be run; it then unregisters this hook.Exclusive VM Access
J9HOOK_VM_INITIALIZE_SEND_TARGETNotifies when J9Methods are to have their send target initialized.Used to set initial invocation counts.
J9HOOK_MM_OMR_LOCAL_GC_STARTNotifies when a local Garbage Collection has started.Local here applies to policies such as GenCon, where only the nursery region is being GC'd.Exclusive VM Access
J9HOOK_MM_OMR_LOCAL_GC_ENDNotifies when a local GC has endedIf Real Time GC is enabled, the JIT registers jitHookLocalGCEnd; otherwise it registers jitHookReleaseCodeLocalGCEnd.Exclusive VM Access
J9HOOK_MM_OMR_GLOBAL_GC_STARTNotifies when a global GC has started.Global here implies a GC of the entire heap.Exclusive VM Access
J9HOOK_MM_OMR_GLOBAL_GC_ENDNotifies when a global GC has ended.If Real Time GC is enabled, the JIT registers jitHookGlobalGCEnd; otherwise it registers jitHookReleaseCodeGlobalGCEnd.Exclusive VM Access
J9HOOK_MM_OMR_GC_CYCLE_ENDNotifies when the GC Cycle has ended.The JIT only registers against this hook if Real Time GC is enabled.Exclusive VM Access
J9HOOK_VM_INTERNAL_CLASS_LOADNotifies when a class has been loaded.Class Table Mutex
J9HOOK_VM_CLASS_PREINITIALIZENotifies the initialization of a class prior to running the <clinit>.Class Table Mutex
J9HOOK_VM_CLASS_INITIALIZENotifies the initialization of a class after running the <clint>.
J9HOOK_VM_CLASS_UNLOADNotifies when a class has been unloaded.Exclusive VM Access
J9HOOK_VM_CLASSES_UNLOADNotifies with the list of classes that have been unloaded.Exclusive VM Access, Class Table Mutex, Compilation Monitor
J9HOOK_VM_ANON_CLASSES_UNLOADNotifies with the list of anonymous classes that have been unloaded.Exclusive VM Access
J9HOOK_VM_CLASS_LOADER_UNLOADNotifies with the list of classloaders that have been unloaded.The JIT uses this hook to clean up the DLT buffers.Exclusive VM Access
J9HOOK_VM_CLASS_LOADERS_UNLOADNotifies for each classloader that is unloaded.Exclusive VM Access
J9HOOK_MM_CLASS_UNLOADING_ENDNotifies that the GC has finished unloading classes.Exclusive VM Access
J9HOOK_MM_INTERRUPT_COMPILATIONNotifies that JIT compilation should be interrupted in order for the GC to unload classes.
J9HOOK_VM_THREAD_CREATEDNotifies when a thread has been created.
J9HOOK_VM_THREAD_DESTROYNotifies when a thread has been destroyed.
J9HOOK_VM_THREAD_STARTEDNotifies when a thread has started running.
J9HOOK_VM_THREAD_ENDNotifies when a thread has finished running.
J9HOOK_VM_THREAD_CRASHNotifies when a thread has crashed.
J9HOOK_VM_JNI_NATIVE_REGISTEREDNotifies when a native has been registered.Assumption Table Mutex

Chronological Order of Hooks

Certain hooks will be invoked in a specific order because they relate to a defined set of events that occur in the VM, for example GC. This section outlines the order in which these hooks get triggered (assuming no failures or error conditions).

Garbage Collection

  1. J9HOOK_MM_OMR_GC_CYCLE_START

  2. J9HOOK_MM_OMR_LOCAL_GC_START or J9HOOK_MM_OMR_GLOBAL_GC_START

  3. J9HOOK_MM_OMR_LOCAL_GC_END or J9HOOK_MM_OMR_GLOBAL_GC_END

  4. J9HOOK_MM_OMR_GC_CYCLE_END

Class Loading/Unloading

  1. J9HOOK_VM_INTERNAL_CLASS_LOAD

  2. J9HOOK_VM_CLASS_PREINITIALIZE

  3. J9HOOK_VM_CLASS_INITIALIZE

  4. J9HOOK_VM_CLASS_UNLOAD

  5. J9HOOK_VM_CLASSES_UNLOAD

  6. J9HOOK_VM_ANON_CLASSES_UNLOAD

  7. J9HOOK_VM_CLASS_LOADERS_UNLOAD

  8. J9HOOK_VM_CLASS_LOADER_UNLOAD

Threads

  1. J9HOOK_VM_THREAD_CREATED

  2. J9HOOK_VM_THREAD_STARTED

  3. J9HOOK_VM_THREAD_END

  4. J9HOOK_VM_THREAD_DESTROY

  1. The locks are not necessarily all held simultaneously.