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Add JIT Hooks Documentation
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| <!-- | ||
| Copyright (c) 2020, 2020 IBM Corp. and others | ||
| This program and the accompanying materials are made available under | ||
| the terms of the Eclipse Public License 2.0 which accompanies this | ||
| distribution and is available at https://www.eclipse.org/legal/epl-2.0/ | ||
| or the Apache License, Version 2.0 which accompanies this distribution and | ||
| is available at https://www.apache.org/licenses/LICENSE-2.0. | ||
| This Source Code may also be made available under the following | ||
| Secondary Licenses when the conditions for such availability set | ||
| forth in the Eclipse Public License, v. 2.0 are satisfied: GNU | ||
| General Public License, version 2 with the GNU Classpath | ||
| Exception [1] and GNU General Public License, version 2 with the | ||
| OpenJDK Assembly Exception [2]. | ||
| [1] https://www.gnu.org/software/classpath/license.html | ||
| [2] http://openjdk.java.net/legal/assembly-exception.html | ||
| SPDX-License-Identifier: EPL-2.0 OR Apache-2.0 OR GPL-2.0 WITH Classpath-exception-2.0 OR LicenseRef-GPL-2.0 WITH Assembly-exception | ||
| --> | ||
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| # Background | ||
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| 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. | ||
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| # 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. | ||
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| |Hook|Description|Comment|Locks Held<sup>1</sup>| | ||
| |--|--|--|--| | ||
| |`J9HOOK_VM_PROFILING_BYTECODE_BUFFER_FULL`|Notifies 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_ID`|Notifies 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_TARGET`|Notifies when J9Methods are to have their send target initialized.|Used to set initial invocation counts.|| | ||
| |`J9HOOK_MM_OMR_LOCAL_GC_START`|Notifies 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_END`|Notifies when a local GC has ended|If Real Time GC is enabled, the JIT registers `jitHookLocalGCEnd`; otherwise it registers `jitHookReleaseCodeLocalGCEnd`.|Exclusive VM Access| | ||
| |`J9HOOK_MM_OMR_GLOBAL_GC_START`|Notifies when a global GC has started.|Global here implies a GC of the entire heap.|Exclusive VM Access| | ||
| |`J9HOOK_MM_OMR_GLOBAL_GC_END`|Notifies 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_END`|Notifies 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_LOAD`|Notifies when a class has been loaded.||Class Table Mutex| | ||
| |`J9HOOK_VM_CLASS_PREINITIALIZE`|Notifies the initialization of a class prior to running the `<clinit>`.||Class Table Mutex| | ||
| |`J9HOOK_VM_CLASS_INITIALIZE`|Notifies the initialization of a class after running the `<clint>`.||| | ||
| |`J9HOOK_VM_CLASS_UNLOAD`|Notifies when a class has been unloaded.||Exclusive VM Access| | ||
| |`J9HOOK_VM_CLASSES_UNLOAD`|Notifies with the list of classes that have been unloaded.||Exclusive VM Access, Class Table Mutex, Compilation Monitor| | ||
| |`J9HOOK_VM_ANON_CLASSES_UNLOAD`|Notifies with the list of anonymous classes that have been unloaded. ||Exclusive VM Access| | ||
| |`J9HOOK_VM_CLASS_LOADER_UNLOAD`|Notifies 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_UNLOAD`|Notifies for each classloader that is unloaded.||Exclusive VM Access| | ||
| |`J9HOOK_MM_CLASS_UNLOADING_END`|Notifies that the GC has finished unloading classes.||Exclusive VM Access| | ||
| |`J9HOOK_MM_INTERRUPT_COMPILATION`|Notifies that JIT compilation should be interrupted in order for the GC to unload classes.||| | ||
| |`J9HOOK_VM_THREAD_CREATED`|Notifies when a thread has been created.||| | ||
| |`J9HOOK_VM_THREAD_DESTROY`|Notifies when a thread has been destroyed.||| | ||
| |`J9HOOK_VM_THREAD_STARTED`|Notifies when a thread has started running.||| | ||
| |`J9HOOK_VM_THREAD_END`|Notifies when a thread has finished running.||| | ||
| |`J9HOOK_VM_THREAD_CRASH`|Notifies when a thread has crashed.||| | ||
| |`J9HOOK_VM_JNI_NATIVE_REGISTERED`|Notifies when a native has been registered.||Assumption Table Mutex| | ||
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| # 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). | ||
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| ## 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` | ||
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| ## 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` | ||
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| ## Threads | ||
| 1. `J9HOOK_VM_THREAD_CREATED` | ||
| 2. `J9HOOK_VM_THREAD_STARTED` | ||
| 3. `J9HOOK_VM_THREAD_END ` | ||
| 4. `J9HOOK_VM_THREAD_DESTROY` | ||
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| <hr/> | ||
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| 1. The locks are not necessarily all held simultaneously. |