108 lines
4.8 KiB
Java
108 lines
4.8 KiB
Java
/*
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* Copyright (C) 2016 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package dalvik.annotation.optimization;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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/**
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* An ART runtime built-in optimization for {@code native} methods to speed up JNI transitions:
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* Compared to normal {@code native} methods, {@code native} methods that are annotated with
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* {@literal @}{@code FastNative} use faster JNI transitions from managed code to the native code
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* and back. Calls from a {@literal @}{@code FastNative} method implementation to JNI functions
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* that access the managed heap or call managed code also have faster internal transitions.
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*
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* <p>
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* While executing a {@literal @}{@code FastNative} method, the garbage collection cannot
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* suspend the thread for essential work and may become blocked. Use with caution. Do not use
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* this annotation for long-running methods, including usually-fast, but generally unbounded,
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* methods. In particular, the code should not perform significant I/O operations or acquire
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* native locks that can be held for a long time. (Some logging or native allocations, which
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* internally acquire native locks for a short time, are generally OK. However, as the cost
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* of several such operations adds up, the {@literal @}{@code FastNative} performance gain
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* can become insignificant and overshadowed by potential GC delays.)
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* Acquiring managed locks is OK as it internally allows thread suspension.
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* </p>
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*
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* <p>
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* For performance critical methods that need this annotation, it is strongly recommended
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* to explicitly register the method(s) with JNI {@code RegisterNatives} instead of relying
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* on the built-in dynamic JNI linking.
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* </p>
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*
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* <p>
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* The {@literal @}{@code FastNative} optimization was implemented for system use since
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* Android 8 and became CTS-tested public API in Android 14. Developers aiming for maximum
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* compatibility should avoid calling {@literal @}{@code FastNative} methods on Android 13-.
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* The optimization is likely to work also on Android 8-13 devices (after all, it was used
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* in the system, albeit without the strong CTS guarantees), especially those that use
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* unmodified versions of ART, such as Android 12+ devices with the official ART Module.
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* The built-in dynamic JNI linking is working only in Android 12+, the explicit registration
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* with JNI {@code RegisterNatives} is strictly required for running on Android versions 8-11.
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* The annotation is ignored on Android 7-.
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* </p>
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*
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* <p>
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* <b>Deadlock Warning:</b> As a rule of thumb, any native locks acquired in a
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* {@literal @}{@link FastNative} call (despite the above warning that this is an unbounded
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* operation that can block GC for a long time) must be released before returning to managed code.
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* </p>
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*
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* <p>
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* Say some code does:
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*
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* <code>
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* fast_jni_call_to_grab_a_lock();
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* does_some_java_work();
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* fast_jni_call_to_release_a_lock();
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* </code>
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*
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* <p>
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* This code can lead to deadlocks. Say thread 1 just finishes
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* {@code fast_jni_call_to_grab_a_lock()} and is in {@code does_some_java_work()}.
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* GC kicks in and suspends thread 1. Thread 2 now is in {@code fast_jni_call_to_grab_a_lock()}
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* but is blocked on grabbing the native lock since it's held by thread 1.
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* Now thread suspension can't finish since thread 2 can't be suspended since it's doing
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* FastNative JNI.
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* </p>
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*
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* <p>
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* Normal JNI doesn't have the issue since once it's in native code,
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* it is considered suspended from java's point of view.
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* FastNative JNI however doesn't do the state transition done by JNI.
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* </p>
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*
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* <p>
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* Note that even in FastNative methods you <b>are</b> allowed to
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* allocate objects and make upcalls into Java code. A call from Java to
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* a FastNative function and back to Java is equivalent to a call from one Java
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* method to another. What's forbidden in a FastNative method is blocking
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* the calling thread in some non-Java code and thereby preventing the thread
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* from responding to requests from the garbage collector to enter the suspended
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* state.
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* </p>
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*
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* <p>
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* Has no effect when used with non-native methods.
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* </p>
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*/
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@Retention(RetentionPolicy.CLASS) // Save memory, don't instantiate as an object at runtime.
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@Target(ElementType.METHOD)
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public @interface FastNative {}
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