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66 lines
3.5 KiB
66 lines
3.5 KiB
4 months ago
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/*
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* Copyright (C) 2014 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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#ifndef ART_RUNTIME_READ_BARRIER_OPTION_H_
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#define ART_RUNTIME_READ_BARRIER_OPTION_H_
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namespace art {
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// Options for performing a read barrier or not.
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//
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// Besides disabled GC and GC's internal usage, there are a few cases where the read
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// barrier is unnecessary and can be avoided to reduce code size and improve performance.
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// In the following cases, the result of the operation or chain of operations shall be the
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// same whether we go through the from-space or to-space:
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//
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// 1. We're reading a reference known to point to an un-reclaimable immune space object.
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// (For example boot image class and string references, read by compiled code from
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// .data.bimg.rel.ro . Similarly, such references constructed using position independent
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// code in the compiled boot image code do not need a read barrier.)
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// 2. We're reading the reference for comparison involving a non-moving space reference.
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// (Whether the non-moving space reference is the one we're reading or the one we shall
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// compare it with, the result is the same with and without read barrier.)
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// 3. We're reading the reference for comparison with null.
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// (Similar to 2 above, given that null is "non-moving".)
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// 4. We're reading a reference to a holder from which we shall read
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// - constant primitive field, or
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// - constant reference field known to point to an un-reclaimable immune space object, or
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// - constant reference field for comparison involving a non-moving space reference, or
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// - constant reference field for comparison with null, or
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// - constant reference fields in a chain leading to one or more of the above purposes;
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// the entire chain needs to be read without read barrier.
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// The term "constant" refers to fields set to their final value between allocating
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// the holder and the next opportunity for the holder to be moved by the GC, i.e.
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// before the first suspend point after the allocation, or seen by another thread.
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// This includes several fields in the Class object, such as the primitive type or
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// component type but not the superclass.
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//
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// References read without a read barrier must not remain live at the next suspend point,
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// with the exception of references to un-reclaimable immune space objects.
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//
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// For un-reclaimable immune space objects, we rely on graying dirty objects in the FlipCallback
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// pause (we try to gray them just before flipping thread roots but the FlipCallback has to re-scan
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// for newly dirtied objects) and clean objects conceptually become black at that point
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// (marking them through is a no-op as all reference fields must also point to immune spaces),
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// so mutator threads can never miss a read barrier as they never see white immune space object.
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enum ReadBarrierOption {
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kWithReadBarrier, // Perform a read barrier.
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kWithoutReadBarrier, // Don't perform a read barrier.
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};
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} // namespace art
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#endif // ART_RUNTIME_READ_BARRIER_OPTION_H_
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