| /* |
| * Copyright (C) 2014 The Android Open Source Project |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); |
| * you may not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| #ifndef ART_RUNTIME_READ_BARRIER_OPTION_H_ |
| #define ART_RUNTIME_READ_BARRIER_OPTION_H_ |
| namespace art { |
| |
| // Options for performing a read barrier or not. |
| // |
| // Besides disabled GC and GC's internal usage, there are a few cases where the read |
| // barrier is unnecessary and can be avoided to reduce code size and improve performance. |
| // In the following cases, the result of the operation or chain of operations shall be the |
| // same whether we go through the from-space or to-space: |
| // |
| // 1. We're reading a reference known to point to an un-reclaimable immune space object. |
| // (For example boot image class and string references, read by compiled code from |
| // .data.bimg.rel.ro . Similarly, such references constructed using position independent |
| // code in the compiled boot image code do not need a read barrier.) |
| // 2. We're reading the reference for comparison involving a non-moving space reference. |
| // (Whether the non-moving space reference is the one we're reading or the one we shall |
| // compare it with, the result is the same with and without read barrier.) |
| // 3. We're reading the reference for comparison with null. |
| // (Similar to 2 above, given that null is "non-moving".) |
| // 4. We're reading a reference to a holder from which we shall read |
| // - constant primitive field, or |
| // - mutable primitive field for testing an invariant, or |
| // - constant reference field known to point to an un-reclaimable immune space object, or |
| // - constant reference field for comparison involving a non-moving space reference, or |
| // - constant reference field for comparison with null, or |
| // - constant reference fields in a chain leading to one or more of the above purposes; |
| // the entire chain needs to be read without read barrier. |
| // The terms "constant" and "invariant" refer to values stored in holder fields before the |
| // holder reference is stored in the location for which we want to avoid the read barrier. |
| // Since the stored holder reference points to an object with the initialized constant or |
| // invariant, when we start a new GC and that holder instance becomes a from-space object |
| // both the from-space and to-space versions shall hold the same constant or invariant. |
| // |
| // While correct inter-thread memory visibility needs to be ensured for these constants and |
| // invariants, it needs to be equally ensured for non-moving GC types, so read barriers or |
| // their avoidance do not place any additional constraints on inter-thread synchronization. |
| // |
| // References read without a read barrier must not remain live at the next suspend point, |
| // with the exception of references to un-reclaimable immune space objects. |
| // |
| // For un-reclaimable immune space objects, we rely on graying dirty objects in the FlipCallback |
| // pause (we try to gray them just before flipping thread roots but the FlipCallback has to re-scan |
| // for newly dirtied objects) and clean objects conceptually become black at that point |
| // (marking them through is a no-op as all reference fields must also point to immune spaces), |
| // so mutator threads can never miss a read barrier as they never see white immune space object. |
| // |
| // Examples: |
| // |
| // The j.l.Class contains many constant fields and invariants: |
| // - primitive type is constant (primitive classes are pre-initialized in the boot image, |
| // or created in early single-threaded stage when running without boot image; non-primitive |
| // classes keep the value 0 from the Class object allocation), |
| // - element type is constant (initialized during array class object allocation, null otherwise), |
| // - access flags are mutable but the proxy class bit is an invariant set during class creation, |
| // - once the class is resolved, the class status is still mutable but it shall remain resolved, |
| // being a resolved is an invariant from that point on, |
| // - once a class becomes erroneous, the class status shall be constant (and unresolved |
| // erroneous class shall not become resolved). |
| // This allows reading a chain of element type references for any number of array dimensions |
| // without read barrier to find the (non-array) element class and check whether it's primitive, |
| // or proxy class. When creating an array class, the element type is already either resolved or |
| // unresolved erroneous and neither shall change, so we can also check these invariants (but not |
| // resolved erroneous because that is not an invariant from the creation of the array class). |
| // |
| // The superclass becomes constant during the ClassStatus::kIdx stage, so it's safe to treat it |
| // as constant when reading from locations that can reference only resolved classes. |
| enum ReadBarrierOption { |
| kWithReadBarrier, // Perform a read barrier. |
| kWithoutReadBarrier, // Don't perform a read barrier. |
| }; |
| |
| } // namespace art |
| |
| #endif // ART_RUNTIME_READ_BARRIER_OPTION_H_ |