1 /* 2 * Copyright (C) 2014 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef ART_RUNTIME_READ_BARRIER_OPTION_H_ 18 #define ART_RUNTIME_READ_BARRIER_OPTION_H_ 19 namespace art { 20 21 // Options for performing a read barrier or not. 22 // 23 // Besides disabled GC and GC's internal usage, there are a few cases where the read 24 // barrier is unnecessary and can be avoided to reduce code size and improve performance. 25 // In the following cases, the result of the operation or chain of operations shall be the 26 // same whether we go through the from-space or to-space: 27 // 28 // 1. We're reading a reference known to point to an un-reclaimable immune space object. 29 // (For example boot image class and string references, read by compiled code from 30 // .data.bimg.rel.ro . Similarly, such references constructed using position independent 31 // code in the compiled boot image code do not need a read barrier.) 32 // 2. We're reading the reference for comparison involving a non-moving space reference. 33 // (Whether the non-moving space reference is the one we're reading or the one we shall 34 // compare it with, the result is the same with and without read barrier.) 35 // 3. We're reading the reference for comparison with null. 36 // (Similar to 2 above, given that null is "non-moving".) 37 // 4. We're reading a reference to a holder from which we shall read 38 // - constant primitive field, or 39 // - constant reference field known to point to an un-reclaimable immune space object, or 40 // - constant reference field for comparison involving a non-moving space reference, or 41 // - constant reference field for comparison with null, or 42 // - constant reference fields in a chain leading to one or more of the above purposes; 43 // the entire chain needs to be read without read barrier. 44 // The term "constant" refers to fields set to their final value between allocating 45 // the holder and the next opportunity for the holder to be moved by the GC, i.e. 46 // before the first suspend point after the allocation, or seen by another thread. 47 // This includes several fields in the Class object, such as the primitive type or 48 // component type but not the superclass. 49 // 50 // References read without a read barrier must not remain live at the next suspend point, 51 // with the exception of references to un-reclaimable immune space objects. 52 // 53 // For un-reclaimable immune space objects, we rely on graying dirty objects in the FlipCallback 54 // pause (we try to gray them just before flipping thread roots but the FlipCallback has to re-scan 55 // for newly dirtied objects) and clean objects conceptually become black at that point 56 // (marking them through is a no-op as all reference fields must also point to immune spaces), 57 // so mutator threads can never miss a read barrier as they never see white immune space object. 58 enum ReadBarrierOption { 59 kWithReadBarrier, // Perform a read barrier. 60 kWithoutReadBarrier, // Don't perform a read barrier. 61 }; 62 63 } // namespace art 64 65 #endif // ART_RUNTIME_READ_BARRIER_OPTION_H_ 66