| // Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file |
| // for details. All rights reserved. Use of this source code is governed by a |
| // BSD-style license that can be found in the LICENSE file. |
| |
| #ifndef RUNTIME_VM_POINTER_TAGGING_H_ |
| #define RUNTIME_VM_POINTER_TAGGING_H_ |
| |
| #include "platform/assert.h" |
| #include "platform/globals.h" |
| |
| // This header defines constants associated with pointer tagging: |
| // |
| // * which bits determine whether or not this is a Smi value or a heap |
| // pointer; |
| // * which bits determine whether this is a pointer into a new or an old |
| // space. |
| |
| namespace dart { |
| |
| // Dart VM aligns all objects by 2 words in in the old space and misaligns them |
| // in new space. This allows to distinguish new and old pointers by their bits. |
| // |
| // Note: these bits depend on the word size. |
| template <intptr_t word_size, intptr_t word_size_log2> |
| struct ObjectAlignment { |
| // Alignment offsets are used to determine object age. |
| static constexpr intptr_t kNewObjectAlignmentOffset = word_size; |
| static constexpr intptr_t kOldObjectAlignmentOffset = 0; |
| static constexpr intptr_t kNewObjectBitPosition = word_size_log2; |
| |
| // Object sizes are aligned to kObjectAlignment. |
| static constexpr intptr_t kObjectAlignment = 2 * word_size; |
| static constexpr intptr_t kObjectAlignmentLog2 = word_size_log2 + 1; |
| static constexpr intptr_t kObjectAlignmentMask = kObjectAlignment - 1; |
| |
| // Discriminate between true and false based on the alignment bit. |
| static constexpr intptr_t kBoolValueBitPosition = kObjectAlignmentLog2; |
| static constexpr intptr_t kBoolValueMask = 1 << kBoolValueBitPosition; |
| |
| static constexpr intptr_t kTrueOffsetFromNull = kObjectAlignment * 2; |
| static constexpr intptr_t kFalseOffsetFromNull = kObjectAlignment * 3; |
| }; |
| |
| using HostObjectAlignment = ObjectAlignment<kWordSize, kWordSizeLog2>; |
| |
| static constexpr intptr_t kNewObjectAlignmentOffset = |
| HostObjectAlignment::kNewObjectAlignmentOffset; |
| static constexpr intptr_t kOldObjectAlignmentOffset = |
| HostObjectAlignment::kOldObjectAlignmentOffset; |
| static constexpr intptr_t kNewObjectBitPosition = |
| HostObjectAlignment::kNewObjectBitPosition; |
| static constexpr intptr_t kObjectAlignment = |
| HostObjectAlignment::kObjectAlignment; |
| static constexpr intptr_t kObjectAlignmentLog2 = |
| HostObjectAlignment::kObjectAlignmentLog2; |
| static constexpr intptr_t kObjectAlignmentMask = |
| HostObjectAlignment::kObjectAlignmentMask; |
| static constexpr intptr_t kBoolValueBitPosition = |
| HostObjectAlignment::kBoolValueBitPosition; |
| static constexpr intptr_t kBoolValueMask = HostObjectAlignment::kBoolValueMask; |
| static constexpr intptr_t kTrueOffsetFromNull = |
| HostObjectAlignment::kTrueOffsetFromNull; |
| static constexpr intptr_t kFalseOffsetFromNull = |
| HostObjectAlignment::kFalseOffsetFromNull; |
| |
| // The largest value of kObjectAlignment across all configurations. |
| static constexpr intptr_t kMaxObjectAlignment = 16; |
| COMPILE_ASSERT(kMaxObjectAlignment >= kObjectAlignment); |
| |
| // On all targets heap pointers are tagged by set least significant bit. |
| // |
| // To recover address of the actual heap object kHeapObjectTag needs to be |
| // subtracted from the tagged pointer value. |
| // |
| // Smi-s (small integers) have least significant bit cleared. |
| // |
| // To recover the integer value tagged pointer value needs to be shifted |
| // right by kSmiTagShift. |
| enum { |
| kSmiTag = 0, |
| kHeapObjectTag = 1, |
| kSmiTagSize = 1, |
| kSmiTagMask = 1, |
| kSmiTagShift = 1, |
| }; |
| |
| } // namespace dart |
| |
| #endif // RUNTIME_VM_POINTER_TAGGING_H_ |