| /* |
| * Copyright (C) 2017 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_SUBTYPE_CHECK_BITS_AND_STATUS_H_ |
| #define ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_ |
| |
| #include "base/bit_struct.h" |
| #include "base/bit_utils.h" |
| #include "base/casts.h" |
| #include "class_status.h" |
| #include "subtype_check_bits.h" |
| |
| namespace art { |
| |
| /* |
| * Enables a highly efficient O(1) subtype comparison by storing extra data |
| * in the unused padding bytes of ClassStatus. |
| */ |
| |
| // TODO: Update BitSizeOf with this version. |
| template <typename T> |
| static constexpr size_t NonNumericBitSizeOf() { |
| return kBitsPerByte * sizeof(T); |
| } |
| |
| /** |
| * MSB (most significant bit) LSB |
| * +---------------+---------------------------------------------------+ |
| * | | | |
| * | ClassStatus | SubtypeCheckBits | |
| * | | | |
| * +---------------+---------------------------------------------------+ |
| * <-- 4 bits --> <----- 28 bits -----> |
| * |
| * Invariants: |
| * |
| * AddressOf(ClassStatus) == AddressOf(SubtypeCheckBitsAndStatus) |
| * BitSizeOf(SubtypeCheckBitsAndStatus) == 32 |
| * |
| * Note that with this representation the "Path To Root" in the MSB of this 32-bit word: |
| * This enables a highly efficient path comparison between any two labels: |
| * |
| * src <: target := |
| * (src & mask) == (target & mask) where mask := (1u << len(path-to-root(target)) - 1u |
| * |
| * In the above example, the `len()` (and thus `mask`) is a function of the depth. |
| * Since the target is known at compile time, it becomes |
| * (src & #imm_mask) == #imm |
| * or |
| * ((src - #imm) << #imm_shift_to_remove_high_bits) == 0 |
| * or a similar expression chosen for the best performance or code size. |
| * |
| * (This requires that path-to-root in `target` is not truncated, i.e. it is in the Assigned state). |
| */ |
| static constexpr size_t kClassStatusBitSize = MinimumBitsToStore(enum_cast<>(ClassStatus::kLast)); |
| static_assert(kClassStatusBitSize == 4u, "ClassStatus should need 4 bits."); |
| BITSTRUCT_DEFINE_START(SubtypeCheckBitsAndStatus, BitSizeOf<BitString::StorageType>()) |
| BitStructField<SubtypeCheckBits, /*lsb*/ 0> subtype_check_info_; |
| BitStructField<ClassStatus, |
| /*lsb*/ SubtypeCheckBits::BitStructSizeOf(), |
| /*width*/ kClassStatusBitSize> status_; |
| BitStructInt</*lsb*/ 0, /*width*/ BitSizeOf<BitString::StorageType>()> int32_alias_; |
| BITSTRUCT_DEFINE_END(SubtypeCheckBitsAndStatus); |
| |
| // Use the spare alignment from "ClassStatus" to store all the new SubtypeCheckInfo data. |
| static_assert(sizeof(SubtypeCheckBitsAndStatus) == sizeof(uint32_t), |
| "All of SubtypeCheckInfo+ClassStatus should fit into 4 bytes"); |
| } // namespace art |
| |
| #endif // ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_ |