1 /* 2 * Copyright (C) 2017 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_SUBTYPE_CHECK_BITS_AND_STATUS_H_ 18 #define ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_ 19 20 #include "base/bit_struct.h" 21 #include "base/bit_utils.h" 22 #include "base/casts.h" 23 #include "class_status.h" 24 #include "subtype_check_bits.h" 25 26 namespace art { 27 28 /* 29 * Enables a highly efficient O(1) subtype comparison by storing extra data 30 * in the unused padding bytes of ClassStatus. 31 */ 32 33 // TODO: Update BitSizeOf with this version. 34 template <typename T> 35 static constexpr size_t NonNumericBitSizeOf() { 36 return kBitsPerByte * sizeof(T); 37 } 38 39 /** 40 * MSB (most significant bit) LSB 41 * +---------------+---------------------------------------------------+ 42 * | | | 43 * | ClassStatus | SubtypeCheckBits | 44 * | | | 45 * +---------------+---------------------------------------------------+ 46 * <-- 4 bits --> <----- 28 bits -----> 47 * 48 * Invariants: 49 * 50 * AddressOf(ClassStatus) == AddressOf(SubtypeCheckBitsAndStatus) 51 * BitSizeOf(SubtypeCheckBitsAndStatus) == 32 52 * 53 * Note that with this representation the "Path To Root" in the MSB of this 32-bit word: 54 * This enables a highly efficient path comparison between any two labels: 55 * 56 * src <: target := 57 * (src & mask) == (target & mask) where mask := (1u << len(path-to-root(target)) - 1u 58 * 59 * In the above example, the `len()` (and thus `mask`) is a function of the depth. 60 * Since the target is known at compile time, it becomes 61 * (src & #imm_mask) == #imm 62 * or 63 * ((src - #imm) << #imm_shift_to_remove_high_bits) == 0 64 * or a similar expression chosen for the best performance or code size. 65 * 66 * (This requires that path-to-root in `target` is not truncated, i.e. it is in the Assigned state). 67 */ 68 static constexpr size_t kClassStatusBitSize = MinimumBitsToStore(enum_cast<>(ClassStatus::kLast)); 69 static_assert(kClassStatusBitSize == 4u, "ClassStatus should need 4 bits."); 70 BITSTRUCT_DEFINE_START(SubtypeCheckBitsAndStatus, BitSizeOf<BitString::StorageType>()) 71 BitStructField<SubtypeCheckBits, /*lsb=*/ 0> subtype_check_info_; 72 BitStructField<ClassStatus, 73 /*lsb=*/ SubtypeCheckBits::BitStructSizeOf(), 74 /*width=*/ kClassStatusBitSize> status_; 75 BitStructInt</*lsb=*/ 0, /*width=*/ BitSizeOf<BitString::StorageType>()> int32_alias_; 76 BITSTRUCT_DEFINE_END(SubtypeCheckBitsAndStatus); 77 78 // Use the spare alignment from "ClassStatus" to store all the new SubtypeCheckInfo data. 79 static_assert(sizeof(SubtypeCheckBitsAndStatus) == sizeof(uint32_t), 80 "All of SubtypeCheckInfo+ClassStatus should fit into 4 bytes"); 81 } // namespace art 82 83 #endif // ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_ 84