/* * 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_