/*
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* Copyright (C) 2017 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
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#define ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
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#include "base/bit_struct.h"
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#include "base/bit_utils.h"
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#include "base/casts.h"
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#include "class_status.h"
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#include "subtype_check_bits.h"
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namespace art {
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/*
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* Enables a highly efficient O(1) subtype comparison by storing extra data
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* in the unused padding bytes of ClassStatus.
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*/
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// TODO: Update BitSizeOf with this version.
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template <typename T>
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static constexpr size_t NonNumericBitSizeOf() {
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return kBitsPerByte * sizeof(T);
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}
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/**
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* MSB (most significant bit) LSB
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* +---------------+---------------------------------------------------+
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* | | |
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* | ClassStatus | SubtypeCheckBits |
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* | | |
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* +---------------+---------------------------------------------------+
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* <-- 4 bits --> <----- 28 bits ----->
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*
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* Invariants:
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*
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* AddressOf(ClassStatus) == AddressOf(SubtypeCheckBitsAndStatus)
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* BitSizeOf(SubtypeCheckBitsAndStatus) == 32
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*
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* Note that with this representation the "Path To Root" in the MSB of this 32-bit word:
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* This enables a highly efficient path comparison between any two labels:
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*
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* src <: target :=
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* (src & mask) == (target & mask) where mask := (1u << len(path-to-root(target)) - 1u
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*
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* In the above example, the `len()` (and thus `mask`) is a function of the depth.
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* Since the target is known at compile time, it becomes
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* (src & #imm_mask) == #imm
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* or
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* ((src - #imm) << #imm_shift_to_remove_high_bits) == 0
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* or a similar expression chosen for the best performance or code size.
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*
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* (This requires that path-to-root in `target` is not truncated, i.e. it is in the Assigned state).
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*/
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static constexpr size_t kClassStatusBitSize = MinimumBitsToStore(enum_cast<>(ClassStatus::kLast));
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static_assert(kClassStatusBitSize == 4u, "ClassStatus should need 4 bits.");
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BITSTRUCT_DEFINE_START(SubtypeCheckBitsAndStatus, BitSizeOf<BitString::StorageType>())
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BitStructField<SubtypeCheckBits, /*lsb=*/ 0> subtype_check_info_;
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BitStructField<ClassStatus,
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/*lsb=*/ SubtypeCheckBits::BitStructSizeOf(),
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/*width=*/ kClassStatusBitSize> status_;
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BitStructInt</*lsb=*/ 0, /*width=*/ BitSizeOf<BitString::StorageType>()> int32_alias_;
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BITSTRUCT_DEFINE_END(SubtypeCheckBitsAndStatus);
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// Use the spare alignment from "ClassStatus" to store all the new SubtypeCheckInfo data.
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static_assert(sizeof(SubtypeCheckBitsAndStatus) == sizeof(uint32_t),
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"All of SubtypeCheckInfo+ClassStatus should fit into 4 bytes");
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} // namespace art
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#endif // ART_RUNTIME_SUBTYPE_CHECK_BITS_AND_STATUS_H_
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