/*
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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_LIBARTBASE_BASE_BIT_STRUCT_H_
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#define ART_LIBARTBASE_BASE_BIT_STRUCT_H_
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#include "bit_struct_detail.h"
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#include "bit_utils.h"
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//
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// Zero-cost, type-safe, well-defined "structs" of bit fields.
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//
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// ---------------------------------------------
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// Usage example:
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// ---------------------------------------------
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//
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// // Definition for type 'Example'
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// BITSTRUCT_DEFINE_START(Example, 10)
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// BitStructUint<0, 2> u2; // Every field must be a BitStruct[*].
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// BitStructInt<2, 7> i7;
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// BitStructUint<9, 1> i1;
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// BITSTRUCT_DEFINE_END(Example);
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//
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// Would define a bit struct with this layout:
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// <- 1 -> <-- 7 --> <- 2 ->
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// +--------+---------------+-----+
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// | i1 | i7 | u2 +
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// +--------+---------------+-----+
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// 10 9 2 0
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//
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// // Read-write just like regular values.
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// Example ex;
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// ex.u2 = 3;
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// ex.i7 = -25;
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// ex.i1 = true;
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// size_t u2 = ex.u2;
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// int i7 = ex.i7;
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// bool i1 = ex.i1;
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//
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// // It's packed down to the smallest # of machine words.
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// assert(sizeof(Example) == 2);
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// // The exact bit pattern is well-defined by the template parameters.
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// uint16_t cast = *reinterpret_cast<uint16_t*>(ex);
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// assert(cast == ((3) | (0b100111 << 2) | (true << 9);
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//
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// ---------------------------------------------
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// Why not just use C++ bitfields?
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// ---------------------------------------------
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//
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// The layout is implementation-defined.
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// We do not know whether the fields are packed left-to-right or
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// right-to-left, so it makes it useless when the memory layout needs to be
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// precisely controlled.
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//
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// ---------------------------------------------
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// More info:
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// ---------------------------------------------
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// Currently uintmax_t is the largest supported underlying storage type,
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// all (kBitOffset + kBitWidth) must fit into BitSizeOf<uintmax_t>();
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//
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// Using BitStruct[U]int will automatically select an underlying type
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// that's the smallest to fit your (offset + bitwidth).
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//
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// BitStructNumber can be used to manually select an underlying type.
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//
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// BitStructField can be used with custom standard-layout structs,
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// thus allowing for arbitrary nesting of bit structs.
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//
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namespace art {
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// Zero-cost wrapper around a struct 'T', allowing it to be stored as a bitfield
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// at offset 'kBitOffset' and width 'kBitWidth'.
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// The storage is plain unsigned int, whose size is the smallest required to fit
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// 'kBitOffset + kBitWidth'. All operations to this become BitFieldExtract/BitFieldInsert
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// operations to the underlying uint.
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//
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// Field memory representation:
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//
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// MSB <-- width --> LSB
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// +--------+------------+--------+
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// | ?????? | u bitfield | ?????? +
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// +--------+------------+--------+
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// offset 0
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//
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// Reading/writing the bitfield (un)packs it into a temporary T:
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//
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// MSB <-- width --> LSB
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// +-----------------+------------+
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// | 0.............0 | T bitfield |
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// +-----------------+------------+
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// 0
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//
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// It's the responsibility of the StorageType to ensure the bit representation
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// of T can be represented by kBitWidth.
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template <typename T,
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size_t kBitOffset,
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size_t kBitWidth = BitStructSizeOf<T>(),
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typename StorageType = typename detail::MinimumTypeUnsignedHelper<kBitOffset + kBitWidth>::type>
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struct BitStructField {
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static_assert(std::is_standard_layout<T>::value, "T must be standard layout");
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operator T() const {
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return Get();
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}
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// Exclude overload when T==StorageType.
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template <typename _ = void,
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typename = std::enable_if_t<std::is_same<T, StorageType>::value, _>>
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explicit operator StorageType() const {
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return GetStorage();
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}
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BitStructField& operator=(T value) {
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return Assign(*this, value);
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}
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static constexpr size_t BitStructSizeOf() {
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return kBitWidth;
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}
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BitStructField& operator=(const BitStructField& other) {
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// Warning. The default operator= will overwrite the entire storage!
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return *this = static_cast<T>(other);
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}
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BitStructField(const BitStructField& other) {
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Assign(*this, static_cast<T>(other));
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}
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BitStructField() = default;
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~BitStructField() = default;
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protected:
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template <typename T2>
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T2& Assign(T2& what, T value) {
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// Since C++ doesn't allow the type of operator= to change out
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// in the subclass, reimplement operator= in each subclass
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// manually and call this helper function.
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static_assert(std::is_base_of<BitStructField, T2>::value, "T2 must inherit BitStructField");
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what.Set(value);
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return what;
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}
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T Get() const {
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ValueStorage vs;
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vs.pod_.val_ = GetStorage();
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return vs.value_;
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}
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void Set(T value) {
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ValueStorage value_as_storage;
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value_as_storage.value_ = value;
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storage_.pod_.val_ = BitFieldInsert(storage_.pod_.val_,
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value_as_storage.pod_.val_,
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kBitOffset,
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kBitWidth);
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}
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private:
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StorageType GetStorage() const {
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return BitFieldExtract(storage_.pod_.val_, kBitOffset, kBitWidth);
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}
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// Underlying value must be wrapped in a separate standard-layout struct.
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// See below for more details.
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struct PodWrapper {
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StorageType val_;
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};
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union ValueStorage {
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// Safely alias pod_ and value_ together.
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//
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// See C++ 9.5.1 [class.union]:
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// If a standard-layout union contains several standard-layout structs that share a common
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// initial sequence ... it is permitted to inspect the common initial sequence of any of
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// standard-layout struct members.
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PodWrapper pod_;
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T value_;
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} storage_;
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// Future work: In theory almost non-standard layout can be supported here,
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// assuming they don't rely on the address of (this).
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// We just have to use memcpy since the union-aliasing would not work.
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};
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// Base class for number-like BitStruct fields.
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// T is the type to store in as a bit field.
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// kBitOffset, kBitWidth define the position and length of the bitfield.
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//
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// (Common usage should be BitStructInt, BitStructUint -- this
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// intermediate template allows a user-defined integer to be used.)
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template <typename T, size_t kBitOffset, size_t kBitWidth>
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struct BitStructNumber : public BitStructField<T, kBitOffset, kBitWidth, /*StorageType*/T> {
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using StorageType = T;
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BitStructNumber& operator=(T value) {
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return BaseType::Assign(*this, value);
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}
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/*implicit*/ operator T() const {
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return Get();
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}
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explicit operator bool() const {
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return static_cast<bool>(Get());
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}
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BitStructNumber& operator++() {
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*this = Get() + 1u;
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return *this;
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}
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StorageType operator++(int) {
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return Get() + 1u;
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}
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BitStructNumber& operator--() {
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*this = Get() - 1u;
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return *this;
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}
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StorageType operator--(int) {
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return Get() - 1u;
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}
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private:
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using BaseType = BitStructField<T, kBitOffset, kBitWidth, /*StorageType*/T>;
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using BaseType::Get;
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};
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// Create a BitStruct field which uses the smallest underlying int storage type,
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// in order to be large enough to fit (kBitOffset + kBitWidth).
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//
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// Values are sign-extended when they are read out.
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template <size_t kBitOffset, size_t kBitWidth>
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using BitStructInt =
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BitStructNumber<typename detail::MinimumTypeHelper<int, kBitOffset + kBitWidth>::type,
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kBitOffset,
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kBitWidth>;
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// Create a BitStruct field which uses the smallest underlying uint storage type,
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// in order to be large enough to fit (kBitOffset + kBitWidth).
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//
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// Values are zero-extended when they are read out.
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template <size_t kBitOffset, size_t kBitWidth>
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using BitStructUint =
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BitStructNumber<typename detail::MinimumTypeHelper<unsigned int, kBitOffset + kBitWidth>::type,
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kBitOffset,
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kBitWidth>;
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// Start a definition for a bitstruct.
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// A bitstruct is defined to be a union with a common initial subsequence
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// that we call 'DefineBitStructSize<bitwidth>'.
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//
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// See top of file for usage example.
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//
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// This marker is required by the C++ standard in order to
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// have a "common initial sequence".
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//
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// See C++ 9.5.1 [class.union]:
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// If a standard-layout union contains several standard-layout structs that share a common
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// initial sequence ... it is permitted to inspect the common initial sequence of any of
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// standard-layout struct members.
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#define BITSTRUCT_DEFINE_START(name, bitwidth) \
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union name { /* NOLINT */ \
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art::detail::DefineBitStructSize<(bitwidth)> _; \
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static constexpr size_t BitStructSizeOf() { return (bitwidth); } \
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name& operator=(const name& other) { _ = other._; return *this; } /* NOLINT */ \
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name(const name& other) : _(other._) {} \
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name() = default; \
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~name() = default;
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// End the definition of a bitstruct, and insert a sanity check
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// to ensure that the bitstruct did not exceed the specified size.
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//
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// See top of file for usage example.
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#define BITSTRUCT_DEFINE_END(name) \
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}; \
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static_assert(art::detail::ValidateBitStructSize<name>(), \
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#name "bitsize incorrect: " \
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"did you insert extra fields that weren't BitStructX, " \
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"and does the size match the sum of the field widths?")
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// Determine the minimal bit size for a user-defined type T.
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// Used by BitStructField to determine how small a custom type is.
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template <typename T>
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static constexpr size_t BitStructSizeOf() {
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return T::BitStructSizeOf();
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}
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} // namespace art
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#endif // ART_LIBARTBASE_BASE_BIT_STRUCT_H_
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