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3708354 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 | // SPDX-FileCopyrightText: 2014 Tony Wasserka
// SPDX-FileCopyrightText: 2014 Dolphin Emulator Project
// SPDX-License-Identifier: BSD-3-Clause AND GPL-2.0-or-later
#pragma once
#include <cstddef>
#include <limits>
#include <type_traits>
#include "common/swap.h"
/*
* Abstract bitfield class
*
* Allows endianness-independent access to individual bitfields within some raw
* integer value. The assembly generated by this class is identical to the
* usage of raw bitfields, so it's a perfectly fine replacement.
*
* For BitField<X,Y,Z>, X is the distance of the bitfield to the LSB of the
* raw value, Y is the length in bits of the bitfield. Z is an integer type
* which determines the sign of the bitfield. Z must have the same size as the
* raw integer.
*
*
* General usage:
*
* Create a new union with the raw integer value as a member.
* Then for each bitfield you want to expose, add a BitField member
* in the union. The template parameters are the bit offset and the number
* of desired bits.
*
* Changes in the bitfield members will then get reflected in the raw integer
* value and vice-versa.
*
*
* Sample usage:
*
* union SomeRegister
* {
* u32 hex;
*
* BitField<0,7,u32> first_seven_bits; // unsigned
* BitField<7,8,u32> next_eight_bits; // unsigned
* BitField<3,15,s32> some_signed_fields; // signed
* };
*
* This is equivalent to the little-endian specific code:
*
* union SomeRegister
* {
* u32 hex;
*
* struct
* {
* u32 first_seven_bits : 7;
* u32 next_eight_bits : 8;
* };
* struct
* {
* u32 : 3; // padding
* s32 some_signed_fields : 15;
* };
* };
*
*
* Caveats:
*
* 1)
* BitField provides automatic casting from and to the storage type where
* appropriate. However, when using non-typesafe functions like printf, an
* explicit cast must be performed on the BitField object to make sure it gets
* passed correctly, e.g.:
* printf("Value: %d", (s32)some_register.some_signed_fields);
*
* 2)
* Not really a caveat, but potentially irritating: This class is used in some
* packed structures that do not guarantee proper alignment. Therefore we have
* to use #pragma pack here not to pack the members of the class, but instead
* to break GCC's assumption that the members of the class are aligned on
* sizeof(StorageType).
* TODO(neobrain): Confirm that this is a proper fix and not just masking
* symptoms.
*/
#pragma pack(1)
template <std::size_t Position, std::size_t Bits, typename T, typename EndianTag = LETag>
struct BitField {
private:
// UnderlyingType is T for non-enum types and the underlying type of T if
// T is an enumeration. Note that T is wrapped within an enable_if in the
// former case to workaround compile errors which arise when using
// std::underlying_type<T>::type directly.
using UnderlyingType = typename std::conditional_t<std::is_enum_v<T>, std::underlying_type<T>,
std::enable_if<true, T>>::type;
// We store the value as the unsigned type to avoid undefined behaviour on value shifting
using StorageType = std::make_unsigned_t<UnderlyingType>;
using StorageTypeWithEndian = typename AddEndian<StorageType, EndianTag>::type;
public:
/// Constants to allow limited introspection of fields if needed
static constexpr std::size_t position = Position;
static constexpr std::size_t bits = Bits;
static constexpr StorageType mask = (((StorageType)~0) >> (8 * sizeof(T) - bits)) << position;
/**
* Formats a value by masking and shifting it according to the field parameters. A value
* containing several bitfields can be assembled by formatting each of their values and ORing
* the results together.
*/
[[nodiscard]] static constexpr StorageType FormatValue(const T& value) {
return (static_cast<StorageType>(value) << position) & mask;
}
/**
* Extracts a value from the passed storage. In most situations prefer use the member functions
* (such as Value() or operator T), but this can be used to extract a value from a bitfield
* union in a constexpr context.
*/
[[nodiscard]] static constexpr T ExtractValue(const StorageType& storage) {
if constexpr (std::numeric_limits<UnderlyingType>::is_signed) {
std::size_t shift = 8 * sizeof(T) - bits;
return static_cast<T>(static_cast<UnderlyingType>(storage << (shift - position)) >>
shift);
} else {
return static_cast<T>((storage & mask) >> position);
}
}
// This constructor and assignment operator might be considered ambiguous:
// Would they initialize the storage or just the bitfield?
// Hence, delete them. Use the Assign method to set bitfield values!
BitField(T val) = delete;
BitField& operator=(T val) = delete;
constexpr BitField() noexcept = default;
constexpr BitField(const BitField&) noexcept = default;
constexpr BitField& operator=(const BitField&) noexcept = default;
constexpr BitField(BitField&&) noexcept = default;
constexpr BitField& operator=(BitField&&) noexcept = default;
constexpr void Assign(const T& value) {
#ifdef _MSC_VER
storage = static_cast<StorageType>((storage & ~mask) | FormatValue(value));
#else
// Explicitly reload with memcpy to avoid compiler aliasing quirks
// regarding optimization: GCC/Clang clobber chained stores to
// different bitfields in the same struct with the last value.
StorageTypeWithEndian storage_;
std::memcpy(&storage_, &storage, sizeof(storage_));
storage = static_cast<StorageType>((storage_ & ~mask) | FormatValue(value));
#endif
}
[[nodiscard]] constexpr T Value() const {
return ExtractValue(storage);
}
template <typename ConvertedToType>
[[nodiscard]] constexpr ConvertedToType As() const {
static_assert(!std::is_same_v<T, ConvertedToType>,
"Unnecessary cast. Use Value() instead.");
return static_cast<ConvertedToType>(Value());
}
[[nodiscard]] constexpr operator T() const {
return Value();
}
[[nodiscard]] constexpr explicit operator bool() const {
return Value() != 0;
}
private:
StorageTypeWithEndian storage;
static_assert(bits + position <= 8 * sizeof(T), "Bitfield out of range");
// And, you know, just in case people specify something stupid like bits=position=0x80000000
static_assert(position < 8 * sizeof(T), "Invalid position");
static_assert(bits <= 8 * sizeof(T), "Invalid number of bits");
static_assert(bits > 0, "Invalid number of bits");
static_assert(std::is_trivially_copyable_v<T>, "T must be trivially copyable in a BitField");
};
#pragma pack()
template <std::size_t Position, std::size_t Bits, typename T>
using BitFieldBE = BitField<Position, Bits, T, BETag>;
template <std::size_t Position, std::size_t Bits, typename T, typename EndianTag = LETag>
inline auto format_as(BitField<Position, Bits, T, EndianTag> bitfield) {
return bitfield.Value();
}
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