// 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;
[[nodiscard]] constexpr operator T() const {
return Value();
}
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);
}
[[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>;
