bit_field.h

// Licensed under GPLv2 or any later version
// Refer to the license.txt file included.

// Copyright 2014 Tony Wasserka
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//
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#pragma once

#include <cstddef>
#include <limits>
#include <type_traits>
#include "common/common_funcs.h"
#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.
     */
    static constexpr FORCE_INLINE StorageType FormatValue(const T& value) {
        return ((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.
     */
    static constexpr FORCE_INLINE 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 operator T() const {
        return Value();
    }

    constexpr void Assign(const T& value) {
        storage = (static_cast<StorageType>(storage) & ~mask) | FormatValue(value);
    }

    constexpr T Value() const {
        return ExtractValue(storage);
    }

    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>;