Lime3DS/src/common/bit_field.h

// Licensed under GPLv2
// Refer to the license.txt file included.


// Copyright 2014 Tony Wasserka
// All rights reserved.
//
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#pragma once

#include <limits>
#include <type_traits>

#include "common/common.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>
struct BitField
{
private:
    // This constructor might be considered ambiguous:
    // Would it initialize the storage or just the bitfield?
    // Hence, delete it. Use the assignment operator to set bitfield values!
    BitField(T val) = delete;

public:
    // Force default constructor to be created
    // so that we can use this within unions
    BitField() = default;

#ifndef _WIN32
    // We explicitly delete the copy assigment operator here, because the
    // default copy assignment would copy the full storage value, rather than
    // just the bits relevant to this particular bit field.
    // Ideally, we would just implement the copy assignment to copy only the
    // relevant bits, but this requires compiler support for unrestricted
    // unions.
    // MSVC 2013 has no support for this, hence we disable this code on
    // Windows (so that the default copy assignment operator will be used).
    // For any C++11 conformant compiler we delete the operator to make sure
    // we never use this inappropriate operator to begin with.
    // TODO: Implement this operator properly once all target compilers
    // support unrestricted unions.
    BitField& operator=(const BitField&) = delete;
#endif

    __forceinline BitField& operator=(T val)
    {
        storage = (storage & ~GetMask()) | (((StorageType)val << position) & GetMask());
        return *this;
    }

    __forceinline operator T() const
    {
        return Value();
    }

    __forceinline T Value() const
    {
        if (std::numeric_limits<T>::is_signed)
        {
            std::size_t shift = 8 * sizeof(T)-bits;
            return (T)(((storage & GetMask()) << (shift - position)) >> shift);
        }
        else
        {
            return (T)((storage & GetMask()) >> position);
        }
    }

private:
    // StorageType 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.
    typedef typename std::conditional < std::is_enum<T>::value,
        std::underlying_type<T>,
        std::enable_if < true, T >> ::type::type StorageType;

    // Unsigned version of StorageType
    typedef typename std::make_unsigned<StorageType>::type StorageTypeU;

    __forceinline StorageType GetMask() const
    {
        return ((~(StorageTypeU)0) >> (8 * sizeof(T)-bits)) << position;
    }

    StorageType 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_standard_layout<T>::value, "Invalid base type");
};
#pragma pack()