dolphin/Source/Core/Common/BitField.h

// Copyright 2014 Dolphin Emulator Project
// Licensed under GPLv2+
// Refer to the license.txt file included.

// Copyright 2014 Tony Wasserka
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above copyright
//       notice, this list of conditions and the following disclaimer in the
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//       be used to endorse or promote products derived from this software
//       without specific prior written permission.
//
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#pragma once

#include <cstddef>
#include <limits>
#include <type_traits>

#include "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
  constexpr BitField() = default;

  // We explicitly delete the copy assignment 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.
  // TODO: Implement this operator properly once all target compilers
  // support unrestricted unions.
  BitField& operator=(const BitField&) = delete;

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

  constexpr T Value() const { return Value(std::is_signed<T>()); }
  constexpr operator T() const { return Value(); }
  constexpr std::size_t StartBit() const { return position; }
  constexpr std::size_t NumBits() const { return bits; }
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.
  using StorageType = typename std::conditional_t<std::is_enum<T>::value, std::underlying_type<T>,
                                                  std::enable_if<true, T>>::type;

  // Unsigned version of StorageType
  using StorageTypeU = std::make_unsigned_t<StorageType>;

  constexpr T Value(std::true_type) const
  {
    using shift_amount = std::integral_constant<size_t, 8 * sizeof(T) - bits>;
    return static_cast<T>((storage << (shift_amount() - position)) >> shift_amount());
  }

  constexpr T Value(std::false_type) const
  {
    return static_cast<T>((storage & GetMask()) >> position);
  }

  static constexpr StorageType GetMask()
  {
    return (std::numeric_limits<StorageTypeU>::max() >> (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");
};
#pragma pack()