// Licensed under GPLv2 or any later version
// 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
// documentation and/or other materials provided with the distribution.
// * Neither the name of the owner nor the names of its contributors may
// be used to endorse or promote products derived from this software
// without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#pragma once
#include <cstddef>
#include <limits>
#include <type_traits>
#include "common/common_funcs.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:
// We hide 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.
// We don't delete it because we want BitField to be trivially copyable.
constexpr BitField& operator=(const BitField&) = default;
// 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>;
public:
/// Constants to allow limited introspection of fields if needed
static constexpr size_t position = Position;
static constexpr size_t bits = Bits;
static constexpr StorageType mask = (((StorageTypeU)~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 (std::numeric_limits<T>::is_signed) {
std::size_t shift = 8 * sizeof(T) - bits;
return (T)((storage << (shift - position)) >> shift);
} else {
return (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;
// Force default constructor to be created
// so that we can use this within unions
constexpr BitField() = default;
constexpr FORCE_INLINE operator T() const {
return Value();
}
constexpr FORCE_INLINE void Assign(const T& value) {
storage = (storage & ~mask) | FormatValue(value);
}
constexpr T Value() const {
return ExtractValue(storage);
}
// TODO: we may want to change this to explicit operator bool() if it's bug-free in VS2015
constexpr FORCE_INLINE bool ToBool() const {
return Value() != 0;
}
private:
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_trivially_copyable_v<T>, "T must be trivially copyable in a BitField");
};
#pragma pack()
