196 lines
7.3 KiB
C++
196 lines
7.3 KiB
C++
// Licensed under GPLv2 or any later version
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// Refer to the license.txt file included.
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// Copyright 2014 Tony Wasserka
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above copyright
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// notice, this list of conditions and the following disclaimer in the
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// documentation and/or other materials provided with the distribution.
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// * Neither the name of the owner nor the names of its contributors may
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// be used to endorse or promote products derived from this software
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// without specific prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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#pragma once
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#include <cstddef>
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#include <limits>
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#include <type_traits>
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#include "common/common_funcs.h"
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/*
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* Abstract bitfield class
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*
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* Allows endianness-independent access to individual bitfields within some raw
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* integer value. The assembly generated by this class is identical to the
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* usage of raw bitfields, so it's a perfectly fine replacement.
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*
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* For BitField<X,Y,Z>, X is the distance of the bitfield to the LSB of the
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* raw value, Y is the length in bits of the bitfield. Z is an integer type
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* which determines the sign of the bitfield. Z must have the same size as the
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* raw integer.
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*
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*
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* General usage:
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*
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* Create a new union with the raw integer value as a member.
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* Then for each bitfield you want to expose, add a BitField member
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* in the union. The template parameters are the bit offset and the number
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* of desired bits.
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*
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* Changes in the bitfield members will then get reflected in the raw integer
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* value and vice-versa.
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*
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*
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* Sample usage:
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*
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* union SomeRegister
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* {
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* u32 hex;
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*
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* BitField<0,7,u32> first_seven_bits; // unsigned
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* BitField<7,8,u32> next_eight_bits; // unsigned
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* BitField<3,15,s32> some_signed_fields; // signed
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* };
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*
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* This is equivalent to the little-endian specific code:
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*
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* union SomeRegister
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* {
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* u32 hex;
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*
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* struct
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* {
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* u32 first_seven_bits : 7;
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* u32 next_eight_bits : 8;
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* };
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* struct
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* {
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* u32 : 3; // padding
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* s32 some_signed_fields : 15;
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* };
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* };
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*
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*
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* Caveats:
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*
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* 1)
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* BitField provides automatic casting from and to the storage type where
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* appropriate. However, when using non-typesafe functions like printf, an
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* explicit cast must be performed on the BitField object to make sure it gets
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* passed correctly, e.g.:
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* printf("Value: %d", (s32)some_register.some_signed_fields);
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*
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* 2)
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* Not really a caveat, but potentially irritating: This class is used in some
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* packed structures that do not guarantee proper alignment. Therefore we have
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* to use #pragma pack here not to pack the members of the class, but instead
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* to break GCC's assumption that the members of the class are aligned on
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* sizeof(StorageType).
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* TODO(neobrain): Confirm that this is a proper fix and not just masking
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* symptoms.
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*/
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#pragma pack(1)
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template <std::size_t Position, std::size_t Bits, typename T>
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struct BitField {
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private:
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// We hide the copy assigment operator here, because the default copy
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// assignment would copy the full storage value, rather than just the bits
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// relevant to this particular bit field.
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// We don't delete it because we want BitField to be trivially copyable.
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constexpr BitField& operator=(const BitField&) = default;
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// StorageType is T for non-enum types and the underlying type of T if
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// T is an enumeration. Note that T is wrapped within an enable_if in the
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// former case to workaround compile errors which arise when using
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// std::underlying_type<T>::type directly.
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using StorageType = typename std::conditional_t<std::is_enum<T>::value, std::underlying_type<T>,
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std::enable_if<true, T>>::type;
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// Unsigned version of StorageType
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using StorageTypeU = std::make_unsigned_t<StorageType>;
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public:
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/// Constants to allow limited introspection of fields if needed
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static constexpr size_t position = Position;
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static constexpr size_t bits = Bits;
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static constexpr StorageType mask = (((StorageTypeU)~0) >> (8 * sizeof(T) - bits)) << position;
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/**
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* Formats a value by masking and shifting it according to the field parameters. A value
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* containing several bitfields can be assembled by formatting each of their values and ORing
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* the results together.
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*/
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static constexpr FORCE_INLINE StorageType FormatValue(const T& value) {
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return ((StorageType)value << position) & mask;
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}
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/**
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* Extracts a value from the passed storage. In most situations prefer use the member functions
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* (such as Value() or operator T), but this can be used to extract a value from a bitfield
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* union in a constexpr context.
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*/
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static constexpr FORCE_INLINE T ExtractValue(const StorageType& storage) {
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if (std::numeric_limits<T>::is_signed) {
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std::size_t shift = 8 * sizeof(T) - bits;
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return (T)((storage << (shift - position)) >> shift);
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} else {
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return (T)((storage & mask) >> position);
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}
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}
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// This constructor and assignment operator might be considered ambiguous:
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// Would they initialize the storage or just the bitfield?
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// Hence, delete them. Use the Assign method to set bitfield values!
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BitField(T val) = delete;
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BitField& operator=(T val) = delete;
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// Force default constructor to be created
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// so that we can use this within unions
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constexpr BitField() = default;
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constexpr FORCE_INLINE operator T() const {
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return Value();
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}
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constexpr FORCE_INLINE void Assign(const T& value) {
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storage = (storage & ~mask) | FormatValue(value);
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}
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constexpr T Value() const {
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return ExtractValue(storage);
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}
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constexpr explicit operator bool() const {
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return Value() != 0;
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}
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private:
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StorageType storage;
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static_assert(bits + position <= 8 * sizeof(T), "Bitfield out of range");
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// And, you know, just in case people specify something stupid like bits=position=0x80000000
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static_assert(position < 8 * sizeof(T), "Invalid position");
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static_assert(bits <= 8 * sizeof(T), "Invalid number of bits");
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static_assert(bits > 0, "Invalid number of bits");
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static_assert(std::is_trivially_copyable_v<T>, "T must be trivially copyable in a BitField");
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};
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#pragma pack()
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