summaryrefslogtreecommitdiffstats
path: root/src/common/bit_field.h
blob: 3ec061e634554d1a14ca947d3e7fe7f4f35f2aab (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
// 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
//       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
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.


#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)
    {
        Assign(val);
        return *this;
    }

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

    __forceinline void Assign(const T& value) {
        storage = (storage & ~GetMask()) | (((StorageType)value << position) & GetMask());
    }

    __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()