1// Copyright (c) 2015-2016 The Khronos Group Inc.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15#ifndef SOURCE_UTIL_BITUTILS_H_
16#define SOURCE_UTIL_BITUTILS_H_
17
18#include <cassert>
19#include <cstdint>
20#include <cstring>
21#include <type_traits>
22
23namespace spvtools {
24namespace utils {
25
26// Performs a bitwise copy of source to the destination type Dest.
27template <typename Dest, typename Src>
28Dest BitwiseCast(Src source) {
29 Dest dest;
30 static_assert(sizeof(source) == sizeof(dest),
31 "BitwiseCast: Source and destination must have the same size");
32 std::memcpy(&dest, &source, sizeof(dest));
33 return dest;
34}
35
36// Calculates the bit width of the integer type |T|.
37template <typename T>
38struct IntegerBitWidth {
39 static_assert(std::is_integral<T>::value, "Integer type required");
40 static const size_t kBitsPerByte = 8;
41 static const size_t get = sizeof(T) * kBitsPerByte;
42};
43
44// SetBits<T, First, Num> returns an integer of type <T> with bits set
45// for position <First> through <First + Num - 1>, counting from the least
46// significant bit. In particular when Num == 0, no positions are set to 1.
47// A static assert will be triggered if First + Num > sizeof(T) * 8, that is,
48// a bit that will not fit in the underlying type is set.
49template <typename T, size_t First = 0, size_t Num = 0>
50struct SetBits {
51 static_assert(First < IntegerBitWidth<T>::get,
52 "Tried to set a bit that is shifted too far.");
53 const static T get = (T(1) << First) | SetBits<T, First + 1, Num - 1>::get;
54};
55
56template <typename T, size_t Last>
57struct SetBits<T, Last, 0> {
58 const static T get = T(0);
59};
60
61// This is all compile-time so we can put our tests right here.
62static_assert(IntegerBitWidth<uint32_t>::get == 32, "IntegerBitWidth mismatch");
63static_assert(IntegerBitWidth<int32_t>::get == 32, "IntegerBitWidth mismatch");
64static_assert(IntegerBitWidth<uint64_t>::get == 64, "IntegerBitWidth mismatch");
65static_assert(IntegerBitWidth<uint8_t>::get == 8, "IntegerBitWidth mismatch");
66
67static_assert(SetBits<uint32_t, 0, 0>::get == uint32_t(0x00000000),
68 "SetBits failed");
69static_assert(SetBits<uint32_t, 0, 1>::get == uint32_t(0x00000001),
70 "SetBits failed");
71static_assert(SetBits<uint32_t, 31, 1>::get == uint32_t(0x80000000),
72 "SetBits failed");
73static_assert(SetBits<uint32_t, 1, 2>::get == uint32_t(0x00000006),
74 "SetBits failed");
75static_assert(SetBits<uint32_t, 30, 2>::get == uint32_t(0xc0000000),
76 "SetBits failed");
77static_assert(SetBits<uint32_t, 0, 31>::get == uint32_t(0x7FFFFFFF),
78 "SetBits failed");
79static_assert(SetBits<uint32_t, 0, 32>::get == uint32_t(0xFFFFFFFF),
80 "SetBits failed");
81static_assert(SetBits<uint32_t, 16, 16>::get == uint32_t(0xFFFF0000),
82 "SetBits failed");
83
84static_assert(SetBits<uint64_t, 0, 1>::get == uint64_t(0x0000000000000001LL),
85 "SetBits failed");
86static_assert(SetBits<uint64_t, 63, 1>::get == uint64_t(0x8000000000000000LL),
87 "SetBits failed");
88static_assert(SetBits<uint64_t, 62, 2>::get == uint64_t(0xc000000000000000LL),
89 "SetBits failed");
90static_assert(SetBits<uint64_t, 31, 1>::get == uint64_t(0x0000000080000000LL),
91 "SetBits failed");
92static_assert(SetBits<uint64_t, 16, 16>::get == uint64_t(0x00000000FFFF0000LL),
93 "SetBits failed");
94
95// Returns number of '1' bits in a word.
96template <typename T>
97size_t CountSetBits(T word) {
98 static_assert(std::is_integral<T>::value,
99 "CountSetBits requires integer type");
100 size_t count = 0;
101 while (word) {
102 word &= word - 1;
103 ++count;
104 }
105 return count;
106}
107
108// Checks if the bit at the |position| is set to '1'.
109// Bits zero-indexed starting at the least significant bit.
110// |position| must be within the bit width of |T|.
111template <typename T>
112bool IsBitAtPositionSet(T word, size_t position) {
113 static_assert(std::is_integral<T>::value, "Integer type required");
114 static_assert(std::is_unsigned<T>::value, "Unsigned type required");
115 assert(position < IntegerBitWidth<T>::get &&
116 "position must be less than the bit width");
117 return word & T(T(1) << position);
118}
119
120// Returns a value obtained by setting a range of adjacent bits of |word| to
121// |value|. Affected bits are within the range:
122// [first_position, first_position + num_bits_to_mutate),
123// assuming zero-based indexing starting at the least
124// significant bit. Bits to mutate must be within the bit width of |T|.
125template <typename T>
126T MutateBits(T word, size_t first_position, size_t num_bits_to_mutate,
127 bool value) {
128 static_assert(std::is_integral<T>::value, "Integer type required");
129 static_assert(std::is_unsigned<T>::value, "Unsigned type required");
130 static const size_t word_bit_width = IntegerBitWidth<T>::get;
131 assert(first_position < word_bit_width &&
132 "Mutated bits must be within bit width");
133 assert(first_position + num_bits_to_mutate <= word_bit_width &&
134 "Mutated bits must be within bit width");
135 if (num_bits_to_mutate == 0) {
136 return word;
137 }
138
139 const T all_ones = ~T(0);
140 const size_t num_unaffected_low_bits = first_position;
141 const T unaffected_low_mask =
142 T(T(all_ones >> num_unaffected_low_bits) << num_unaffected_low_bits);
143
144 const size_t num_unaffected_high_bits =
145 word_bit_width - (first_position + num_bits_to_mutate);
146 const T unaffected_high_mask =
147 T(T(all_ones << num_unaffected_high_bits) >> num_unaffected_high_bits);
148
149 const T mutation_mask = unaffected_low_mask & unaffected_high_mask;
150 if (value) {
151 return word | mutation_mask;
152 }
153 return word & T(~mutation_mask);
154}
155
156// Returns a value obtained by setting the |num_bits_to_set| highest bits to
157// '1'. |num_bits_to_set| must be not be greater than the bit width of |T|.
158template <typename T>
159T SetHighBits(T word, size_t num_bits_to_set) {
160 if (num_bits_to_set == 0) {
161 return word;
162 }
163 const size_t word_bit_width = IntegerBitWidth<T>::get;
164 assert(num_bits_to_set <= word_bit_width &&
165 "Can't set more bits than bit width");
166 return MutateBits(word, word_bit_width - num_bits_to_set, num_bits_to_set,
167 true);
168}
169
170// Returns a value obtained by setting the |num_bits_to_set| highest bits to
171// '0'. |num_bits_to_set| must be not be greater than the bit width of |T|.
172template <typename T>
173T ClearHighBits(T word, size_t num_bits_to_set) {
174 if (num_bits_to_set == 0) {
175 return word;
176 }
177 const size_t word_bit_width = IntegerBitWidth<T>::get;
178 assert(num_bits_to_set <= word_bit_width &&
179 "Can't clear more bits than bit width");
180 return MutateBits(word, word_bit_width - num_bits_to_set, num_bits_to_set,
181 false);
182}
183
184} // namespace utils
185} // namespace spvtools
186
187#endif // SOURCE_UTIL_BITUTILS_H_
188