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The C and C++ Include Header Files
/usr/include/python3.12/internal/pycore_bitutils.h
$ cat -n /usr/include/python3.12/internal/pycore_bitutils.h 1 /* Bit and bytes utilities. 2 3 Bytes swap functions, reverse order of bytes: 4 5 - _Py_bswap16(uint16_t) 6 - _Py_bswap32(uint32_t) 7 - _Py_bswap64(uint64_t) 8 */ 9 10 #ifndef Py_INTERNAL_BITUTILS_H 11 #define Py_INTERNAL_BITUTILS_H 12 #ifdef __cplusplus 13 extern "C" { 14 #endif 15 16 #ifndef Py_BUILD_CORE 17 # error "this header requires Py_BUILD_CORE define" 18 #endif 19 20 #if defined(__GNUC__) \ 21 && ((__GNUC__ >= 5) || (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)) 22 /* __builtin_bswap16() is available since GCC 4.8, 23 __builtin_bswap32() is available since GCC 4.3, 24 __builtin_bswap64() is available since GCC 4.3. */ 25 # define _PY_HAVE_BUILTIN_BSWAP 26 #endif 27 28 #ifdef _MSC_VER 29 /* Get _byteswap_ushort(), _byteswap_ulong(), _byteswap_uint64() */ 30 # include <intrin.h> 31 #endif 32 33 static inline uint16_t 34 _Py_bswap16(uint16_t word) 35 { 36 #if defined(_PY_HAVE_BUILTIN_BSWAP) || _Py__has_builtin(__builtin_bswap16) 37 return __builtin_bswap16(word); 38 #elif defined(_MSC_VER) 39 Py_BUILD_ASSERT(sizeof(word) == sizeof(unsigned short)); 40 return _byteswap_ushort(word); 41 #else 42 // Portable implementation which doesn't rely on circular bit shift 43 return ( ((word & UINT16_C(0x00FF)) << 8) 44 | ((word & UINT16_C(0xFF00)) >> 8)); 45 #endif 46 } 47 48 static inline uint32_t 49 _Py_bswap32(uint32_t word) 50 { 51 #if defined(_PY_HAVE_BUILTIN_BSWAP) || _Py__has_builtin(__builtin_bswap32) 52 return __builtin_bswap32(word); 53 #elif defined(_MSC_VER) 54 Py_BUILD_ASSERT(sizeof(word) == sizeof(unsigned long)); 55 return _byteswap_ulong(word); 56 #else 57 // Portable implementation which doesn't rely on circular bit shift 58 return ( ((word & UINT32_C(0x000000FF)) << 24) 59 | ((word & UINT32_C(0x0000FF00)) << 8) 60 | ((word & UINT32_C(0x00FF0000)) >> 8) 61 | ((word & UINT32_C(0xFF000000)) >> 24)); 62 #endif 63 } 64 65 static inline uint64_t 66 _Py_bswap64(uint64_t word) 67 { 68 #if defined(_PY_HAVE_BUILTIN_BSWAP) || _Py__has_builtin(__builtin_bswap64) 69 return __builtin_bswap64(word); 70 #elif defined(_MSC_VER) 71 return _byteswap_uint64(word); 72 #else 73 // Portable implementation which doesn't rely on circular bit shift 74 return ( ((word & UINT64_C(0x00000000000000FF)) << 56) 75 | ((word & UINT64_C(0x000000000000FF00)) << 40) 76 | ((word & UINT64_C(0x0000000000FF0000)) << 24) 77 | ((word & UINT64_C(0x00000000FF000000)) << 8) 78 | ((word & UINT64_C(0x000000FF00000000)) >> 8) 79 | ((word & UINT64_C(0x0000FF0000000000)) >> 24) 80 | ((word & UINT64_C(0x00FF000000000000)) >> 40) 81 | ((word & UINT64_C(0xFF00000000000000)) >> 56)); 82 #endif 83 } 84 85 86 // Population count: count the number of 1's in 'x' 87 // (number of bits set to 1), also known as the hamming weight. 88 // 89 // Implementation note. CPUID is not used, to test if x86 POPCNT instruction 90 // can be used, to keep the implementation simple. For example, Visual Studio 91 // __popcnt() is not used this reason. The clang and GCC builtin function can 92 // use the x86 POPCNT instruction if the target architecture has SSE4a or 93 // newer. 94 static inline int 95 _Py_popcount32(uint32_t x) 96 { 97 #if (defined(__clang__) || defined(__GNUC__)) 98 99 #if SIZEOF_INT >= 4 100 Py_BUILD_ASSERT(sizeof(x) <= sizeof(unsigned int)); 101 return __builtin_popcount(x); 102 #else 103 // The C standard guarantees that unsigned long will always be big enough 104 // to hold a uint32_t value without losing information. 105 Py_BUILD_ASSERT(sizeof(x) <= sizeof(unsigned long)); 106 return __builtin_popcountl(x); 107 #endif 108 109 #else 110 // 32-bit SWAR (SIMD Within A Register) popcount 111 112 // Binary: 0 1 0 1 ... 113 const uint32_t M1 = 0x55555555; 114 // Binary: 00 11 00 11. .. 115 const uint32_t M2 = 0x33333333; 116 // Binary: 0000 1111 0000 1111 ... 117 const uint32_t M4 = 0x0F0F0F0F; 118 119 // Put count of each 2 bits into those 2 bits 120 x = x - ((x >> 1) & M1); 121 // Put count of each 4 bits into those 4 bits 122 x = (x & M2) + ((x >> 2) & M2); 123 // Put count of each 8 bits into those 8 bits 124 x = (x + (x >> 4)) & M4; 125 // Sum of the 4 byte counts. 126 // Take care when considering changes to the next line. Portability and 127 // correctness are delicate here, thanks to C's "integer promotions" (C99 128 // §6.3.1.1p2). On machines where the `int` type has width greater than 32 129 // bits, `x` will be promoted to an `int`, and following C's "usual 130 // arithmetic conversions" (C99 §6.3.1.8), the multiplication will be 131 // performed as a multiplication of two `unsigned int` operands. In this 132 // case it's critical that we cast back to `uint32_t` in order to keep only 133 // the least significant 32 bits. On machines where the `int` type has 134 // width no greater than 32, the multiplication is of two 32-bit unsigned 135 // integer types, and the (uint32_t) cast is a no-op. In both cases, we 136 // avoid the risk of undefined behaviour due to overflow of a 137 // multiplication of signed integer types. 138 return (uint32_t)(x * 0x01010101U) >> 24; 139 #endif 140 } 141 142 143 // Return the index of the most significant 1 bit in 'x'. This is the smallest 144 // integer k such that x < 2**k. Equivalent to floor(log2(x)) + 1 for x != 0. 145 static inline int 146 _Py_bit_length(unsigned long x) 147 { 148 #if (defined(__clang__) || defined(__GNUC__)) 149 if (x != 0) { 150 // __builtin_clzl() is available since GCC 3.4. 151 // Undefined behavior for x == 0. 152 return (int)sizeof(unsigned long) * 8 - __builtin_clzl(x); 153 } 154 else { 155 return 0; 156 } 157 #elif defined(_MSC_VER) 158 // _BitScanReverse() is documented to search 32 bits. 159 Py_BUILD_ASSERT(sizeof(unsigned long) <= 4); 160 unsigned long msb; 161 if (_BitScanReverse(&msb, x)) { 162 return (int)msb + 1; 163 } 164 else { 165 return 0; 166 } 167 #else 168 const int BIT_LENGTH_TABLE[32] = { 169 0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 170 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5 171 }; 172 int msb = 0; 173 while (x >= 32) { 174 msb += 6; 175 x >>= 6; 176 } 177 msb += BIT_LENGTH_TABLE[x]; 178 return msb; 179 #endif 180 } 181 182 183 #ifdef __cplusplus 184 } 185 #endif 186 #endif /* !Py_INTERNAL_BITUTILS_H */
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