1 //===----------------------------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 // TODO: __builtin_clzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can 10 // refactor this code to exclusively use __builtin_clzg. 11 12 #ifndef _LIBCPP___CXX03___BIT_COUNTL_H 13 #define _LIBCPP___CXX03___BIT_COUNTL_H 14 15 #include <__cxx03/__bit/rotate.h> 16 #include <__cxx03/__config> 17 #include <__cxx03/__type_traits/is_unsigned_integer.h> 18 #include <__cxx03/limits> 19 20 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 21 # pragma GCC system_header 22 #endif 23 24 _LIBCPP_PUSH_MACROS 25 #include <__cxx03/__undef_macros> 26 27 _LIBCPP_BEGIN_NAMESPACE_STD 28 __libcpp_clz(unsigned __x)29_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(unsigned __x) _NOEXCEPT { return __builtin_clz(__x); } 30 __libcpp_clz(unsigned long __x)31_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(unsigned long __x) _NOEXCEPT { 32 return __builtin_clzl(__x); 33 } 34 __libcpp_clz(unsigned long long __x)35_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(unsigned long long __x) _NOEXCEPT { 36 return __builtin_clzll(__x); 37 } 38 39 #ifndef _LIBCPP_HAS_NO_INT128 __libcpp_clz(__uint128_t __x)40inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(__uint128_t __x) _NOEXCEPT { 41 # if __has_builtin(__builtin_clzg) 42 return __builtin_clzg(__x); 43 # else 44 // The function is written in this form due to C++ constexpr limitations. 45 // The algorithm: 46 // - Test whether any bit in the high 64-bits is set 47 // - No bits set: 48 // - The high 64-bits contain 64 leading zeros, 49 // - Add the result of the low 64-bits. 50 // - Any bits set: 51 // - The number of leading zeros of the input is the number of leading 52 // zeros in the high 64-bits. 53 return ((__x >> 64) == 0) ? (64 + __builtin_clzll(static_cast<unsigned long long>(__x))) 54 : __builtin_clzll(static_cast<unsigned long long>(__x >> 64)); 55 # endif 56 } 57 #endif // _LIBCPP_HAS_NO_INT128 58 59 template <class _Tp> __countl_zero(_Tp __t)60_LIBCPP_HIDE_FROM_ABI int __countl_zero(_Tp __t) _NOEXCEPT { 61 static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_zero requires an unsigned integer type"); 62 #if __has_builtin(__builtin_clzg) 63 return __builtin_clzg(__t, numeric_limits<_Tp>::digits); 64 #else // __has_builtin(__builtin_clzg) 65 if (__t == 0) 66 return numeric_limits<_Tp>::digits; 67 68 if (sizeof(_Tp) <= sizeof(unsigned int)) 69 return std::__libcpp_clz(static_cast<unsigned int>(__t)) - 70 (numeric_limits<unsigned int>::digits - numeric_limits<_Tp>::digits); 71 else if (sizeof(_Tp) <= sizeof(unsigned long)) 72 return std::__libcpp_clz(static_cast<unsigned long>(__t)) - 73 (numeric_limits<unsigned long>::digits - numeric_limits<_Tp>::digits); 74 else if (sizeof(_Tp) <= sizeof(unsigned long long)) 75 return std::__libcpp_clz(static_cast<unsigned long long>(__t)) - 76 (numeric_limits<unsigned long long>::digits - numeric_limits<_Tp>::digits); 77 else { 78 int __ret = 0; 79 int __iter = 0; 80 const unsigned int __ulldigits = numeric_limits<unsigned long long>::digits; 81 while (true) { 82 __t = std::__rotl(__t, __ulldigits); 83 if ((__iter = std::__countl_zero(static_cast<unsigned long long>(__t))) != __ulldigits) 84 break; 85 __ret += __iter; 86 } 87 return __ret + __iter; 88 } 89 #endif // __has_builtin(__builtin_clzg) 90 } 91 92 _LIBCPP_END_NAMESPACE_STD 93 94 _LIBCPP_POP_MACROS 95 96 #endif // _LIBCPP___CXX03___BIT_COUNTL_H 97