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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-exception6//7//===----------------------------------------------------------------------===//8 9// TODO: __builtin_clzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can10// refactor this code to exclusively use __builtin_clzg.11 12#ifndef _LIBCPP___CXX03___BIT_COUNTL_H13#define _LIBCPP___CXX03___BIT_COUNTL_H14 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_header22#endif23 24_LIBCPP_PUSH_MACROS25#include <__cxx03/__undef_macros>26 27_LIBCPP_BEGIN_NAMESPACE_STD28 29_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(unsigned __x) _NOEXCEPT { return __builtin_clz(__x); }30 31_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(unsigned long __x) _NOEXCEPT {32 return __builtin_clzl(__x);33}34 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_INT12840inline _LIBCPP_HIDE_FROM_ABI int __libcpp_clz(__uint128_t __x) _NOEXCEPT {41# if __has_builtin(__builtin_clzg)42 return __builtin_clzg(__x);43# else44 // 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 set47 // - 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 leading52 // 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# endif56}57#endif // _LIBCPP_HAS_NO_INT12858 59template <class _Tp>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_STD93 94_LIBCPP_POP_MACROS95 96#endif // _LIBCPP___CXX03___BIT_COUNTL_H97