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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-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
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_INT128
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
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_STD
93
94_LIBCPP_POP_MACROS
95
96#endif // _LIBCPP___CXX03___BIT_COUNTL_H
97

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source code of libcxx/include/__cxx03/__bit/countl.h