libcxx/include/__cxx03/__bit/countl.h

   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___BIT_COUNTL_H
  13 #define _LIBCPP___BIT_COUNTL_H
  14
  15 #include <__cxx03/__bit/rotate.h>
  16 #include <__cxx03/__concepts/arithmetic.h>
  17 #include <__cxx03/__config>
  18 #include <__cxx03/__type_traits/is_unsigned_integer.h>
  19 #include <__cxx03/limits>
  20
  21 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  22 #  pragma GCC system_header
  23 #endif
  24
  25 _LIBCPP_PUSH_MACROS
  26 #include <__cxx03/__undef_macros>
  27
  28 _LIBCPP_BEGIN_NAMESPACE_STD
  29
  30 _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(unsigned __x) _NOEXCEPT {
  31   return __builtin_clz(__x);
  32 }
  33
  34 _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(unsigned long __x) _NOEXCEPT {
  35   return __builtin_clzl(__x);
  36 }
  37
  38 _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(unsigned long long __x) _NOEXCEPT {
  39   return __builtin_clzll(__x);
  40 }
  41
  42 #ifndef _LIBCPP_HAS_NO_INT128
  43 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(__uint128_t __x) _NOEXCEPT {
  44 #  if __has_builtin(__builtin_clzg)
  45   return __builtin_clzg(__x);
  46 #  else
  47   // The function is written in this form due to C++ constexpr limitations.
  48   // The algorithm:
  49   // - Test whether any bit in the high 64-bits is set
  50   // - No bits set:
  51   //   - The high 64-bits contain 64 leading zeros,
  52   //   - Add the result of the low 64-bits.
  53   // - Any bits set:
  54   //   - The number of leading zeros of the input is the number of leading
  55   //     zeros in the high 64-bits.
  56   return ((__x >> 64) == 0) ? (64 + __builtin_clzll(static_cast<unsigned long long>(__x)))
  57                             : __builtin_clzll(static_cast<unsigned long long>(__x >> 64));
  58 #  endif
  59 }
  60 #endif // _LIBCPP_HAS_NO_INT128
  61
  62 template <class _Tp>
  63 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countl_zero(_Tp __t) _NOEXCEPT {
  64   static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_zero requires an unsigned integer type");
  65 #if __has_builtin(__builtin_clzg)
  66   return __builtin_clzg(__t, numeric_limits<_Tp>::digits);
  67 #else  // __has_builtin(__builtin_clzg)
  68   if (__t == 0)
  69     return numeric_limits<_Tp>::digits;
  70
  71   if (sizeof(_Tp) <= sizeof(unsigned int))
  72     return std::__libcpp_clz(static_cast<unsigned int>(__t)) -
  73            (numeric_limits<unsigned int>::digits - numeric_limits<_Tp>::digits);
  74   else if (sizeof(_Tp) <= sizeof(unsigned long))
  75     return std::__libcpp_clz(static_cast<unsigned long>(__t)) -
  76            (numeric_limits<unsigned long>::digits - numeric_limits<_Tp>::digits);
  77   else if (sizeof(_Tp) <= sizeof(unsigned long long))
  78     return std::__libcpp_clz(static_cast<unsigned long long>(__t)) -
  79            (numeric_limits<unsigned long long>::digits - numeric_limits<_Tp>::digits);
  80   else {
  81     int __ret                      = 0;
  82     int __iter                     = 0;
  83     const unsigned int __ulldigits = numeric_limits<unsigned long long>::digits;
  84     while (true) {
  85       __t = std::__rotl(__t, __ulldigits);
  86       if ((__iter = std::__countl_zero(static_cast<unsigned long long>(__t))) != __ulldigits)
  87         break;
  88       __ret += __iter;
  89     }
  90     return __ret + __iter;
  91   }
  92 #endif // __has_builtin(__builtin_clzg)
  93 }
  94
  95 #if _LIBCPP_STD_VER >= 20
  96
  97 template <__libcpp_unsigned_integer _Tp>
  98 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr int countl_zero(_Tp __t) noexcept {
  99   return std::__countl_zero(__t);
 100 }
 101
 102 template <__libcpp_unsigned_integer _Tp>
 103 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr int countl_one(_Tp __t) noexcept {
 104   return __t != numeric_limits<_Tp>::max() ? std::countl_zero(static_cast<_Tp>(~__t)) : numeric_limits<_Tp>::digits;
 105 }
 106
 107 #endif // _LIBCPP_STD_VER >= 20
 108
 109 _LIBCPP_END_NAMESPACE_STD
 110
 111 _LIBCPP_POP_MACROS
 112
 113 #endif // _LIBCPP___BIT_COUNTL_H