Replace __libcpp_ctz with __builtin_ctzg

Author: winner245

libcxx/include/__algorithm/sort.h

@@ -357,9 +357,9 @@ inline _LIBCPP_HIDE_FROM_ABI void __swap_bitmap_pos(
   // Swap one pair on each iteration as long as both bitsets have at least one
   // element for swapping.
   while (__left_bitset != 0 && __right_bitset != 0) {
-    difference_type __tz_left  = __libcpp_ctz(__left_bitset);
+    difference_type __tz_left  = __builtin_ctzg(__left_bitset);
     __left_bitset              = __libcpp_blsr(__left_bitset);
-    difference_type __tz_right = __libcpp_ctz(__right_bitset);
+    difference_type __tz_right = __builtin_ctzg(__right_bitset);
     __right_bitset             = __libcpp_blsr(__right_bitset);
     _Ops::iter_swap(__first + __tz_left, __last - __tz_right);
   }

libcxx/include/__bit/countr.h

@@ -6,14 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
-// TODO: __builtin_ctzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
-//  refactor this code to exclusively use __builtin_ctzg.
-
 #ifndef _LIBCPP___BIT_COUNTR_H
 #define _LIBCPP___BIT_COUNTR_H
 
 #include <__assert>
-#include <__bit/rotate.h>
 #include <__concepts/arithmetic.h>
 #include <__config>
 #include <__type_traits/is_unsigned.h>
@@ -28,55 +24,10 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_ctz(unsigned __x) _NOEXCEPT {
-  return __builtin_ctz(__x);
-}
-
-[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_ctz(unsigned long __x) _NOEXCEPT {
-  return __builtin_ctzl(__x);
-}
-
-[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_ctz(unsigned long long __x) _NOEXCEPT {
-  return __builtin_ctzll(__x);
-}
-
-// A constexpr implementation for C++11 and later (using clang extensions for constexpr support)
-// Precondition: __t != 0 (the caller __countr_zero handles __t == 0 as a special case)
-template <class _Tp>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __countr_zero_impl(_Tp __t) _NOEXCEPT {
-  _LIBCPP_ASSERT_INTERNAL(__t != 0, "__countr_zero_impl called with zero value");
-  static_assert(is_unsigned<_Tp>::value, "__countr_zero_impl only works with unsigned types");
-  if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned int)) {
-    return std::__libcpp_ctz(static_cast<unsigned int>(__t));
-  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long)) {
-    return std::__libcpp_ctz(static_cast<unsigned long>(__t));
-  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long long)) {
-    return std::__libcpp_ctz(static_cast<unsigned long long>(__t));
-  } else {
-#if _LIBCPP_STD_VER == 11
-    unsigned long long __ull       = static_cast<unsigned long long>(__t);
-    const unsigned int __ulldigits = numeric_limits<unsigned long long>::digits;
-    return __ull == 0ull ? __ulldigits + std::__countr_zero_impl<_Tp>(__t >> __ulldigits) : std::__libcpp_ctz(__ull);
-#else
-    int __ret                      = 0;
-    const unsigned int __ulldigits = numeric_limits<unsigned long long>::digits;
-    while (static_cast<unsigned long long>(__t) == 0uLL) {
-      __ret += __ulldigits;
-      __t >>= __ulldigits;
-    }
-    return __ret + std::__libcpp_ctz(static_cast<unsigned long long>(__t));
-#endif
-  }
-}
-
 template <class _Tp>
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __countr_zero(_Tp __t) _NOEXCEPT {
   static_assert(is_unsigned<_Tp>::value, "__countr_zero only works with unsigned types");
-#if __has_builtin(__builtin_ctzg) // TODO (LLVM 21): This can be dropped once we only support Clang >= 19.
   return __builtin_ctzg(__t, numeric_limits<_Tp>::digits);
-#else
-  return __t != 0 ? std::__countr_zero_impl(__t) : numeric_limits<_Tp>::digits;
-#endif
 }
 
 #if _LIBCPP_STD_VER >= 20

libcxx/include/__bit_reference

@@ -19,7 +19,6 @@
 #include <__algorithm/rotate.h>
 #include <__algorithm/swap_ranges.h>
 #include <__assert>
-#include <__bit/countr.h>
 #include <__compare/ordering.h>
 #include <__config>
 #include <__cstddef/ptrdiff_t.h>
@@ -165,7 +164,7 @@ public:
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void flip() _NOEXCEPT { *__seg_ ^= __mask_; }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator<_Cp, false> operator&() const _NOEXCEPT {
-    return __bit_iterator<_Cp, false>(__seg_, static_cast<unsigned>(std::__libcpp_ctz(__mask_)));
+    return __bit_iterator<_Cp, false>(__seg_, static_cast<unsigned>(__builtin_ctzg(__mask_)));
   }
 
 private:
@@ -234,7 +233,7 @@ public:
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator<_Cp, true> operator&() const _NOEXCEPT {
-    return __bit_iterator<_Cp, true>(__seg_, static_cast<unsigned>(std::__libcpp_ctz(__mask_)));
+    return __bit_iterator<_Cp, true>(__seg_, static_cast<unsigned>(__builtin_ctzg(__mask_)));
   }
 
 private: