[libc++] Replace __libcpp_popcount by __builtin_popcountg (#133937)

`__libcpp_popcount` was previously used as a fallback for `__builtin_popcountg` to ensure compatibility with older compilers (Clang 18 and earlier), as `__builtin_popcountg` became available in Clang 19. Now that support for Clang 18 has been officially dropped in #130142, we can now safely  replace all instances of `__libcpp_popcount` with `__builtin_popcountg` and eliminate the fallback logic.

Author: winner245

libcxx/include/__bit/popcount.h

@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// TODO: __builtin_popcountg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
-//  refactor this code to exclusively use __builtin_popcountg.
-
 #ifndef _LIBCPP___BIT_POPCOUNT_H
 #define _LIBCPP___BIT_POPCOUNT_H
 
@@ -27,50 +24,10 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned __x) _NOEXCEPT {
-  return __builtin_popcount(__x);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned long __x) _NOEXCEPT {
-  return __builtin_popcountl(__x);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned long long __x) _NOEXCEPT {
-  return __builtin_popcountll(__x);
-}
-
-template <class _Tp>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount_impl(_Tp __t) _NOEXCEPT {
-  if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned int)) {
-    return std::__libcpp_popcount(static_cast<unsigned int>(__t));
-  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long)) {
-    return std::__libcpp_popcount(static_cast<unsigned long>(__t));
-  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long long)) {
-    return std::__libcpp_popcount(static_cast<unsigned long long>(__t));
-  } else {
-#if _LIBCPP_STD_VER == 11
-    return __t != 0 ? std::__libcpp_popcount(static_cast<unsigned long long>(__t)) +
-                          std::__popcount_impl<_Tp>(__t >> numeric_limits<unsigned long long>::digits)
-                    : 0;
-#else
-    int __ret = 0;
-    while (__t != 0) {
-      __ret += std::__libcpp_popcount(static_cast<unsigned long long>(__t));
-      __t >>= std::numeric_limits<unsigned long long>::digits;
-    }
-    return __ret;
-#endif
-  }
-}
-
 template <class _Tp>
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount(_Tp __t) _NOEXCEPT {
   static_assert(is_unsigned<_Tp>::value, "__popcount only works with unsigned types");
-#if __has_builtin(__builtin_popcountg) // TODO (LLVM 21): This can be dropped once we only support Clang >= 19.
   return __builtin_popcountg(__t);
-#else
-  return std::__popcount_impl(__t);
-#endif
 }
 
 #if _LIBCPP_STD_VER >= 20

libcxx/include/__stop_token/atomic_unique_lock.h

@@ -28,7 +28,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // and LockedBit is the value of State when the lock bit is set, e.g  1 << 2
 template <class _State, _State _LockedBit>
 class _LIBCPP_AVAILABILITY_SYNC __atomic_unique_lock {
-  static_assert(std::__libcpp_popcount(static_cast<unsigned long long>(_LockedBit)) == 1,
+  static_assert(std::__popcount(static_cast<unsigned long long>(_LockedBit)) == 1,
                 "LockedBit must be an integer where only one bit is set");
 
   std::atomic<_State>& __state_;