Put implementation details into __popcount_impl

Author: winner245

libcxx/include/__bit/popcount.h

@@ -40,8 +40,7 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned lo
 }
 
 template <class _Tp>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount(_Tp __t) _NOEXCEPT {
-  static_assert(is_unsigned<_Tp>::value, "__popcount only works with unsigned types");
+[[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)) {
@@ -64,15 +63,21 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount(_Tp __t) _NOEXCEPT {
   }
 }
 
+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
 
 template <__libcpp_unsigned_integer _Tp>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noexcept {
-#  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(__t);
-#  endif
 }
 
 #endif