[libc++] Use _BitInt and __builtin_popcountg in bitset::count()

Author: philnik777

libcxx/include/__bit/popcount.h

@@ -21,6 +21,8 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+static_assert(__is_unsigned_integer_v<unsigned _BitInt(10)>);
+
 template <class _Tp>
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount(_Tp __t) _NOEXCEPT {
   static_assert(__is_unsigned_integer_v<_Tp>, "__popcount only works with unsigned types");

libcxx/include/bitset

@@ -867,7 +867,16 @@ bitset<_Size>::to_string(char __zero, char __one) const {
 
 template <size_t _Size>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 size_t bitset<_Size>::count() const _NOEXCEPT {
-  return static_cast<size_t>(std::count(__base::__make_iter(0), __base::__make_iter(_Size), true));
+#  if defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(_LIBCPP_CXX03_LANG)
+  if constexpr (_Size == 0) {
+    return 0;
+  } else if constexpr (_Size <= __base::__bits_per_word) {
+    return __builtin_popcountg(static_cast<unsigned _BitInt(_Size)>(__base::__first_));
+  } else
+#  endif
+  {
+    return static_cast<size_t>(std::count(__base::__make_iter(0), __base::__make_iter(_Size), true));
+  }
 }
 
 template <size_t _Size>