Apply @frederick-vs-ja suggestions to support 16-bit platforms

Author: winner245

libcxx/include/bitset

@@ -281,14 +281,23 @@ inline _LIBCPP_HIDE_FROM_ABI void __bitset<_N_words, _Size>::__init(unsigned lon
 template <size_t _N_words, size_t _Size>
 inline _LIBCPP_CONSTEXPR __bitset<_N_words, _Size>::__bitset(unsigned long long __v) _NOEXCEPT
 #  ifndef _LIBCPP_CXX03_LANG
-#    if __SIZEOF_SIZE_T__ == 8
-    : __first_{__v}
-#    elif __SIZEOF_SIZE_T__ == 4
+#    if (__SIZEOF_LONG_LONG__ + __SIZEOF_SIZE_T__ - 1) / __SIZEOF_SIZE_T__ == 1
+    : __first_{static_cast<__storage_type>(__v)}
+#    elif (__SIZEOF_LONG_LONG__ + __SIZEOF_SIZE_T__ - 1) / __SIZEOF_SIZE_T__ == 2
+    : __first_{static_cast<__storage_type>(__v), static_cast<__storage_type>(__v >> __bits_per_word)}
+#    elif (__SIZEOF_LONG_LONG__ + __SIZEOF_SIZE_T__ - 1) / __SIZEOF_SIZE_T__ == 4
+#      if _N_words == 2
+    : __first_{static_cast<__storage_type>(__v), static_cast<__storage_type>(__v >> __bits_per_word)}
+#      elif _N_words == 3
     : __first_{static_cast<__storage_type>(__v),
-               _Size >= 2 * __bits_per_word
-                   ? static_cast<__storage_type>(__v >> __bits_per_word)
-                   : static_cast<__storage_type>((__v >> __bits_per_word) &
-                                                 (__storage_type(1) << (_Size - __bits_per_word)) - 1)}
+               static_cast<__storage_type>(__v >> __bits_per_word),
+               static_cast<__storage_type>(__v >> (__bits_per_word * 2))}
+#      else
+    : __first_{static_cast<__storage_type>(__v),
+               static_cast<__storage_type>(__v >> __bits_per_word),
+               static_cast<__storage_type>(__v >> (__bits_per_word * 2)),
+               static_cast<__storage_type>(__v >> (__bits_per_word * 3))}
+#      endif
 #    else
 #      error This constructor has not been ported to this platform
 #    endif
@@ -379,13 +388,13 @@ __bitset<_N_words, _Size>::to_ullong(true_type, false_type) const {
 template <size_t _N_words, size_t _Size>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long long
 __bitset<_N_words, _Size>::to_ullong(true_type, true_type) const {
-  unsigned long long __r = __first_[0];
+  unsigned long long __r = static_cast<unsigned long long>(__first_[0]);
   _LIBCPP_DIAGNOSTIC_PUSH
   _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wshift-count-overflow")
   const size_t __ull_words = sizeof(unsigned long long) / sizeof(__storage_type);
-  const size_t __n_words = _N_words < __ull_words ? _N_words : __ull_words;
+  const size_t __n_words   = _N_words < __ull_words ? _N_words : __ull_words;
   for (size_t __i = 1; __i < __n_words; ++__i)
-    __r |= static_cast<unsigned long long>(__first_[__i]) << (sizeof(__storage_type) * CHAR_BIT * __i);
+    __r |= static_cast<unsigned long long>(__first_[__i]) << (__bits_per_word * __i);
   _LIBCPP_DIAGNOSTIC_POP
   return __r;
 }
@@ -626,7 +635,8 @@ public:
 
   // 23.3.5.1 constructors:
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bitset() _NOEXCEPT {}
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bitset(unsigned long long __v) _NOEXCEPT : __base(__v) {}
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bitset(unsigned long long __v) _NOEXCEPT
+      : __base(sizeof(unsigned long long) * CHAR_BIT <= _Size ? __v : __v & ((1ULL << _Size) - 1)) {}
   template <class _CharT, __enable_if_t<_IsCharLikeType<_CharT>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 explicit bitset(
       const _CharT* __str,