[libc++] Remove unnecessary division and modulo operations in bitset (#121312)

The PR removes the unnecessary division and modulo operations in the
one-word specialization `__bitset<1, _Size>`. The reason is that for the
one-word specialization, we have `__pos < __bits_per_word` (as
`__bitset<1, _Size>` is an implementation detail only used by the public
`bitset`). So `__pos / __bits_per_word == 0` and `__pos / __pos %
__bits_per_word == __pos`.

Author: winner245

libcxx/include/__bit_reference

@@ -329,6 +329,7 @@ public:
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator*() const _NOEXCEPT {
+    _LIBCPP_ASSERT_INTERNAL(__ctz_ < __bits_per_word, "Dereferencing an invalid __bit_iterator.");
     return __conditional_t<_IsConst, __bit_const_reference<_Cp>, __bit_reference<_Cp> >(
         __seg_, __storage_type(1) << __ctz_);
   }
@@ -453,7 +454,10 @@ private:
   _LIBCPP_HIDE_FROM_ABI
   _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __bit_iterator(__storage_pointer __s, unsigned __ctz) _NOEXCEPT
       : __seg_(__s),
-        __ctz_(__ctz) {}
+        __ctz_(__ctz) {
+    _LIBCPP_ASSERT_INTERNAL(
+        __ctz_ < __bits_per_word, "__bit_iterator initialized with an invalid number of trailing zeros.");
+  }
 
   friend typename _Cp::__self;
 

libcxx/include/bitset

@@ -464,10 +464,14 @@ protected:
     return __const_reference(&__first_, __storage_type(1) << __pos);
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __iterator __make_iter(size_t __pos) _NOEXCEPT {
-    return __iterator(&__first_ + __pos / __bits_per_word, __pos % __bits_per_word);
+    // Allow the == case to accommodate the past-the-end iterator.
+    _LIBCPP_ASSERT_INTERNAL(__pos <= __bits_per_word, "Out of bounds access in the single-word bitset implementation.");
+    return __pos != __bits_per_word ? __iterator(&__first_, __pos) : __iterator(&__first_ + 1, 0);
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __const_iterator __make_iter(size_t __pos) const _NOEXCEPT {
-    return __const_iterator(&__first_ + __pos / __bits_per_word, __pos % __bits_per_word);
+    // Allow the == case to accommodate the past-the-end iterator.
+    _LIBCPP_ASSERT_INTERNAL(__pos <= __bits_per_word, "Out of bounds access in the single-word bitset implementation.");
+    return __pos != __bits_per_word ? __const_iterator(&__first_, __pos) : __const_iterator(&__first_ + 1, 0);
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 void operator&=(const __bitset& __v) _NOEXCEPT;