[libcxx-commits] [libcxx] [libc++] Simplify vector<bool>::__construct_at_end (Reopend) (PR #119632)

[Peng Liu via libcxx-commits]

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@@ -552,36 +552,29 @@ vector<bool, _Allocator>::__recommend(size_type __new_size) const {
 }
 
 //  Default constructs __n objects starting at __end_
-//  Precondition:  __n > 0
 //  Precondition:  size() + __n <= capacity()
 //  Postcondition:  size() == size() + __n
 template <class _Allocator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 vector<bool, _Allocator>::__construct_at_end(size_type __n, bool __x) {
-  size_type __old_size = this->__size_;
+  _LIBCPP_ASSERT_INTERNAL(
+      capacity() >= size() + __n, "vector<bool>::__construct_at_end called with insufficient capacity");
+  std::fill_n(end(), __n, __x);
   this->__size_ += __n;
-  if (__old_size == 0 || ((__old_size - 1) / __bits_per_word) != ((this->__size_ - 1) / __bits_per_word)) {
-    if (this->__size_ <= __bits_per_word)
-      this->__begin_[0] = __storage_type(0);
-    else
-      this->__begin_[(this->__size_ - 1) / __bits_per_word] = __storage_type(0);
-  }
-  std::fill_n(__make_iter(__old_size), __n, __x);
+  if (end().__ctz_ != 0) // Ensure uninitialized leading bits in the last word are set to zero
+    std::fill_n(end(), __bits_per_word - end().__ctz_, 0);
----------------
winner245 wrote:

In constant evaluation, we have no uninitialized bits at all because `__vallocate` guarantees that the allocated memory is default-constructed. In non-constant evaluation, we effectively only reads from bits within the bounds of `vector<bool>` since we utilize bit masks. So in this case, the purpose of the initialization is to make sanitizer tools happy. According to my test, sanitizers do not complain as long as the unused bits in the last word are initialized to 0 within `__construct_at_end`, regardless of whether this occurs before or after the initialization of other words. However, without the initialization of unused bits, subsequent read from `vector<bool>` would trigger a Msan CI failure. 

https://github.com/llvm/llvm-project/pull/119632