[libc++] Simplify vector<bool>::flip() and add new tests (#119607)

This PR simplifies the internal bitwise logic of the `flip()` function
for `vector<bool>`, and creates new tests to validate the changes.

Author: winner245

libcxx/include/__vector/vector_bool.h

@@ -1049,18 +1049,11 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<bool, _Allocator>::resize(size_type __
 
 template <class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<bool, _Allocator>::flip() _NOEXCEPT {
-  // do middle whole words
-  size_type __n         = __size_;
+  // Flip each storage word entirely, including the last potentially partial word.
+  // The unused bits in the last word are safe to flip as they won't be accessed.
   __storage_pointer __p = __begin_;
-  for (; __n >= __bits_per_word; ++__p, __n -= __bits_per_word)
+  for (size_type __n = __external_cap_to_internal(size()); __n != 0; ++__p, --__n)
     *__p = ~*__p;
-  // do last partial word
-  if (__n > 0) {
-    __storage_type __m = ~__storage_type(0) >> (__bits_per_word - __n);
-    __storage_type __b = *__p & __m;
-    *__p &= ~__m;
-    *__p |= ~__b & __m;
-  }
 }
 
 template <class _Allocator>

libcxx/test/std/containers/sequences/vector.bool/flip.pass.cpp

@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// void flip();
+
+#include <cassert>
+#include <memory>
+#include <vector>
+
+#include "min_allocator.h"
+#include "test_allocator.h"
+#include "test_macros.h"
+
+template <typename Allocator>
+TEST_CONSTEXPR_CXX20 void test_vector_flip(std::size_t n, Allocator a) {
+  std::vector<bool, Allocator> v(n, false, a);
+  for (std::size_t i = 0; i < n; ++i)
+    v[i] = i & 1;
+  std::vector<bool, Allocator> original = v;
+  v.flip();
+  for (size_t i = 0; i < n; ++i)
+    assert(v[i] == !original[i]);
+  v.flip();
+  assert(v == original);
+}
+
+TEST_CONSTEXPR_CXX20 bool tests() {
+  // Test small vectors with different allocators
+  test_vector_flip(3, std::allocator<bool>());
+  test_vector_flip(3, min_allocator<bool>());
+  test_vector_flip(3, test_allocator<bool>(5));
+
+  // Test large vectors with different allocators
+  test_vector_flip(1000, std::allocator<bool>());
+  test_vector_flip(1000, min_allocator<bool>());
+  test_vector_flip(1000, test_allocator<bool>(5));
+
+  return true;
+}
+
+int main(int, char**) {
+  tests();
+#if TEST_STD_VER >= 20
+  static_assert(tests());
+#endif
+  return 0;
+}