Simplify flip() for std::bitset (llvm#120807)

This PR simplifies the internal bitwise logic of the `flip()` function
for `std::bitset`.

Author: winner245

libcxx/include/bitset

@@ -329,12 +329,10 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 void __bitset<_N_words, _Siz
   for (; __n >= __bits_per_word; ++__p, __n -= __bits_per_word)
     *__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;
-  }
+  // Ensure trailing padding bits are zeroed as part of the ABI for consistent hashing behavior. std::hash<bitset>
+  // assumes trailing bits are zeroed; otherwise, identical bitsets could hash differently.
+  if (__n > 0)
+    *__p ^= (__storage_type(1) << __n) - 1;
 }
 
 template <size_t _N_words, size_t _Size>
@@ -514,9 +512,7 @@ __bitset<1, _Size>::operator^=(const __bitset& __v) _NOEXCEPT {
 
 template <size_t _Size>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 void __bitset<1, _Size>::flip() _NOEXCEPT {
-  __storage_type __m = ~__storage_type(0) >> (__bits_per_word - _Size);
-  __first_           = ~__first_;
-  __first_ &= __m;
+  __first_ ^= ~__storage_type(0) >> (__bits_per_word - _Size);
 }
 
 template <size_t _Size>

libcxx/test/std/utilities/template.bitset/bitset.members/flip_all.pass.cpp

@@ -18,19 +18,21 @@
 
 template <std::size_t N>
 TEST_CONSTEXPR_CXX23 void test_flip_all() {
-    std::vector<std::bitset<N> > const cases = get_test_cases<N>();
-    for (std::size_t c = 0; c != cases.size(); ++c) {
-        std::bitset<N> v1 = cases[c];
-        std::bitset<N> v2 = v1;
-        v2.flip();
-        for (std::size_t i = 0; i < v1.size(); ++i)
-            assert(v2[i] == ~v1[i]);
-    }
+  std::vector<std::bitset<N> > const cases = get_test_cases<N>();
+  for (std::size_t c = 0; c != cases.size(); ++c) {
+    std::bitset<N> v1 = cases[c];
+    std::bitset<N> v2 = v1;
+    v2.flip();
+    for (std::size_t i = 0; i < v1.size(); ++i)
+      assert(v2[i] == ~v1[i]);
+  }
 }
 
 TEST_CONSTEXPR_CXX23 bool test() {
   test_flip_all<0>();
   test_flip_all<1>();
+  test_flip_all<2>();
+  test_flip_all<5>();
   test_flip_all<31>();
   test_flip_all<32>();
   test_flip_all<33>();

libcxx/test/std/utilities/template.bitset/bitset_test_cases.h

@@ -43,6 +43,28 @@ TEST_CONSTEXPR_CXX23 inline std::vector<std::bitset<2> > get_test_cases<2>() {
   return cases;
 }
 
+template <>
+TEST_CONSTEXPR_CXX23 inline std::vector<std::bitset<5> > get_test_cases<5>() {
+  std::vector<std::bitset<5> > cases;
+  cases.push_back(std::bitset<5>("00000"));
+  cases.push_back(std::bitset<5>("00001"));
+  cases.push_back(std::bitset<5>("10000"));
+  cases.push_back(std::bitset<5>("00010"));
+  cases.push_back(std::bitset<5>("01000"));
+  cases.push_back(std::bitset<5>("00011"));
+  cases.push_back(std::bitset<5>("11000"));
+  cases.push_back(std::bitset<5>("00100"));
+  cases.push_back(std::bitset<5>("11011"));
+  cases.push_back(std::bitset<5>("00101"));
+  cases.push_back(std::bitset<5>("10100"));
+  cases.push_back(std::bitset<5>("00110"));
+  cases.push_back(std::bitset<5>("01100"));
+  cases.push_back(std::bitset<5>("00111"));
+  cases.push_back(std::bitset<5>("11100"));
+  cases.push_back(std::bitset<5>("11111"));
+  return cases;
+}
+
 template <>
 TEST_CONSTEXPR_CXX23 inline std::vector<std::bitset<31> > get_test_cases<31>() {
   std::vector<std::bitset<31> > cases;