Optimize __insert_with_sentinel Function in std::vector

Author: winner245

libcxx/include/__vector/vector.h

@@ -1250,30 +1250,29 @@ vector<_Tp, _Allocator>::__insert_with_sentinel(const_iterator __position, _Inpu
   difference_type __off = __position - begin();
   pointer __p           = this->__begin_ + __off;
   pointer __old_last    = this->__end_;
-  for (; this->__end_ != this->__cap_ && __first != __last; ++__first) {
+  for (; this->__end_ != this->__cap_ && __first != __last; ++__first)
     __construct_one_at_end(*__first);
+
+  if (__first == __last)
+    (void)std::rotate(__p, __old_last, this->__end_);
+  else {
+    __split_buffer<value_type, allocator_type&> __v(__alloc_);
+    auto __guard = std::__make_exception_guard(
+        _AllocatorDestroyRangeReverse<allocator_type, pointer>(__alloc_, __old_last, this->__end_));
+    __v.__construct_at_end_with_sentinel(std::move(__first), std::move(__last));
+    __split_buffer<value_type, allocator_type&> __merged(__recommend(size() + __v.size()), __off, __alloc_);
+    std::__uninitialized_allocator_relocate(
+        __alloc_, std::__to_address(__old_last), std::__to_address(this->__end_), std::__to_address(__merged.__end_));
+    __guard.__complete(); // Release the guard once objects in [__old_last_, __end_) have been successfully relocated.
+    __merged.__end_ += this->__end_ - __old_last;
+    this->__end_ = __old_last;
+    std::__uninitialized_allocator_relocate(
+        __alloc_, std::__to_address(__v.__begin_), std::__to_address(__v.__end_), std::__to_address(__merged.__end_));
+    __merged.__end_ += __v.size();
+    __v.__end_   = __v.__begin_;
+    __p          = __swap_out_circular_buffer(__merged, __p);
   }
-  __split_buffer<value_type, allocator_type&> __v(this->__alloc_);
-  if (__first != __last) {
-#if _LIBCPP_HAS_EXCEPTIONS
-    try {
-#endif // _LIBCPP_HAS_EXCEPTIONS
-      __v.__construct_at_end_with_sentinel(std::move(__first), std::move(__last));
-      difference_type __old_size = __old_last - this->__begin_;
-      difference_type __old_p    = __p - this->__begin_;
-      reserve(__recommend(size() + __v.size()));
-      __p        = this->__begin_ + __old_p;
-      __old_last = this->__begin_ + __old_size;
-#if _LIBCPP_HAS_EXCEPTIONS
-    } catch (...) {
-      erase(__make_iter(__old_last), end());
-      throw;
-    }
-#endif // _LIBCPP_HAS_EXCEPTIONS
-  }
-  __p = std::rotate(__p, __old_last, this->__end_);
-  insert(__make_iter(__p), std::make_move_iterator(__v.begin()), std::make_move_iterator(__v.end()));
-  return begin() + __off;
+  return __make_iter(__p);
 }
 
 template <class _Tp, class _Allocator>

libcxx/test/std/containers/sequences/vector/vector.modifiers/insert_iter_iter_iter.pass.cpp

@@ -46,6 +46,46 @@ TEST_CONSTEXPR_CXX20 bool tests()
         for (; j < 105; ++j)
             assert(v[j] == 0);
     }
+    {   // Vector may or may not need to reallocate because of the insertion -- test both cases.
+      { // The input range is shorter than the remaining capacity of the vector -- ensure no reallocation happens.
+        typedef std::vector<int> V;
+        V v(100);
+        v.reserve(v.size() + 10);
+        int a[]     = {1, 2, 3, 4, 5};
+        const int N = sizeof(a) / sizeof(a[0]);
+        V::iterator i =
+            v.insert(v.cbegin() + 10, cpp17_input_iterator<const int*>(a), cpp17_input_iterator<const int*>(a + N));
+        assert(v.size() == 100 + N);
+        assert(is_contiguous_container_asan_correct(v));
+        assert(i == v.begin() + 10);
+        int j;
+        for (j = 0; j < 10; ++j)
+          assert(v[j] == 0);
+        for (std::size_t k = 0; k < N; ++j, ++k)
+          assert(v[j] == a[k]);
+        for (; j < 105; ++j)
+          assert(v[j] == 0);
+      }
+      { // The input range is longer than the remaining capacity of the vector -- ensure reallocation happens.
+        typedef std::vector<int> V;
+        V v(100);
+        v.reserve(v.size() + 2);
+        int a[]     = {1, 2, 3, 4, 5};
+        const int N = sizeof(a) / sizeof(a[0]);
+        V::iterator i =
+            v.insert(v.cbegin() + 10, cpp17_input_iterator<const int*>(a), cpp17_input_iterator<const int*>(a + N));
+        assert(v.size() == 100 + N);
+        assert(is_contiguous_container_asan_correct(v));
+        assert(i == v.begin() + 10);
+        int j;
+        for (j = 0; j < 10; ++j)
+          assert(v[j] == 0);
+        for (std::size_t k = 0; k < N; ++j, ++k)
+          assert(v[j] == a[k]);
+        for (; j < 105; ++j)
+          assert(v[j] == 0);
+      }
+    }
     {
         typedef std::vector<int> V;
         V v(100);