[libcxx-commits] [libcxx] [libc++] Optimize vector growing of trivially relocatable types (PR #76657)

[Nikolas Klauser via libcxx-commits]

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@@ -982,37 +982,54 @@ template <ranges::input_range _Range,
 vector(from_range_t, _Range&&, _Alloc = _Alloc()) -> vector<ranges::range_value_t<_Range>, _Alloc>;
 #endif
 
+// __swap_out_circular_buffer relocates the objects in [__begin_, __end_) into the front of __v and swaps the buffers of
+// *this and __v. It is assumed that __v provides space for exactly (__end_ - __begin_) objects in the front. This
+// function has a strong exception guarantee.
 template <class _Tp, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 vector<_Tp, _Allocator>::__swap_out_circular_buffer(__split_buffer<value_type, allocator_type&>& __v) {
   __annotate_delete();
-  using _RevIter = std::reverse_iterator<pointer>;
-  __v.__begin_   = std::__uninitialized_allocator_move_if_noexcept(
-                     __alloc(), _RevIter(__end_), _RevIter(__begin_), _RevIter(__v.__begin_))
-                     .base();
+  auto __new_begin = __v.__begin_ - (__end_ - __begin_);
+  std::__uninitialized_allocator_relocate(
+      __alloc(), std::__to_address(__begin_), std::__to_address(__end_), std::__to_address(__new_begin));
+  __v.__begin_ = __new_begin;
+  __end_       = __begin_; // All the objects have been destroyed by relocating them.
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philnik777 wrote:

At least currently UBSan and Asan seem to be happy: https://godbolt.org/z/d4d4vMjsn

https://github.com/llvm/llvm-project/pull/76657