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

[Louis Dionne 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.
   std::swap(this->__begin_, __v.__begin_);
   std::swap(this->__end_, __v.__end_);
   std::swap(this->__end_cap(), __v.__end_cap());
   __v.__first_ = __v.__begin_;
   __annotate_new(size());
 }
 
+// __swap_out_circular_buffer relocates the objects in [__begin_, __p) into the front of __v, the objects in
+// [__p, __end_) into the back of __v and swaps the buffers of *this and __v. It is assumed that __v provides space for
+// exactly (__p - __begin_) objects in the front and space for at least (__end_ - __p) objects in the back. This
+// function has a strong exception guarantee if __begin_ == __p || __end_ == __p.
 template <class _Tp, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 typename vector<_Tp, _Allocator>::pointer
 vector<_Tp, _Allocator>::__swap_out_circular_buffer(__split_buffer<value_type, allocator_type&>& __v, pointer __p) {
   __annotate_delete();
-  pointer __r    = __v.__begin_;
-  using _RevIter = std::reverse_iterator<pointer>;
-  __v.__begin_   = std::__uninitialized_allocator_move_if_noexcept(
-                     __alloc(), _RevIter(__p), _RevIter(__begin_), _RevIter(__v.__begin_))
-                     .base();
-  __v.__end_ = std::__uninitialized_allocator_move_if_noexcept(__alloc(), __p, __end_, __v.__end_);
+  pointer __ret = __v.__begin_;
+
+  // Relocate [__p, __end_) first to avoid having a hole in [__begin_, __end_)
+  // in case something in [__begin_, __p) throws.
+  std::__uninitialized_allocator_relocate(
----------------
ldionne wrote:

In the patch description, please call out that you're not doing this in reverse order anymore. We just talked about it and agree it should be fine, but it's worth calling out at least in the commit message.

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