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

[Louis Dionne via libcxx-commits]

================
@@ -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(
+      __alloc(), std::__to_address(__p), std::__to_address(__end_), std::__to_address(__v.__end_));
+  __v.__end_ += (__end_ - __p);
+  __end_           = __p; // The objects in [__p, __end_) have been destroyed by relocating them.
+  auto __new_begin = __v.__begin_ - (__p - __begin_);
+
+  std::__uninitialized_allocator_relocate(
+      __alloc(), std::__to_address(__begin_), std::__to_address(__p), 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());
-  return __r;
+  return __ret;
 }
 
 template <class _Tp, class _Allocator>
----------------
ldionne wrote:

Not attached to this line: Please add tests for this stuff. At the minimum we need a libc++ specific test like `.compile.pass.cpp` that checks the trivial relocatability of various types.

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