[libcxx-commits] [libcxx] [lldb] [libcxx] adds size-based `__split_buffer` representation to unstable ABI (PR #139632)

[Christopher Di Bella via libcxx-commits]

================
@@ -428,50 +653,60 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void __split_buffer<_Tp, _Allocator>::shrink_to_fi
   }
 }
 
-template <class _Tp, class _Allocator>
+template <class _Tp, class _Allocator, template <class, class, class> class _Layout>
 template <class... _Args>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 void __split_buffer<_Tp, _Allocator>::emplace_front(_Args&&... __args) {
-  if (__begin_ == __first_) {
-    if (__end_ < __cap_) {
-      difference_type __d = __cap_ - __end_;
+_LIBCPP_CONSTEXPR_SINCE_CXX20 void __split_buffer<_Tp, _Allocator, _Layout>::emplace_front(_Args&&... __args) {
+  if (__front_spare() == 0) {
+    pointer __end = end();
+    if (__back_spare() > 0) {
+      // The elements are pressed up against the front of the buffer: we need to move them back a
+      // little bit to make `emplace_front` have amortised O(1) complexity.
+      difference_type __d = __back_spare();
       __d                 = (__d + 1) / 2;
-      __begin_            = std::move_backward(__begin_, __end_, __end_ + __d);
-      __end_ += __d;
+      __set_begin(std::move_backward(begin(), __end, __end + __d));
+
+      // `begin()` was moved further into the buffer, so we need to update the pointer-based
+      // layout's end with it. We don't need to do anything for the size-based layout here, because
+      // the number of elements hasn't changed yet.
+      __set_size_if_pointer_based(__d);
     } else {
-      size_type __c = std::max<size_type>(2 * static_cast<size_type>(__cap_ - __first_), 1);
-      __split_buffer<value_type, __alloc_rr&> __t(__c, (__c + 3) / 4, __alloc_);
-      __t.__construct_at_end(move_iterator<pointer>(__begin_), move_iterator<pointer>(__end_));
-      std::swap(__first_, __t.__first_);
-      std::swap(__begin_, __t.__begin_);
-      std::swap(__end_, __t.__end_);
-      std::swap(__cap_, __t.__cap_);
+      size_type __c = std::max<size_type>(2 * capacity(), 1);
+      __split_buffer<value_type, __alloc_rr&> __t(__c, (__c + 3) / 4, this->__alloc_);
+      __t.__construct_at_end(move_iterator<pointer>(begin()), move_iterator<pointer>(__end));
+      __swap_without_allocator(__t);
     }
   }
-  __alloc_traits::construct(__alloc_, std::__to_address(__begin_ - 1), std::forward<_Args>(__args)...);
-  --__begin_;
+
+  __alloc_traits::construct(this->__alloc_, std::__to_address(begin() - 1), std::forward<_Args>(__args)...);
+  __set_begin(begin() - 1);
+
+  // Now that we've added an element to the front of the buffer, we need to update the size-based
+  // layout's size. We don't need to do anything for the pointer-based layout here, because the end
+  // pointer was never updated.
+  __set_end_if_size_based();
----------------
cjdb wrote:

Done. I agree that combining `__set_begin` and `__set_sentinel` is a good move.

https://github.com/llvm/llvm-project/pull/139632