[libcxx-commits] [libcxx] [libc++] Optimize copy construction and assignment of __tree (PR #151304)

[Louis Dionne via libcxx-commits]

================
@@ -1213,6 +1213,80 @@ private:
     __node_pointer __cache_root_;
     __node_pointer __cache_elem_;
   };
+
+  class __tree_deleter {
+    __node_allocator& __alloc_;
+
+  public:
+    using pointer = __node_pointer;
+
+    _LIBCPP_HIDE_FROM_ABI __tree_deleter(__node_allocator& __alloc) : __alloc_(__alloc) {}
+
+    void operator()(__node_pointer __ptr) {
+      if (!__ptr)
+        return;
+
+      (*this)(static_cast<__node_pointer>(__ptr->__left_));
+
+      auto __right = __ptr->__right_;
+
+      __node_traits::destroy(__alloc_, std::addressof(__ptr->__value_));
+      __node_traits::deallocate(__alloc_, __ptr, 1);
+
+      (*this)(static_cast<__node_pointer>(__right));
+    }
+  };
+
+  _LIBCPP_HIDE_FROM_ABI __node_pointer __copy_construct_tree(__node_pointer __src) {
+    if (!__src)
+      return nullptr;
+
+    __node_holder __new_node = __construct_node(__src->__value_);
+
+    unique_ptr<__node, __tree_deleter> __left(
+        __copy_construct_tree(static_cast<__node_pointer>(__src->__left_)), __node_alloc_);
+    __node_pointer __right = __copy_construct_tree(static_cast<__node_pointer>(__src->__right_));
+
+    __node_pointer __new_node_ptr = __new_node.release();
+
+    __new_node_ptr->__is_black_ = __src->__is_black_;
+    __new_node_ptr->__left_     = static_cast<__node_base_pointer>(__left.release());
+    __new_node_ptr->__right_    = static_cast<__node_base_pointer>(__right);
+    if (__new_node_ptr->__left_)
+      __new_node_ptr->__left_->__parent_ = static_cast<__end_node_pointer>(__new_node_ptr);
+    if (__new_node_ptr->__right_)
+      __new_node_ptr->__right_->__parent_ = static_cast<__end_node_pointer>(__new_node_ptr);
+    return __new_node_ptr;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI __node_pointer __copy_assign_tree(__node_pointer __assign_to, __node_pointer __src) {
+    if (!__src) {
+      destroy(__assign_to);
+      return nullptr;
+    }
+
+    __assign_value(__assign_to->__value_, __src->__value_);
+
+    if (__assign_to->__left_) {
+      __assign_to->__left_ = static_cast<__node_base_pointer>(__copy_assign_tree(
+          static_cast<__node_pointer>(__assign_to->__left_), static_cast<__node_pointer>(__src->__left_)));
+    } else if (__src->__left_) {
+      auto __new_left       = __copy_construct_tree(static_cast<__node_pointer>(__src->__left_));
+      __assign_to->__left_  = static_cast<__node_base_pointer>(__new_left);
+      __new_left->__parent_ = static_cast<__end_node_pointer>(__assign_to);
+    }
----------------
ldionne wrote:

```suggestion
    // If we already have a left node in the destination tree, reuse it and copy-assign recursively
    if (__assign_to->__left_) {
      __assign_to->__left_ = static_cast<__node_base_pointer>(__copy_assign_tree(
          static_cast<__node_pointer>(__assign_to->__left_), static_cast<__node_pointer>(__src->__left_)));
    // Otherwise, we must create new nodes so copy-construct from here on
    } else if (__src->__left_) {
      auto __new_left       = __copy_construct_tree(static_cast<__node_pointer>(__src->__left_));
      __assign_to->__left_  = static_cast<__node_base_pointer>(__new_left);
      __new_left->__parent_ = static_cast<__end_node_pointer>(__assign_to);
    }
```

https://github.com/llvm/llvm-project/pull/151304