RFR: 8349211: Add support for intrusive trees to the utilities red-black tree [v10]

[Casper Norrbin]

On Thu, 20 Feb 2025 15:02:26 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

>> Hi everyone,
>> 
>> The recently integrated red-black tree can be made more flexible by adding support of intrusive trees. In an intrusive tree, the user has full control over node allocation and placement instead of having the tree manage it internally.
>> 
>> Two key changes enable this feature:
>> 1. Nodes can now be created outside of the tree's internal allocation mechanism, enabling users to allocate and prepare nodes before inserting them into the tree.
>> 2. Cursors have been added to simplify navigation and iteration over the tree. These cursors are when inserting and removing nodes in an intrusive tree, where the internal tree allocator is not used. Additionally, cursors enable iteration over the tree and provide a convenient way to access node values.
>> 
>> 
>> Many of the auxiliary tree functions have been updated to use these new features, resulting in simplified and cleaned-up code. More tests have also been added to cover both new and existing functionality.
>> 
>> An example of how you could use the intrusive tree is found below:
>> 
>> ```c++
>>     struct MyIntrusiveStructure {
>>       Node node; // The tree node is part of an external structure
>>       int data;
>> 
>>       MyIntrusiveStructure(int data, Node node) : node(node), data(data) {}
>>       Node* get_node() { return &node; }
>>       static MyIntrusiveStructure* cast_to_self(Node* node) { return (MyIntrusiveStructure*)node; }
>>     };
>> 
>>     Tree my_intrusive_tree;
>> 
>>     Cursor insert_cursor = my_intrusive_tree.cursor_find(0);
>>     Node insert_node = Node(0);
>> 
>>     // Custom allocation here is just malloc
>>     MyIntrusiveStructure* place = (MyIntrusiveStructure*)os::malloc(sizeof(MyIntrusiveStructure), mtTest);
>>     new (place) MyIntrusiveStructure(0, insert_node);
>> 
>>     my_intrusive_tree.insert_at_cursor(place->get_node(), insert_cursor);
>> 
>>     Cursor find_cursor = my_intrusive_tree.cursor_find(0);
>>     int found_data = MyIntrusiveStructure::cast_to_self(find_cursor.node())->data;
>> 
>> 
>> 
>> Please let me know any feedback or concerns!
>
> Casper Norrbin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:
> 
>  - separate rbnode and normal tree subclass
>  - Merge branch 'master' into rb-tree-intrusive-v2
>  - renamed non-value upsert to insert
>  - johan feedback
>  - empty base optimization reference
>  - Merge branch 'master' into rb-tree-intrusive-v2
>  - initialize node on insert + more tests
>  - windows build
>  - build fix
>  - reduced diff
>  - ... and 2 more: https://git.openjdk.org/jdk/compare/f1258f9e...7d4a9ccc

I just pushed a change addressing some of the points mentioned:
- Separated `RBNode` into its own class
- Separated the 'normal' `RBTree` (the parts using an allocator) to its own subclass, with the the intrusive tree essentially becoming the base.
- Added `insert_node(Node)` and `remove(Node)` so that you can use the intrusive parts without necessary having to use cursors.
- Added an optional `hint_node` parameter to insert and lookup functions that if given, starts searching for the intended node/place starting at that node instead of the tree root

Any feedback would be appreciated!
Thank you both for the comprehensive comments, I'll continue working on this.

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PR Comment: https://git.openjdk.org/jdk/pull/23416#issuecomment-2671795419