Improve Tree performance

[aaronp64]

Added TreeItem::last_child to avoid needing to iterate through all children to get to the end. This mainly helps in cases where one TreeItem has many children (1000s), and new children are added to the end, as each add had to iterate through all previously added children.

Gdscript example to compare creating TreeItems, time reduced from 1640ms to 66ms:

func _ready() -> void:
	var root = $Tree.create_item()
	root.set_text(0, "Root")
	var start_time = Time.get_ticks_msec()
	for i in 20000:
		var item = $Tree.create_item()
		item.set_text(0, "Item " + str(i))
	var end_time = Time.get_ticks_msec()
	print("Time: %dms" % (end_time - start_time))

This also impacts SceneTreeEditor. I created a Node2D scene with 10,000 empty Node2D children to test with. Opening this scene took 4.28 seconds before this change, 1.55 seconds after. At 20,000 children, time was reduced from 16 seconds to 2.65 seconds.

EditorDebuggerTree has similar slowness in adding items, but also is repeatedly updated when running game from the editor. Using the gdscript below to create 30,000 Node2Ds at runtime, the editor is significantly more responsive when using the remote scene tree while the parent node stays collapsed. Once the parent node is expanded, more time is spent handling the tree's text/heights, which gets pretty slow, but is still faster than before.

func _ready() -> void:
	for i in 30000:
		add_child(Node2D.new())

Also added some unit tests. All tests pass with both old and new code, except for the Clear items test, which failed on old code due to children_cache not being cleared in clear_children(). Looks like we avoided needing to use the cache in existing code where clear_children() is called, so this hasn't been an issue.

[Calinou]

Tested locally, it works as expected.

Testing project: test-tree.zip

Benchmark

Using an optimized editor build (MSVC 2022 optimize=speed). Running the MRP above which contains the first code sample from OP.

Before	After
745 ms	38 ms

PC specifications

• CPU: Intel Core i9-13900K
• GPU: NVIDIA GeForce RTX 4090
• RAM: 64 GB (2×32 GB DDR5-5800 C30)
• SSD: Solidigm P44 Pro 2 TB
• OS: Windows 11 23H2

[AThousandShips]

Should be moved down to the #ifndef ADVANCED_GUI_DISABLED part as it is advanced UI

[aaronp64]

Moved to advanced gui section

[Jimmio92]

Such a shame this came too late for 4.3; speed improvements to scene addition across the board like this are a huge help to those developing/attempting larger titles with Godot. Thanks for finding this.

[Mickeon]

It can probably be cherrypicked later, assuming lack of unforeseen consequences

[Saul2022]

Wouldn't this fix #94648 (comment) along with rthe closing pr?

[aaronp64]

Wouldn't this fix #94648 (comment) along with rthe closing pr?

No, I ran into this when looking into that issue, but most of the time spent in that case is in DynamicBVH functions. From their screenshots, I think the reason Tree wasn't an issue there is that the block nodes were internal to the map, so they weren't being added to the SceneTreeEditor

[akien-mga]

Thanks!

[AThousandShips]

This broke trees in at least EditorHelpSearch, will try to identify what was broken

Edit: Fixed, see:

• [Tree] Fix error when removing child from Tree #96206