FTI - Optimize SceneTree traversal

Author: lawnjelly

doc/classes/ProjectSettings.xml

@@ -2482,6 +2482,12 @@
 			[b]Dummy[/b] is a 3D physics server that does nothing and returns only dummy values, effectively disabling all 3D physics functionality.
 			Third-party extensions and modules can add other physics engines to select with this setting.
 		</member>
+		<member name="physics/3d/physics_interpolation/scene_traversal" type="String" setter="" getter="" default="&quot;DEFAULT&quot;">
+			The approach used for 3D scene traversal when physics interpolation is enabled.
+			- [code]DEFAULT[/code]: The default optimized method.
+			- [code]Legacy[/code]: The previous reference method used for scene tree traversal, which is slower.
+			- [code]Debug[/code]: Swaps between [code]DEFAULT[/code] and [code]Legacy[/code] methods on alternating frames, and provides logging information (which in turn makes it slower). Intended for debugging only; you should use the [code]DEFAULT[/code] method in most cases.
+		</member>
 		<member name="physics/3d/run_on_separate_thread" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], the 3D physics server runs on a separate thread, making better use of multi-core CPUs. If [code]false[/code], the 3D physics server runs on the main thread. Running the physics server on a separate thread can increase performance, but restricts API access to only physics process.
 			[b]Note:[/b] When [member physics/3d/physics_engine] is set to [code]Jolt Physics[/code], enabling this setting will prevent the 3D physics server from being able to provide any context when reporting errors and warnings, and will instead always refer to nodes as [code]&lt;unknown&gt;[/code].

scene/3d/node_3d.cpp

@@ -270,6 +270,10 @@ void Node3D::_notification(int p_what) {
 			// unless they need to perform specific tasks (like changing process modes).
 			fti_pump_xform();
 			fti_pump_property();
+
+			// Detect whether we are using an identity transform.
+			// This is an optimization for faster tree transform concatenation.
+			data.fti_is_identity_xform = data.local_transform == Transform3D();
 		} break;
 		case NOTIFICATION_SUSPENDED:
 		case NOTIFICATION_PAUSED: {
@@ -1448,6 +1452,8 @@ Node3D::Node3D() :
 	data.fti_on_tick_property_list = false;
 	data.fti_global_xform_interp_set = false;
 	data.fti_frame_xform_force_update = false;
+	data.fti_is_identity_xform = false;
+	data.fti_processed = false;
 
 #ifdef TOOLS_ENABLED
 	data.gizmos_disabled = false;

scene/3d/node_3d.h

@@ -51,6 +51,7 @@ class Node3D : public Node {
 	GDCLASS(Node3D, Node);
 
 	friend class SceneTreeFTI;
+	friend class SceneTreeFTITests;
 
 public:
 	// Edit mode for the rotation.
@@ -143,6 +144,8 @@ class Node3D : public Node {
 		bool fti_on_tick_property_list : 1;
 		bool fti_global_xform_interp_set : 1;
 		bool fti_frame_xform_force_update : 1;
+		bool fti_is_identity_xform : 1;
+		bool fti_processed : 1;
 
 		RID visibility_parent;
 

scene/main/node.h

@@ -176,7 +176,7 @@ class Node : public Object {
 		mutable int internal_children_back_count_cache = 0;
 		mutable int external_children_count_cache = 0;
 		mutable int index = -1; // relative to front, normal or back.
-		int depth = -1;
+		int32_t depth = -1;
 		int blocked = 0; // Safeguard that throws an error when attempting to modify the tree in a harmful way while being traversed.
 		StringName name;
 		SceneTree *tree = nullptr;
@@ -379,6 +379,7 @@ class Node : public Object {
 
 	void _set_use_identity_transform(bool p_enable) { data.use_identity_transform = p_enable; }
 	bool _is_using_identity_transform() const { return data.use_identity_transform; }
+	int32_t _get_scene_tree_depth() const { return data.depth; }
 
 	//call from SceneTree
 	void _call_input(const Ref<InputEvent> &p_event);