[3.x] FTI - Optimize SceneTree traversal

doc/classes/ProjectSettings.xml

@@ -1418,6 +1418,12 @@
 			Sets which physics engine to use for 3D physics.
 			"DEFAULT" is currently the [url=https://bulletphysics.org]Bullet[/url] physics engine. The "GodotPhysics" engine is still supported as an alternative.
 		</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/smooth_trimesh_collision" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], smooths out collision with trimesh shapes ([ConcavePolygonShape]) by telling the Bullet physics engine to generate internal edge information for every trimesh shape created.
 			[b]Note:[/b] Only effective if [member physics/3d/physics_engine] is set to [code]DEFAULT[/code] or [code]Bullet[/code], [i]not[/i] [code]GodotPhysics[/code].

scene/3d/spatial.cpp

@@ -285,6 +285,10 @@ void Spatial::_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 == Transform();
 		} break;
 
 		case NOTIFICATION_PAUSED: {
@@ -1127,6 +1131,8 @@ Spatial::Spatial() :
 	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;
 
 	data.merging_mode = MERGING_MODE_INHERIT;
 

scene/3d/spatial.h

@@ -53,6 +53,7 @@ class Spatial : public Node {
 	OBJ_CATEGORY("3D");
 
 	friend class SceneTreeFTI;
+	friend class SceneTreeFTITests;
 
 public:
 	enum MergingMode : unsigned int {
@@ -129,6 +130,8 @@ class Spatial : 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;
 
 		bool merging_allowed : 1;
 

scene/main/node.h

@@ -111,8 +111,8 @@ class Node : public Object {
 		HashMap<StringName, Node *> owned_unique_nodes;
 		bool unique_name_in_owner = false;
 
+		int32_t depth;
 		int pos;
-		int depth;
 		int blocked; // safeguard that throws an error when attempting to modify the tree in a harmful way while being traversed.
 		StringName name;
 		SceneTree *tree;
@@ -266,6 +266,7 @@ class Node : public Object {
 	bool _is_physics_interpolation_reset_requested() const { return data.physics_interpolation_reset_requested; }
 	void _set_use_identity_transform(bool p_enable);
 	bool _is_using_identity_transform() const { return data.use_identity_transform; }
+	int32_t _get_scene_tree_depth() const { return data.depth; }
 
 public:
 	enum {