Depsgraph: Avoid hash lookup for every evaluated scene query

Cache pointer to evaluated scene datablock on relations build time,
that pointer never changes after that.

source/blender/depsgraph/intern/depsgraph.cc

@@ -102,7 +102,8 @@ Depsgraph::Depsgraph(Scene *scene,
     scene(scene),
     view_layer(view_layer),
     mode(mode),
-    ctime(BKE_scene_frame_get(scene))
+    ctime(BKE_scene_frame_get(scene)),
+    scene_cow(NULL)
 {
 	BLI_spin_init(&lock);
 	id_hash = BLI_ghash_ptr_new("Depsgraph id hash");

source/blender/depsgraph/intern/depsgraph.h

@@ -197,6 +197,11 @@ struct Depsgraph {
 
 	/* Time at which dependency graph is being or was last evaluated. */
 	float ctime;
+
+	/* Evaluated version of datablocks we access a lot.
+	 * Stored here to save us form doing hash lookup.
+	 */
+	Scene *scene_cow;
 };
 
 }  // namespace DEG

source/blender/depsgraph/intern/depsgraph_build.cc

@@ -260,6 +260,9 @@ void DEG_graph_build_from_view_layer(Depsgraph *graph,
 	/* Relations are up to date. */
 	deg_graph->need_update = false;
 
+	/* Store pointers to commonly used valuated datablocks. */
+	deg_graph->scene_cow = (Scene *)deg_graph->get_cow_id(&deg_graph->scene->id);
+
 	if (need_on_visible_update) {
 		DEG_graph_on_visible_update(bmain, graph);
 	}

source/blender/depsgraph/intern/depsgraph_query.cc

@@ -105,9 +105,7 @@ Scene *DEG_get_evaluated_scene(const Depsgraph *graph)
 {
 	const DEG::Depsgraph *deg_graph =
 	        reinterpret_cast<const DEG::Depsgraph *>(graph);
-	Scene *scene_orig = deg_graph->scene;
-	Scene *scene_cow =
-	        reinterpret_cast<Scene *>(deg_graph->get_cow_id(&scene_orig->id));
+	Scene *scene_cow = deg_graph->scene_cow;
 	/* TODO(sergey): Shall we expand datablock here? Or is it OK to assume
 	 * that calleer is OK with just a pointer in case scene is not up[dated
 	 * yet?