From 05b089200772fd880b6b27196e6ff8622edb4fdb Mon Sep 17 00:00:00 2001
From: Robert Swain <robert.swain@gmail.com>
Date: Tue, 29 Mar 2022 03:05:01 +0000
Subject: [PATCH] bevy_transform: Use Changed in the query for much faster
 transform_propagate_system (#4180)

# Objective

- Improve transform propagation performance

## Solution

- Use `Changed<Transform>` as part of the `root_query` and `transform_query` to avoid the indirection of having to look up the `Entity` in the `changed_transform_query`
- Get rid of the `changed_transform_query` entirely
- `transform_propagate_system` execution time for `many_cubes -- sphere` dropped from 1.07ms to 0.159ms, an 85% reduction for this system. Frame rate increased from ~42fps to ~44fps
---
 crates/bevy_transform/src/systems.rs | 35 +++++++++++++++++-----------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/crates/bevy_transform/src/systems.rs b/crates/bevy_transform/src/systems.rs
index 9fb2e9a2d0ef9a..bc40d58bb61df5 100644
--- a/crates/bevy_transform/src/systems.rs
+++ b/crates/bevy_transform/src/systems.rs
@@ -1,7 +1,8 @@
 use crate::components::{GlobalTransform, Transform};
 use bevy_ecs::{
     entity::Entity,
-    query::{Changed, With, Without},
+    prelude::Changed,
+    query::{With, Without},
     system::Query,
 };
 use bevy_hierarchy::{Children, Parent};
@@ -10,16 +11,23 @@ use bevy_hierarchy::{Children, Parent};
 /// [`Transform`] component.
 pub fn transform_propagate_system(
     mut root_query: Query<
-        (Entity, Option<&Children>, &Transform, &mut GlobalTransform),
+        (
+            Option<&Children>,
+            &Transform,
+            Changed<Transform>,
+            &mut GlobalTransform,
+        ),
         Without<Parent>,
     >,
-    mut transform_query: Query<(&Transform, &mut GlobalTransform), With<Parent>>,
-    changed_transform_query: Query<Entity, Changed<Transform>>,
+    mut transform_query: Query<
+        (&Transform, Changed<Transform>, &mut GlobalTransform),
+        With<Parent>,
+    >,
     children_query: Query<Option<&Children>, (With<Parent>, With<GlobalTransform>)>,
 ) {
-    for (entity, children, transform, mut global_transform) in root_query.iter_mut() {
+    for (children, transform, transform_changed, mut global_transform) in root_query.iter_mut() {
         let mut changed = false;
-        if changed_transform_query.get(entity).is_ok() {
+        if transform_changed {
             *global_transform = GlobalTransform::from(*transform);
             changed = true;
         }
@@ -28,7 +36,6 @@ pub fn transform_propagate_system(
             for child in children.iter() {
                 propagate_recursive(
                     &global_transform,
-                    &changed_transform_query,
                     &mut transform_query,
                     &children_query,
                     *child,
@@ -41,16 +48,19 @@ pub fn transform_propagate_system(
 
 fn propagate_recursive(
     parent: &GlobalTransform,
-    changed_transform_query: &Query<Entity, Changed<Transform>>,
-    transform_query: &mut Query<(&Transform, &mut GlobalTransform), With<Parent>>,
+    transform_query: &mut Query<
+        (&Transform, Changed<Transform>, &mut GlobalTransform),
+        With<Parent>,
+    >,
     children_query: &Query<Option<&Children>, (With<Parent>, With<GlobalTransform>)>,
     entity: Entity,
     mut changed: bool,
 ) {
-    changed |= changed_transform_query.get(entity).is_ok();
-
     let global_matrix = {
-        if let Ok((transform, mut global_transform)) = transform_query.get_mut(entity) {
+        if let Ok((transform, transform_changed, mut global_transform)) =
+            transform_query.get_mut(entity)
+        {
+            changed |= transform_changed;
             if changed {
                 *global_transform = parent.mul_transform(*transform);
             }
@@ -64,7 +74,6 @@ fn propagate_recursive(
         for child in children.iter() {
             propagate_recursive(
                 &global_matrix,
-                changed_transform_query,
                 transform_query,
                 children_query,
                 *child,