From a81c8f97448714f968eead7b159d9b3301b178a6 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Mon, 9 Dec 2024 19:31:17 -0800
Subject: [PATCH] Don't unconditionally create temporary render entities for
 visible objects. (#16723)

PR #15756 made us create temporary render entities for all visible
objects, even if they had no render world counterpart. This regressed
our `many_cubes` time from about 3.59 ms/frame to 4.66 ms/frame.

This commit changes that behavior to use `Entity::PLACEHOLDER` instead
of creating a temporary render entity. This improves our `many_cubes`
time from 5.66 ms/frame to 3.96 ms/frame, a 43% speedup.

I tested 3D, 2D gizmos, and UI and they seem to work.

See the following graph of `many_cubes` frame time (lower is better). PR
#15756 is the one in October.

![Time (ms_frame) vs
Date(3)](https://github.com/user-attachments/assets/2c31a893-97bd-40f6-9e89-d2195a44cf40)
---
 crates/bevy_pbr/src/render/light.rs     | 13 +++++--------
 crates/bevy_render/src/camera/camera.rs |  6 ++----
 2 files changed, 7 insertions(+), 12 deletions(-)

diff --git a/crates/bevy_pbr/src/render/light.rs b/crates/bevy_pbr/src/render/light.rs
index 57ed8f2686..fe14688a24 100644
--- a/crates/bevy_pbr/src/render/light.rs
+++ b/crates/bevy_pbr/src/render/light.rs
@@ -31,7 +31,7 @@ use bevy_render::{
 };
 use bevy_render::{
     mesh::allocator::SlabId,
-    sync_world::{MainEntity, RenderEntity, TemporaryRenderEntity},
+    sync_world::{MainEntity, RenderEntity},
 };
 use bevy_transform::{components::GlobalTransform, prelude::Transform};
 #[cfg(feature = "trace")]
@@ -286,7 +286,7 @@ pub fn extract_lights(
         let render_cubemap_visible_entities = RenderCubemapVisibleEntities {
             data: cubemap_visible_entities
                 .iter()
-                .map(|v| create_render_visible_mesh_entities(&mut commands, &mapper, v))
+                .map(|v| create_render_visible_mesh_entities(&mapper, v))
                 .collect::<Vec<_>>()
                 .try_into()
                 .unwrap(),
@@ -343,7 +343,7 @@ pub fn extract_lights(
                 continue;
             }
             let render_visible_entities =
-                create_render_visible_mesh_entities(&mut commands, &mapper, visible_entities);
+                create_render_visible_mesh_entities(&mapper, visible_entities);
 
             let texel_size =
                 2.0 * ops::tan(spot_light.outer_angle) / directional_light_shadow_map.size as f32;
@@ -430,7 +430,7 @@ pub fn extract_lights(
                 cascade_visible_entities.insert(
                     entity,
                     v.iter()
-                        .map(|v| create_render_visible_mesh_entities(&mut commands, &mapper, v))
+                        .map(|v| create_render_visible_mesh_entities(&mapper, v))
                         .collect(),
                 );
             } else {
@@ -469,7 +469,6 @@ pub fn extract_lights(
 }
 
 fn create_render_visible_mesh_entities(
-    commands: &mut Commands,
     mapper: &Extract<Query<RenderEntity>>,
     visible_entities: &VisibleMeshEntities,
 ) -> RenderVisibleMeshEntities {
@@ -477,9 +476,7 @@ fn create_render_visible_mesh_entities(
         entities: visible_entities
             .iter()
             .map(|e| {
-                let render_entity = mapper
-                    .get(*e)
-                    .unwrap_or_else(|_| commands.spawn(TemporaryRenderEntity).id());
+                let render_entity = mapper.get(*e).unwrap_or(Entity::PLACEHOLDER);
                 (render_entity, MainEntity::from(*e))
             })
             .collect(),
diff --git a/crates/bevy_render/src/camera/camera.rs b/crates/bevy_render/src/camera/camera.rs
index f0603157fb..1221c66a79 100644
--- a/crates/bevy_render/src/camera/camera.rs
+++ b/crates/bevy_render/src/camera/camera.rs
@@ -6,7 +6,7 @@ use crate::{
     render_asset::RenderAssets,
     render_graph::{InternedRenderSubGraph, RenderSubGraph},
     render_resource::TextureView,
-    sync_world::{RenderEntity, SyncToRenderWorld, TemporaryRenderEntity},
+    sync_world::{RenderEntity, SyncToRenderWorld},
     texture::GpuImage,
     view::{
         ColorGrading, ExtractedView, ExtractedWindows, GpuCulling, Msaa, RenderLayers,
@@ -1098,9 +1098,7 @@ pub fn extract_cameras(
                                     .get(*entity)
                                     .cloned()
                                     .map(|entity| entity.id())
-                                    .unwrap_or_else(|_e| {
-                                        commands.spawn(TemporaryRenderEntity).id()
-                                    });
+                                    .unwrap_or(Entity::PLACEHOLDER);
                                 (render_entity, (*entity).into())
                             })
                             .collect();