Stepping disabled performance fix (#11959)

# Objective

* Fixes #11932 (performance impact when stepping is disabled)

## Solution

The `Option<FixedBitSet>` argument added to `ScheduleExecutor::run()` in
#8453 caused a measurable performance impact even when stepping is
disabled. This can be seen by the benchmark of running `Schedule:run()`
on an empty schedule in a tight loop
(https://github.com/bevyengine/bevy/issues/11932#issuecomment-1950970236).

I was able to get the same performance results as on 0.12.1 by changing
the argument
`ScheduleExecutor::run()` from `Option<FixedBitSet>` to
`Option<&FixedBitSet>`. The down-side of this change is that
`Schedule::run()` now takes about 6% longer (3.7319 ms vs 3.9855ns) when
stepping is enabled

---

## Changelog
* Change `ScheduleExecutor::run()` `_skipped_systems` from
`Option<FixedBitSet>` to `Option<&FixedBitSet>`
* Added a few benchmarks to measure `Schedule::run()` performance with
various executors

benches/benches/bevy_ecs/scheduling/mod.rs

@@ -19,4 +19,5 @@ criterion_group!(
     contrived,
     schedule,
     build_schedule,
+    empty_schedule_run,
 );

benches/benches/bevy_ecs/scheduling/schedule.rs

@@ -118,3 +118,28 @@ pub fn build_schedule(criterion: &mut Criterion) {
 
     group.finish();
 }
+
+pub fn empty_schedule_run(criterion: &mut Criterion) {
+    let mut app = bevy_app::App::default();
+
+    let mut group = criterion.benchmark_group("run_empty_schedule");
+
+    let mut schedule = Schedule::default();
+    schedule.set_executor_kind(bevy_ecs::schedule::ExecutorKind::SingleThreaded);
+    group.bench_function("SingleThreaded", |bencher| {
+        bencher.iter(|| schedule.run(&mut app.world));
+    });
+
+    let mut schedule = Schedule::default();
+    schedule.set_executor_kind(bevy_ecs::schedule::ExecutorKind::MultiThreaded);
+    group.bench_function("MultiThreaded", |bencher| {
+        bencher.iter(|| schedule.run(&mut app.world));
+    });
+
+    let mut schedule = Schedule::default();
+    schedule.set_executor_kind(bevy_ecs::schedule::ExecutorKind::Simple);
+    group.bench_function("Simple", |bencher| {
+        bencher.iter(|| schedule.run(&mut app.world));
+    });
+    group.finish();
+}

crates/bevy_ecs/src/schedule/executor/mod.rs

@@ -21,8 +21,8 @@ pub(super) trait SystemExecutor: Send + Sync {
     fn run(
         &mut self,
         schedule: &mut SystemSchedule,
-        skip_systems: Option<FixedBitSet>,
         world: &mut World,
+        skip_systems: Option<&FixedBitSet>,
     );
     fn set_apply_final_deferred(&mut self, value: bool);
 }

crates/bevy_ecs/src/schedule/executor/multi_threaded.rs

@@ -166,8 +166,8 @@ impl SystemExecutor for MultiThreadedExecutor {
     fn run(
         &mut self,
         schedule: &mut SystemSchedule,
-        _skip_systems: Option<FixedBitSet>,
         world: &mut World,
+        _skip_systems: Option<&FixedBitSet>,
     ) {
         // reset counts
         self.num_systems = schedule.systems.len();
@@ -189,26 +189,18 @@ impl SystemExecutor for MultiThreadedExecutor {
         // If stepping is enabled, make sure we skip those systems that should
         // not be run.
         #[cfg(feature = "bevy_debug_stepping")]
-        if let Some(mut skipped_systems) = _skip_systems {
+        if let Some(skipped_systems) = _skip_systems {
             debug_assert_eq!(skipped_systems.len(), self.completed_systems.len());
             // mark skipped systems as completed
-            self.completed_systems |= &skipped_systems;
+            self.completed_systems |= skipped_systems;
             self.num_completed_systems = self.completed_systems.count_ones(..);
 
             // signal the dependencies for each of the skipped systems, as
             // though they had run
             for system_index in skipped_systems.ones() {
                 self.signal_dependents(system_index);
+                self.ready_systems.set(system_index, false);
             }
-
-            // Finally, we need to clear all skipped systems from the ready
-            // list.
-            //
-            // We invert the skipped system mask to get the list of systems
-            // that should be run.  Then we bitwise AND it with the ready list,
-            // resulting in a list of ready systems that aren't skipped.
-            skipped_systems.toggle_range(..);
-            self.ready_systems &= skipped_systems;
         }
 
         let thread_executor = world

crates/bevy_ecs/src/schedule/executor/simple.rs

@@ -33,15 +33,15 @@ impl SystemExecutor for SimpleExecutor {
     fn run(
         &mut self,
         schedule: &mut SystemSchedule,
-        _skip_systems: Option<FixedBitSet>,
         world: &mut World,
+        _skip_systems: Option<&FixedBitSet>,
     ) {
         // If stepping is enabled, make sure we skip those systems that should
         // not be run.
         #[cfg(feature = "bevy_debug_stepping")]
         if let Some(skipped_systems) = _skip_systems {
             // mark skipped systems as completed
-            self.completed_systems |= &skipped_systems;
+            self.completed_systems |= skipped_systems;
         }
 
         for system_index in 0..schedule.systems.len() {

crates/bevy_ecs/src/schedule/executor/single_threaded.rs

@@ -41,15 +41,15 @@ impl SystemExecutor for SingleThreadedExecutor {
     fn run(
         &mut self,
         schedule: &mut SystemSchedule,
-        _skip_systems: Option<FixedBitSet>,
         world: &mut World,
+        _skip_systems: Option<&FixedBitSet>,
     ) {
         // If stepping is enabled, make sure we skip those systems that should
         // not be run.
         #[cfg(feature = "bevy_debug_stepping")]
         if let Some(skipped_systems) = _skip_systems {
             // mark skipped systems as completed
-            self.completed_systems |= &skipped_systems;
+            self.completed_systems |= skipped_systems;
         }
 
         for system_index in 0..schedule.systems.len() {

crates/bevy_ecs/src/schedule/schedule.rs

@@ -333,15 +333,18 @@ impl Schedule {
             .unwrap_or_else(|e| panic!("Error when initializing schedule {:?}: {e}", self.label));
 
         #[cfg(not(feature = "bevy_debug_stepping"))]
-        let skip_systems = None;
+        self.executor.run(&mut self.executable, world, None);
 
         #[cfg(feature = "bevy_debug_stepping")]
+        {
             let skip_systems = match world.get_resource_mut::<Stepping>() {
                 None => None,
                 Some(mut stepping) => stepping.skipped_systems(self),
             };
 
-        self.executor.run(&mut self.executable, skip_systems, world);
+            self.executor
+                .run(&mut self.executable, world, skip_systems.as_ref());
+        }
     }
 
     /// Initializes any newly-added systems and conditions, rebuilds the executable schedule,