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Use new run_without_applying_deferred method in SingleThreadedExecutor #18684

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Merged
merged 3 commits into from
May 6, 2025
Merged

Use new run_without_applying_deferred method in SingleThreadedExecutor #18684

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e9d2956
Use new `run_without_applying_deferred` method in `SingleThreadedExec…
chescock Mar 14, 2025
defb225
`run_unsafe` and `readonly_run_unsafe` are only used when `std` is en…
chescock Apr 2, 2025
6058b7b
Add explanation for `black_box` calls.
chescock Apr 2, 2025
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27 changes: 22 additions & 5 deletions crates/bevy_ecs/src/schedule/executor/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -267,38 +267,54 @@ impl IntoSystemSet<()> for ApplyDeferred {
mod __rust_begin_short_backtrace {
use core::hint::black_box;

#[cfg(feature = "std")]
use crate::world::unsafe_world_cell::UnsafeWorldCell;
use crate::{
error::Result,
system::{ReadOnlySystem, ScheduleSystem},
world::{unsafe_world_cell::UnsafeWorldCell, World},
world::World,
};

/// # Safety
/// See `System::run_unsafe`.
// This is only used by `MultiThreadedExecutor`, and would be dead code without `std`.
#[cfg(feature = "std")]
#[inline(never)]
pub(super) unsafe fn run_unsafe(system: &mut ScheduleSystem, world: UnsafeWorldCell) -> Result {
let result = system.run_unsafe((), world);
// Call `black_box` to prevent this frame from being tail-call optimized away
black_box(());
result
}

/// # Safety
/// See `ReadOnlySystem::run_unsafe`.
#[cfg_attr(
not(feature = "std"),
expect(dead_code, reason = "currently only used with the std feature")
)]
// This is only used by `MultiThreadedExecutor`, and would be dead code without `std`.
#[cfg(feature = "std")]
#[inline(never)]
pub(super) unsafe fn readonly_run_unsafe<O: 'static>(
system: &mut dyn ReadOnlySystem<In = (), Out = O>,
world: UnsafeWorldCell,
) -> O {
// Call `black_box` to prevent this frame from being tail-call optimized away
black_box(system.run_unsafe((), world))
}

#[inline(never)]
pub(super) fn run(system: &mut ScheduleSystem, world: &mut World) -> Result {
let result = system.run((), world);
// Call `black_box` to prevent this frame from being tail-call optimized away
black_box(());
result
}

#[inline(never)]
pub(super) fn run_without_applying_deferred(
system: &mut ScheduleSystem,
world: &mut World,
) -> Result {
let result = system.run_without_applying_deferred((), world);
// Call `black_box` to prevent this frame from being tail-call optimized away
black_box(());
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This is the first time I've seen black_box used like this, and after some searching I still don't know why this would be beneficial. Can you explain or link me to somewhere this is explained?

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I'm not actually sure! I just copied it from the other methods.

... Okay, I think I found the source. It comes from the __rust_begin_short_backtrace method in std, which has a comment "prevent this frame from being tail-call optimised away".

https://github.com/rust-lang/rust/blob/ae9173d7dd4a31806c950c90dcc331f1508b4d17/library/std/src/sys/backtrace.rs#L154-L155

I'm going to add that as a comment to the black_box calls here, even though it's not strictly related to this PR.

result
}
Expand All @@ -308,6 +324,7 @@ mod __rust_begin_short_backtrace {
system: &mut dyn ReadOnlySystem<In = (), Out = O>,
world: &mut World,
) -> O {
// Call `black_box` to prevent this frame from being tail-call optimized away
black_box(system.run((), world))
}
}
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37 changes: 10 additions & 27 deletions crates/bevy_ecs/src/schedule/executor/single_threaded.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -128,33 +128,16 @@ impl SystemExecutor for SingleThreadedExecutor {
}

let f = AssertUnwindSafe(|| {
if system.is_exclusive() {
if let Err(err) = __rust_begin_short_backtrace::run(system, world) {
error_handler(
err,
ErrorContext::System {
name: system.name(),
last_run: system.get_last_run(),
},
);
}
} else {
// Use run_unsafe to avoid immediately applying deferred buffers
let world = world.as_unsafe_world_cell();
system.update_archetype_component_access(world);
// SAFETY: We have exclusive, single-threaded access to the world and
// update_archetype_component_access is being called immediately before this.
unsafe {
if let Err(err) = __rust_begin_short_backtrace::run_unsafe(system, world) {
error_handler(
err,
ErrorContext::System {
name: system.name(),
last_run: system.get_last_run(),
},
);
}
};
if let Err(err) =
__rust_begin_short_backtrace::run_without_applying_deferred(system, world)
{
error_handler(
err,
ErrorContext::System {
name: system.name(),
last_run: system.get_last_run(),
},
);
}
});

Expand Down