Miscompilation of Bevy (and some wgpu) apps resulting in segfault on macOS.

[inodentry]

Recent rustc seems to miscompile something somewhere in the dependency tree of Bevy, causing apps to crash with a segfault.

The issue seems to only manifest when optimizations are enabled and codegen units are > 1. The default cargo release configuration triggers it. Unoptimized builds are fine. Setting codegen-units = 1 with opt-level = 3 also seems to be fine.

I am on macOS 14.1 Sonoma, on Apple M1 Pro hardware.

Code

You can reproduce with any Bevy application. It is a huge code base, I know, I'm sorry I can't find a more minimal example.

Clone the Bevy repo: https://github.com/bevyengine/bevy

all these revs have the issue:

• commit bf4f4e42da3da07b0e84a4d6e40077f7398aea8b (i had that one laying around)
• commit 749f3d74305d35b4a453a80e45b22f08d5189a48 (current latest)
• tag v0.12.0
• tag v0.11.3

Run some example in release mode:

cargo run --example bevymark --release

Version it worked on

The previous 1.74.0-beta.7 did not have the issue. I am not sure which nightly introduced the regression.

Version with regression

I first noticed the issue when I installed the 2023-11-11 nightly, though it might also be present in earlier nightlies. I just installed 1.75.0-beta.1 and updated my nightly, and can confirm the issue is present there, too.

rustc 1.75.0-beta.1 (782883f60 2023-11-12)
binary: rustc
commit-hash: 782883f609713fe9617ba64d90086742ec62d374
commit-date: 2023-11-12
host: aarch64-apple-darwin
release: 1.75.0-beta.1
LLVM version: 17.0.4

rustc 1.76.0-nightly (ba7c7a301 2023-11-13)
binary: rustc
commit-hash: ba7c7a301984967c8c13adb580ef9b86ba706a83
commit-date: 2023-11-13
host: aarch64-apple-darwin
release: 1.76.0-nightly
LLVM version: 17.0.4

[Noratrieb]

I cloned the repo and ran the command you provided, but got errors:

error[E0463]: can't find crate for `bevy_reflect_derive`
 --> crates/bevy_reflect/src/type_uuid.rs:1:9
  |
1 | pub use bevy_reflect_derive::TypeUuid;
  |         ^^^^^^^^^^^^^^^^^^^ can't find crate

with both stable and nightly on bevy commit 749f3d74305d35b4a453a80e45b22f08d5189a48
did I do something wrong? what's your bevy commit?

[inodentry]

@Nilstrieb I specifically tried:

• commit bf4f4e42da3da07b0e84a4d6e40077f7398aea8b (i had that one laying around)
• commit 749f3d74305d35b4a453a80e45b22f08d5189a48 (current latest, same as you)
• tag v0.12.0
• tag v0.11.3

Yes, the previous 2 releases of Bevy (at least) are affected, too.

I don't get your error, everything compiles without errors on my end. 🤔

Have you tried cargo clean followed by cargo update, to ensure you have a clean target directory and cargo isn't getting confused by outdated deps?

[Noratrieb]

uhhh, I have run cargo clean and both cargo update.... wtf cargo?? I tried it on another computer and it compiles. let's dismiss that as a me issue. I can't try it on this other computer for unrelated reasons though so I can't try reproducing it on linux as well to rule out macos-specific issues.

[Noratrieb]

It would be useful if you could use cargo-bisect-rustc to bisect the regression to a single PR, which would make it easier to figure out what's happening and whether it really is a miscompilation or just UB that's now going wrong in bevy's dep tree.

[Noratrieb]

I was not able to reproduce this on Windows. Marking as O-MacOS, but I suspect that the problem is platform-independent and just only manifests in platform-specific bevy code.

[inodentry]

OK. TIL about cargo-bisect-rustc. Installing it and reading the documentation now. Will try it in a moment.

Edit: I created a simple bevy project that should automatically exit after the first frame is rendered to the screen. This should be scriptable. It will exit successfully on older rustc, or segfault on new rustc. cargo bisect-rustc is running now, as I am typing this.

Here is the bevy code if anyone else wants to try it:

use bevy::prelude::*;
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, exit)
        .run();
}
fn exit(mut evw: EventWriter<bevy::app::AppExit>) {
    evw.send(bevy::app::AppExit);
}