RFC: Rust considering switching to using WebAssembly exception handling

[hoodmane]

Rust is considering switching to using WebAssembly exception handling. See this major change proposal. We will initially leave in nightly support for opting out. How would such a change affect this project?

[PgBiel]

Hello,
In principle, if I understood correctly, we have been using your workaround for Wasm exceptions at rust-lang/rust#112195 (comment) ever since Godot 4.3 started compiling with Wasm exceptions (#438 (comment)), requiring GDExtension users to do so as well. Therefore, a change to further promote Wasm exceptions would likely either not affect or maybe even benefit this project, if they become the default, as that would mean one less flag that we would need to compile to Wasm, which is always welcome.

Something that would make a larger difference would be full support for panic=unwind, but I'm not really sure what's blocking that right now (I think std support? Not sure). But I'm not sure if that's related.

[hoodmane]

You can try it out now with RUSTFLAGS=-Zemscripten-wasm-eh cargo build -Zbuild-std if you are interested.

we have been using your workaround for Wasm exceptions at rust-lang/rust#112195 (comment)

I didn't think that workaround was good enough since code gen for the try intrinsic comes out wrong.

Something that would make a larger difference would be full support for panic=unwind

As far as I am aware, panic=unwind is fully supported. But perhaps this is related to the fact that the workaround linked above doesn't actually work as far as I'm aware? Could you say more about what problems you are having?

[PgBiel]

You can try it out now with RUSTFLAGS=-Zemscripten-wasm-eh cargo build -Zbuild-std if you are interested.

Thanks. This compiles, but gives me the following error on run:

Chromium:

Uncaught (in promise) LinkError: WebAssembly.instantiate(): Import #5 "env" "__cpp_exception": tag import requires a WebAssembly.Tag

Firefox:

Uncaught (in promise) LinkError: import object field '__cpp_exception' is not a Tag

The error still occurs even if I add -Cllvm-args=-enable-emscripten-cxx-exceptions=0 from your workaround alongside -Zemscripten-wasm-eh. It only goes away and works if I remove -Zemscripten-wasm-eh and keep the former, as before.

Do you have any ideas on this?

Flags used were as follows

[target.wasm32-unknown-emscripten]
rustflags = [
    "-C", "link-args=-sSIDE_MODULE=2",
    "-Zlink-native-libraries=no",
    "-Zemscripten-wasm-eh"
    # "-Cllvm-args=-enable-emscripten-cxx-exceptions=0", # uncommenting doesn't help
]

we have been using your workaround for Wasm exceptions at rust-lang/rust#112195 (comment)

I didn't think that workaround was good enough since code gen for the try intrinsic comes out wrong.

Something that would make a larger difference would be full support for panic=unwind

(...) But perhaps this is related to the fact that the workaround linked above doesn't actually work as far as I'm aware? Could you say more about what problems you are having?

The workaround very much worked for our purposes. The problem is that upstream Godot switched to Wasm exceptions, so our code compiled with old / js exceptions wouldn't even run because it would expect functions named invoke_(signature) to exist for each function declared on the Rust side, but since our code would run as a library / side module and Godot would run as the entrypoint program (which i suppose is where such functions would be created for deduplication reasons), these functions would never be generated by emscripten.

By using your workaround, the compiled code would successfully run alongside Godot, as it wouldn't generate those calls to invoke_xxxx anymore.

That is, solving unwinding panic was only secondary compared to making the code run in the first place, haha.

As far as I am aware, panic=unwind is fully supported.

I just tested it and it's still aborting regardless, but I'm not using -Zemscripten-wasm-eh yet as it doesn't run due to the above. If we can find a way to run it, I can try again and post my results.

[Bromeon]

@hoodmane do you have any feedback regarding @PgBiel's comment above?

[hoodmane]

Thanks for the ping @Bromeon and for trying it out @PgBiel. I will look into it. It's very surprising to me that the old workaround worked for you folks because it never worked for me...

[hoodmane]

Well I built it successfully with:

export RUSTFLAGS="-Clink-args=-sSIDE_MODULE=2 -Zemscripten-wasm-eh"
cargo build --target wasm32-unknown-emscripten -F experimental-wasm

-Zlink-native-libraries=no is no longer necessary since we merged this fix to rust/libc. How do I run it?

[PgBiel]

How do I run it?

Do you have a Godot project ready? The instructions are here (particularly "Godot editor setup" to run it):

https://godot-rust.github.io/book/toolchain/export-web.html (this assumes you have a project: https://godot-rust.github.io/book/intro/index.html)

Let me know if you need help. If needed, I can try to upload my own project soon. You can also try to start with this (now outdated) base project: https://github.com/PgBiel/hello-gdext-wasm

[Bromeon]

From a project management PoV, is there something concrete and actionable for godot-rust in the near future? Otherwise I'd probably close this issue, to focus the task tracker on active problems to work on.

The discussion can of course continue here, either way! 🙂

[PgBiel]

In principle I'd wait for feedback by @hoodmane before further decision. It'd be nice if we could get this to work, but let's see how their testing goes.

[hoodmane]

I'm still planning to get back to this and test it. If someone could give me a known currently working hello world project to build that would be helpful. Because as I understand it the problem only occurs at load time. I think the problem is that the main Godot engine binary also needs to be built with wasm error handling and not legacy error handling.

Wasm eh has smaller code, better performance, and fewer bugs so it would be helpful to everyone if we can get it working.

[PgBiel]

Sounds good. While https://github.com/PgBiel/hello-gdext-wasm is a good reference project, it is currently not up-to-date with Godot (made with Godot 4.2, latest version is 4.4). I'm currently very busy with other tasks, but in the upcoming week I'll try to update it and let you know.