gc VS exception-handling

It may take some time for WasmGC to be usable by .NET. Based on the discussions the first version of WasmGC does not have a good way to handle a few .NET specific scenarios, and said scenarios are "post-post-mvp". [0]

My concern, of course, is that there is not much incentive for those features to be added if .NET is the only platform that needs them... at that point having a form of 'include' (to where a specific GC version can just be cached and loaded by another WASM assembly) would be more useful, despite the pain it would create.

[0] - https://github.com/WebAssembly/gc/issues/77

Yes, that's definitely true: a single GC will not be optimal for everything, or even possible. Atm interior pointers are not supported at all, for example, but they are on the roadmap for later:

https://github.com/WebAssembly/gc/blob/main/proposals/gc/Pos...

What launched now is enough WasmGC to support a big and useful set of languages (Java, Kotlin, Dart, OCaml, Scheme), but a lot more work will be required here!

GC proposal is from 2018: https://github.com/WebAssembly/proposals/issues/16 and there’s code: https://github.com/WebAssembly/gc/blob/master/proposals/gc/O...

Seems like an awefully long time for progress to be made, given all the possibilities it would unlock.

First is the size. Writing a server-side and client-side program is possible with Rust, and the resulting WASM package will be small enough. At the same time, Microsoft Blazor converts C# code to WASM, but the client delivery has to include the reduced .NET runtime, taking several megabytes for a script. The same is true for GoLang, even with an attempt to reduce the runtime delivery in TinyGo WASM. Developers want to work with their favorite languages, whether it is Java, Kotlin, Dart, C#, F#, Swift, Ruby, Python, C, C++, GoLang, or Rust. These languages produce groups of runtimes. For example, JVM and .NET have many common parts, Ruby and Python are dynamically interpreted at runtime, and all mentioned depend on automatic garbage collection. For smaller WASM packages, browser vendors can include extended runtime implementations, for example, by delivering a general garbage collector as part of WASM. Garbage collection support by WASM is currently in progress: WASM GC, .NET WASM Notes.

My understanding is that the main limitation is technical. WASM doens't do GC or the host system calling conventions and cannot interact directly with object from Javascript because of this. However, this is being worked[0] on and will be solved eventually. Even without this the performance overhead of bridging to JS is low enough that WASM frameworks can beat out React.

0: https://github.com/WebAssembly/gc/blob/main/proposals/gc/Ove...

I’ve just watched a video on YouTube from Google I/O 2023 on Flutter for the web. Kevin Moore explains that Flutter can compile to Wasm, but now that GC support has been added to the standard and WasmGC is supported in Chromium and Firefox, I’m quite intrigued.

I have found the main thread about using WebAssembly GC in C#: https://github.com/WebAssembly/gc/issues/77. If I understand it correctly, it is not possible to use the current prototype version of GC in C#.

The .net team has given their notes on it, the concern is more on the memory layout from what I remember. Though it may be possible still. The runtime would likely still ship some gc code, but only a subset for cases not supported by the wasm gc itself and a few more for interfacing with the gc service, which overall should still result on smaller payloads compared to current sizes.

I assume that's one of the parts of the work done at https://github.com/WebAssembly/gc - not happening any soon yet, but it'll eventually be done.

Interesting article, thanks!

Notes on the issues mentioned there:

* The need for a manual shadow stack: This is fixed in WasmGC (in the same way it works in JS, as the link mentions).

* Lack of try-catch: This is fixed by the Wasm exception handling proposal, which has already shipped in browsers, https://github.com/WebAssembly/exception-handling/blob/main/...

* Null checks: Mostly fixed by WasmGC. The spec defines non-nullable local types, and VMs can use the techniques the article mentions to optimize them using signals (Wizard does, for example).

* Class initialization: This is a difficult problem, as the article says. J2Wasm and Binaryen are working to optimize it through static analysis at the toolchain level. Here is a recent PR I wrote that makes progress there: https://github.com/WebAssembly/binaryen/pull/6061

* The vtable overhead issue the article mentions may be a problem. I'm not aware of good measurements on it, through. There are some ideas on post-MVP solutions for method dispatch that might help, but nothing concrete yet.

* Checks for null and trapping: There has been discussion of variants on the GC instructions that throw instead of trap. Measurements, however, have not shown it to be a big problem atm, so it is low priority.

The author is right that stack walking, signals, and memory control are important areas that could help here.

Overall with WasmGC and exceptions we are in a pretty good place for Java as emitted by J2Wasm today: it is usually faster than J2CL which compiles Java to JavaScript. But there is definitely room for improvement.

It uses https://github.com/WebAssembly/exception-handling/blob/master/proposals/exception-handling/Exceptions.md which is actually supported by all major browsers already.

Yep, you're right. It's also more than just the DOM, it's web APIs in general, such as fetch, audio, webgl/webgpu, etc. WASM still needs GC, exceptions, and WASI to be able to fully interop with any host without any of the current limitations. This'll take a few years. I'm looking forward to the future in which I will be shipping WASM-only web apps to my users.

Its a part of the wasm plan to support gc https://github.com/WebAssembly/gc exceptions https://github.com/WebAssembly/exception-handling

As of Node.js 17, the v8 JavaScript engine has been updated to v9.5. The changes in this release are primarily aimed at expanding internationalization for dates and calendars as well as for the output of time zones. It also implements the WebAssembly Exception Handling proposal, designed to reduce overhead compared to current JavaScript-based workarounds.

Agreed, although at some point in a not very far feature most of those missing features will resolved. So in my mind is just a matter of time. The Wasm Community group is doing an awesome work on that :)

Here are a few examples of what needs move forward in Wasm:

* [1] Wasm Exceptions Handling: Right now Wasm is missing a way to handle exceptions natively (C++ programs can only compile to Wasm using the asyncify or longjmp/setjmp tricks via Js try/catch)

* [2] Wasm GC: Wasm Binary files are quite big (specially in interpreted languages). This is partially caused by the GC being included in the Binary itself. The GC proposal will solve this while also providing faster execution.

* [3] Wasm 64-bit Memory: currently Wasm can only operate with 32-bit data. In some contexts you may want you operate with more than 4GB of memory (for example, when operating over terabytes of data). The 64-bit memory proposal will solve that.

[1]: https://github.com/WebAssembly/exception-handling