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22 November 2022 — by Cheng Shao
WebAssembly backend merged into GHC

Tweag has been working on a GHC WebAssembly backend for some time. Recently, the WebAssembly backend merge request has landed in GHC, and is on course to appear in the upcoming 9.6 release series. This post will give a quick demonstration of how to try it out locally, and explain what comes in this patch and what will be coming next.

Playing with WASM locally

If you’re using nix on x86_64-linux, compiling a Haskell program to a self-contained wasm module is as simple as:

$ nix shell
$ echo 'main = putStrLn "hello world"' > hello.hs
$ wasm32-wasi-ghc hello.hs -o hello.wasm
[1 of 2] Compiling Main             ( hello.hs, hello.o )
[2 of 2] Linking hello.wasm
$ wasmtime ./hello.wasm
hello world

There’s also a non-nix installation script. Check the ghc-wasm-meta repo’s README for details.

What’s interesting about the example above? It doesn’t need any companion JavaScript code, and runs on a variety of wasm engines that support wasi, including but not limited to: wasmtime, wasmedge, wasmer and wasm3. Compared to the legacy asterius project, there are also a few other serious benefits:

  • The killer feature is being able to use GHC’s own RTS code for garbage collection and other runtime functionality. The GHC RTS is way more robust, feature-complete and performant than asterius’s legacy JavaScript runtime. Lots of Haskell features that never worked in asterius (e.g. STM or profiling) now work out of the box.
  • It has proper support for compiling and linking C/C++ code. Terms and conditions apply here, but there’s still a high chance the cbits in your packages will work out of the box.
  • Since it uses LLVM for linking, the linking step is orders of magnitudes faster than asterius, which uses a custom object format and linking logic.
  • GHC CI tests a program that uses the GHC API to parse a Haskell module. ghc is a big package and depends on everything in the boot libraries, so even having only a part of GHC frontend working in pure wasm is already pretty cool, and it certainly provides more assurance than a simple “hello world”. asterius never had ghc in its boot libraries.

What is in this merge request

The GHC wasm backend merge request’s commit history is carefully structured to contain mostly small and easy to review patches. The changeset can be roughly grouped into:

  • Enhancing the build system, making it aware of the wasm32-wasi target, and avoid compiling stuff not supported on that target
  • Avoiding the usage of POSIX features not supported on wasm32-wasi – various places need to be patched, like the RTS, base or unix
  • Doing various other RTS fixes, for issues that didn’t break other GHC targets by pure luck
  • Enhancing the GHC driver with certain wasm-specific logic – most of the time due to the need to workaround some upstream issues in LLVM
  • Modeling the wasm structured control flow, and implementing the algorithm to translate arbitrary Cmm control flow graphs to it – this part of the work was done by my colleague Norman Ramsey, and well explained in his ICFP 2022 paper
  • Implementing the wasm native code generator (NCG), which translates Cmm to assembly code – unlike NCGs for other targets, the wasm NCG uses a dependently-typed IR to preserve type safety of the wasm value stack, and this has proved to be helpful in catching some errors early on when writing the NCG
  • Serving the binary distributions as CI artifacts, and there’s already some basic testing

GHC is a rapidly evolving project, and merging the wasm backend does not make it immune to potential future breakages. For me, it’s not just an honor to implement wasm support, but also a personal commitment to maintain it, prevent bit-rotting, and make sure that the bus factor of this work goes beyond 1 in the future. This is made possible by Tweag’s long term support.

What comes next

JavaScript FFI

asterius had a rich JavaScript FFI implementation, allowing one to import JavaScript functions into Haskell, pass arbitrary JavaScript values as first-class Haskell values, and export Haskell functions to be called by JavaScript. Furthermore, the JavaScript async functions worked naturally with the Haskell threading system, so that when a Haskell thread is blocked on an async JavaScript call, the runtime executes other threads instead of blocking completely.

This is the first of asterius main features that I plan to port to GHC’s wasm backend. You don’t pay for JavaScript if you don’t use it. We’ve already gained good experience with wasm/js interoperability, but this time I will need to do non-trivial refactorings in the GHC RTS storage manager and scheduler to achieve the same. So this will take some time and may not make it into GHC 9.6.1.

Template Haskell

asterius had limited support for Template Haskell. Template Haskell requires dynamically linking Haskell code, but how dynamic linking is supposed to work in wasm is still unclear, so asterius cheated by doing static linking each time a TH splice was evaluated. Since the runtime heap state isn’t preserved between splice evals, when the TH splices are stateful, this approach won’t work, but it’s been proven to work surprisingly well for a lot of TH splices in the wild.

I plan to add Template Haskell support for GHC’s wasm backend in a similar way. Pure TH splices (e.g. generating optics for datatypes) are likely to work, and work much faster than asterius thanks to the much improved linking performance. But splices with side effects (e.g. gitrev that needs to spawn a git subprocess), may not work if the side effect isn’t a supported WASI operation.

Since implementing proper dynamic linking isn’t planned yet, ghci wouldn’t work in GHC’s wasm backend in the near future.

More things to come

There are also other things planned in addition to the above features, including but not limited to:

  • Using the GHC issue tracker for bugfixes/feature planning and discussions, for better transparency of my work
  • Running the full GHC testsuite and nofib benchmarks
  • Supporting cross-compiling to wasm from more host systems
  • Wasm-related patches to common Hackage dependencies, or a Hackage overlay for wasm
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