Scala to WebAssembly: How and Why Rikito Taniguchi (@tanishiking) Scala Compiler Engineer @ VirtusLab

2 Scala の Wasm サポートがくる！ WebAssembly support is coming!

3 ● What is Wasm ● Current Wasm Support in Scala ● Why Wasm (in future) ● How to Wasm ● Future

4 What is WebAssembly (Wasm) WebAssembly は安全・高速・環境や言語に非依存なプログラム形式 WebAssembly はバイナリのプログラム形式 高速な起動と、サンドボックス実行などによる安全性 “WebAssembly is a binary instruction format for a stack-based virtual machine…” Fast (startup) ● Decoding Wasm binary is faster than parsing JS ● Decode, validate, compile in stream / in parallel Secure ● Runs in a sandboxed environment ● Wasm design eliminates dangerous features from its execution semantic

5 What is WebAssembly (Wasm) WebAssembly は環境や言語に非依存なプログラム形式

6 ● What is Wasm ● Current Wasm Support in Scala ● Why Wasm (in future) ● How to Wasm ● Future

7 ScalaのWasmサポートは Scala.js のバックエンドとして実装 Scala.js の中間言語から JS ではなく Wasm + JS helper を生成する Dialect of Scala targeting JavaScript platform ● Frontend compiles Scala code to Scala.js IR (SJSIR) ● Linker backend generates Wasm from SJSIR (instead of JS) Wasm Linker backend in Scala.js NEW!

8 Scala.js により生成される Wasm は JS ヘルパーも含まれる JS 環境で実行できるが、non-js runtime では実行できない Generated Wasm runs on JS environment, but it doesn’t run on non-JS runtimes (wasmedge, wasmtime…) Wasm Linker backend in Scala.js NEW! Doesn’t work on non-JS runtime yet

9 ● Fast startup time ● Fast execution (depends on app) ● Small code size ● Sandboxed environment that’s all? WasmはScala.jsに 高速起動・(アプリの性質によるが)実行速度・小さいコート サイズ・サンドボックス環境を提供。それだけ? Wasm provides Scala.js…

10 ● What is Wasm ● Current Wasm Support in Scala ● Why Wasm (in future) ● Wasm is beyond browsers ● Wasm component Model ● Evolving Landsacape ● How to Wasm ● Future

11 ● Serverless/FaaS (Fastly edge, Fermyon Spin, CF workers) ● Containers/k8s (containerd-wasm-shim) ● Plugins/Extensions (VSCode, Envoy, UDF in TiDB) Wasm is beyond browser 近年では Serverless, containers, plugins の実行環境など ブラウザに限らない環境で活躍 Why? - Wasm’s fast startup time, sandboxing, and portability

12 Wasm module access host environment through a Standardized interfaces (WASI) to access system resources WASI (WebAssembly System Interface) WASI はシステムリソースにアクセスするための標準化されたインタフェース

13 WebAssembly Component Model Wasm module は i32 などの限られた型のみを使って import / export 文字列などの高度な型を表現する標準的なABIがない There’s no standard way to serialize high-level types across modules Wasm modules import/export functions only with i32, i64, f32, f64

14 WebAssembly Component Model Wasm component model は Wasm component 間で高度な型を使って直接 やり取りする方法を提供 Defines standardized way to directly communicate between Wasm components using high-level types.

15 Canonical ABI A standardized way to represent data types in binary WIT (Wasm Interface Types) A language for describing the interfaces and data types. Tools generate glue code that conforms to the ABI from WIT. Component Model under the hood ABI とインターフェース定義言語により構成される

16 Proposals ● WebAssembly SIMD proposal ● Stack Switching proposal ○ Native Delimited Control support (for green threads, etc) Tools ● Warg (Wasm component registry) ● wasm-opt Evolving LandScape SIMD サポート・軽量スレッドなど component model 周りのツールの進化

17 ● What is Wasm ● Current Wasm Support in Scala ● Why Wasm (in future) ● How to Wasm ● Future

18 Wasm native GC was not well supported in 2019 Without Wasm native GC, there’s a problem with interoperability between JavaScript and WebAssembly. (cycle collection problem) Why Scala hadn’t support Wasm? G “Scala.js and WebAssembly a tale of the dangers of the sea” from ScalaDays 2019 Lausanne, by Sébastien Doeraene (author of Scala.js) https://www.youtube.com/watch?v=QsOHofFJpig Wasmは2019年から考えられていたが、当時WasmGCが未成熟。適切にJSと の相互呼び出しができない

19 Compile VM to Wasm ● Module size tends to huge ● Good for legacy applications Embed own GC ● GC need to inspect the stack to find GC roots, which isn’t possible in Wasm, requires a shadow stack. ● Cycle-collection problem Compile Scala to Wasm in 2019 WasmGC以前にWasm対応するとしたら? JVM on Wasm・GCをWasm moduleに埋め込む。どちらにも問題がある

20 GC-managed (by Wasm runtime) data structures and instructions Wasm Garbage Collection (WasmGC) Directly compile GC-languages to Wasm without embedding GC Kotlin, Java, OCaml, Dart Wasm1.0には数値型のみ。WasmGCはstructやarrayなどのWasm runtime にGC管理されたデータ型を導入する。

21 Many Wasm runtimes support WasmGC by default in 2024 ● JS engines ○ V8 (Deno 1.38 / Node.js 22 / Chrome 119) ○ SpiderMonkey (Firefox 120) ○ 🚧 JavaScriptCore (Safari, soon?) ● Stand-alone Wasm VMs ○ WasmEdge (0.14.0) ○ 🚧 Wasmtime ○ 🚧 WAMR WasmGC support status 2019年当時はWasmGCはearly stageだった。 今では多くの Wasm runtime が WasmGC をサポートしている。 It’s right time to support Wasm in Scala!

22 Scala の Wasm サポートがくる！ We compile Scala.js to Wasm with WasmGC

23 ● Support non-JS Wasm ○ Get rid of JS interop semantics ○ WASI support ○ Re-implement JS-dependent libs ● Wasm Component Model Support ● More optimizations ○ Scala.js optimizer ○ Better code generation ○ wasm-opt Future 現状の実装はJS環境でのみ動作。将来的にはクラウド環境での利用・Wasm Component による任意の言語との interoperability を考えている

24 ● Wasm is fast, safe, polyglot, and portable binary program format ● Scala is compiling to WasmGC via Scala.js IR ○ Currently, Scala supports Wasm on browser ● Wasm has potential in non-JS environment + inter-communicate with different source languages. Conclusion Wasmはブラウザ外でも活躍し言語間のinteropのための新しいレイヤ Scala.jsからWasmGCを使って効率の良いバイナリを生成

25 ScalaNative compiles Scala to LLVM IR ● Relies on LLVM, Emscripten, and WASI-SDK to support Wasm! ● ScalaNative libs work out of the box in stand-alone Wasm runtime but… ● LLVM doesn’t support WasmGC ○ Need to embed GC to Wasm module, that is expensive ○ GC cycle-collection problem Appendix: How about Scala Native? ScalaNative は LLVM を活用、LLVM経由でWasmサポートの検討 LLVM は WasmGC 対応しておらず、GCを組み込むことになる。