Experimental Scala.js linker backend for WasmGC

Prerequisites

This project requires Node.js >= 22 to get enough support of WasmGC.

If you are using NVM, you can instal Node.js 22 as follows:

# Install Node.js v22
$ nvm install 22
# Switch to Node.js 22
$ nvm use 22

Setup

Before doing anything else, run npm install.

Run the sample

In sample/src/main/scala/Sample.scala you can find a sandbox to play around.

You can build and run it like any other Scala.js project from sbt:

• sample/fastLinkJS compiles and links the project with the WebAssembly backend.
• sample/run runs the sample linked to WebAssembly with node.

You may want to look at the output in sample/target/scala-2.12/sample-fastopt/ to convince yourself that it was compiled to WebAssembly.

In that directory, you will also find a main.wat file, which is not used for execution. It contains the WebAsembly Text Format representation of main.wasm, for exploratory and debugging purposes. This is only true by default for the sample.

⚠️ If you modify the linker code, you need to reload and sample/clean for your changes to take effect on the sample.

You can also use the run.mjs script to play with @JSExportTopLevel exports.

• Run from the command line with node --experimental-wasm-exnref run.mjs.
• Run from the command line with DENO_V8_FLAGS=--experimental-wasm-exnref deno run --allow-read run.mjs.
• Run from the browser by starting an HTTP server (e.g., python -m http.server) and navigate to testrun.html.

If you encounter the Invalid opcode 0x1f error with Node.js, you need to use a Node.js >= 22.

Unit test suite

Run the unit test suite with tests/test.

• tests/test will
  • Link every test suite from testSuite with the WebAssembly backend
  • Run the produced WebAssembly module and check for any uncaught error
• Each Scala program in test-suite should have a def main(): Unit function. The test passes if the function successfully executes without throwing.
• When you add a test,
  • Add a file under test-suite
  • Add a test case to tests/src/test/scala/tests/TestSuites.scala

By default, .wat files are not generated, as they are quite big (several hundreds of KB for most of the tests). You can enable them by adding withPrettyPrint(true) to the linker configuration in tests/src/test/scala/tests/CoreTests.scala.

Scala.js integration test suite

Run the entire Scala.js test suite, linked with the WebAssembly backend, with:

> scalajs-test-suite/test

When you modify the linker, you need to reload and scalajs-test-suite/clean for your changes to take effect. Since recompiling the test suite from scratch every time is slow, you can replace the clean by manually removing only the linker output with:

$ rm -r scalajs-test-suite/target/scala-2.12/scalajs-test-suite-test-fastopt/

By default, .wat files are not generated for the Scala.js test suite, as they are very big (they exceed 100 MB). It is usually easier to minimize an issue in sample/test, but if you really want the big .wat file for the Scala.js test suite, you can enable it with

> set `scalajs-test-suite`/scalaJSLinkerConfig ~= { _.withPrettyPrint(true) }

Debugging tools

• The WasmGC reference interpreter can be used to validate and convert between the binary and text form:
  • https://github.com/WebAssembly/gc/tree/main/interpreter
  • Use docker image for it https://github.com/tanishiking/wasmgc-docker