At Stepstone, as part of our internal brown bag session, we talked about polyglot capabilities of GraalVM. We know, that GraalVM can run javascript, python, llvm and especially WebAssembly. But...

Can it run Doom?

So, we took Doom port compiled to WebAssembly created by @diekmann and ran it on top of GraalVM. See Internals for implementation details.

GraalVM version

This program was written on GraalVM 17+ v22.3. You must install wasm support by running gu install wasm.

Running

⚠️ To run correctly, you must increase stack size with -Xss4m

From IDE - just run com.stepstone.jc.demo.Doom class

From the command line - compile with mvn package and then run with:

java -Xss4m -jar doom-wasm-1.0-SNAPSHOT-jar-with-dependencies.jar

After this, you should see a Java Swing window and you can start killing monsters

Internals

The process of porting Doom to WASM is described in the original repo.

GraalVM's support for WASM, for now, is very basic. One important missing feature is the ability to inject java object with bindings, which will be called by the wasm program. Just running context.eval("wasm", doomWasmBytes) will lead to unresolved dependency one env and js modules. There is no direct equivalent of javascript's WebAssembly.instantiate accessible from standard GraalVM polyglot context. That's why we need to do some hacking. We use methods accessible by WebAssembly bindings to read and instantiate the WASM module, and to create an instance of WASM memory.

What we need to do:

• create an object, that will be used as WASM<->GraalVM binding and will contain js and env objects inside (com.stepstone.jc.demo.Bindings)
• load doom.wasm module using com.stepstone.jc.demo.WASMModule.module_decode
• instantiate this module with bindings using com.stepstone.jc.demo.WASMModule.module_instantiate
• having this, we can run Doom's main() function to setup game
• then we just need to run periodically main game loop using doom_loop_step()

Native image

It is also possible to compile GraalVM-WASM-based Doom into the native image.

First, you need to run mvn package and then run native-image:

native-image --language:wasm --initialize-at-build-time=sun.awt.Toolkit -Djava.awt.headless=false -H:ConfigurationFileDirectories=native-config -H:DynamicProxyConfigurationFiles=native-config\dynamic-proxy.json --no-fallback  -jar target\doom-wasm-1.0-SNAPSHOT-jar-with-dependencies.jar  

This way you will have Doom compiled to LLVM, compiled to WASM, wrapped in the Java Swing app, and compiled to native code. Simple, right? ¯_(ツ)_/¯