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linux-packaging-mono/external/corert/Documentation/how-to-build-WebAssembly.md
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Build WebAssembly

Currently, building WebAssembly is only possible on Windows.

  1. Install Emscripten by following the instructions here.
  2. Follow the instructions here to update Emscripten to the latest version.
  3. Get CoreRT set up by following the Visual Studio instructions.
  4. Build the WebAssembly runtime by running build.cmd wasm from the repo root.
  5. Run the WebAssembly "Hello World" test by running C:\corert\tests\runtest.cmd wasm.

To debug compiling WebAssembly:

  1. Set the ILCompiler startup command line to @C:\corert\tests\src\Simple\HelloWasm\obj\Debug\wasm\native\HelloWasm.ilc.rsp. That will generate HelloWasm.bc, an LLVM bitcode file.
  2. To compile that to WebAssembly, run emcc HelloWasm.bc -s ALLOW_MEMORY_GROWTH=1 C:\corert\bin\WebAssembly.wasm.Debug\sdk\libPortableRuntime.bc C:\corert\bin\WebAssembly.wasm.Debug\sdk\libbootstrappercpp.bc -s WASM=1 -o HelloWasm.html (if emcc isn't on your path, you'll need to launch an Emscripten command prompt to do this). That will generate a .wasm file with your code as well as html and js files to run it.

To run a WebAssembly application

  1. Ensure you have Edge 41 or above or Firefox.
  2. Open the generated html file in Edge or Firefox and look at the on-screen console for output.

Useful tips:

  • To manually make ILC compile to WebAssembly, add --wasm to the command line.
  • Add -g3 to the emcc command line to generate more debuggable output and a .wast file with the text form of the WebAssembly.
  • Omit -s WASM=1 from the emcc command line to generate asm.js. Browser debuggers currently work better with asm.js and it's often a bit more readable than wast.
  • Add -O2 --llvm-lto 2 to the emcc command line to enable optimizations. This makes the generated WebAssembly as much as 75% smaller as well as more efficient.