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# Build WebAssembly #
Currently, building WebAssembly is only possible on Windows.
1. Install Emscripten by following the instructions [here](https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html).
2. Follow the instructions [here](https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html#updating-the-sdk) to update Emscripten to the latest version.
3. Get CoreRT set up by following the [Visual Studio instructions](how-to-build-and-run-ilcompiler-in-visual-studio.md).
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](https://www.getfirefox.com).
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.
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