Compiling for the Web


To compile export templates for the Web, the following is required:

Building export templates

Before starting, confirm that the Emscripten configuration file exists and specifies all settings correctly. This file is available as ~/.emscripten on UNIX-like systems and %USERPROFILE%\.emscripten on Windows. It’s usually written by the Emscripten SDK, e.g. when invoking emsdk activate latest, or by your package manager. It’s also created when starting Emscripten’s emcc program if the file doesn’t exist.

Open a terminal and navigate to the root directory of the engine source code. Then instruct SCons to build the JavaScript platform. Specify target as either release for a release build or release_debug for a debug build:

scons platform=javascript tools=no target=release
scons platform=javascript tools=no target=release_debug

By default, the JavaScript singleton will be built into the engine. Since eval() calls can be a security concern, the javascript_eval option can be used to build without the singleton:

scons platform=javascript tools=no target=release javascript_eval=no
scons platform=javascript tools=no target=release_debug javascript_eval=no

The engine will now be compiled to WebAssembly by Emscripten. Once finished, the resulting file will be placed in the bin subdirectory. Its name is for release or for debug.

Finally, rename the zip archive to for the release template:

mv bin/ bin/

And for the debug template:

mv bin/ bin/

Building per asm.js translation or LLVM backend

WebAssembly can be compiled in two ways: The default is to first compile to asm.js, a highly optimizable subset of JavaScript, using Emscripten’s fastcomp fork of LLVM. This code is then translated to WebAssembly using a tool called asm2wasm. Emscripten automatically takes care of both processes, we simply run SCons.

The other method uses LLVM’s WebAssembly backend. This backend is not yet available in release versions of LLVM, only in development builds built with LLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly. Compiling with this backend outputs files in LLVM’s .s format, which is translated into actual WebAssembly using a tool called s2wasm. Emscripten manages these processes as well, so we just invoke SCons.

In order to choose one of the two methods, the LLVM_ROOT variable in the Emscripten configuration file is used. If it points to a directory containing binaries of Emscripten’s fastcomp fork of clang, asm2wasm is used. This is the default in a normal Emscripten installation. Otherwise, LLVM binaries built with the WebAssembly backend will be expected and s2wasm is used. On Windows, make sure to escape backslashes of paths within this file as double backslashes \\ or use Unix-style paths with a single forward slash /.