We also considered using LLVM for this, but the idea died. I guess we didn't want to switch compilers at that point (it would probably involve us switching to Clang). The one Josh talks about was actually an ENIGMA engine made in JS, for which game code was converted from EDL/GML to JS instead of C++. I think compiling to asm.js via Emscripten would create a lot faster code, than writing the whole thing in pure JS. Technically we could try that, but right now the biggest problem is that we don't have GLES support. Emscripten converts GLES to WebGL (which is essentially GLES), so it wouldn't probably convert our GL3 graphics system.