What is LLVM?

• a compiler system
• it does a lot! e.g. analysis tools of interest to people who design languages
• it has an intermediate low-level language to compile to, which can then generate native code for different targets
  • e.g. instead of compiling directly from C++ to x86, the LLVM chain goes C++ to LLVM to x86
  • clang is a C and C++ compiler that makes use of LLVM, for example

What is LLVM?

• it is not a virtual machine
  • it can be compiled JIT, but LLVM is not meant to be executed/interpreted like, say, Java or CLR is
• other developers have made frontends other than C/C++, supposedly (e.g. PHP or Lua or Haskell all compiled into LLVM)
• code generation targets other low-level languages, such as x86, x64, MIPS...

That's neat and all

(but what can we actually do?)

• It depends on what languages you have that can compile into LLVM and what targets you can generate assembly for!
• This brings me to the reason I wanted to talk about this: a subset of javascript known as asm.js, and a compiler toolchain known as emscripten.

asm.js and Emscripten

• asm.js -- the javascript spec used as a compile target
  • ​"a strict subset of JavaScript that can be used as a low-level, efficient target language for compilers. This sublanguage effectively describes a sandboxed virtual machine for memory-unsafe languages like C or C++."
• ​emscripten -- the full compiler toolchain
  • "​Emscripten is an LLVM-based project that compiles C and C++ into highly-optimizable JavaScript in asm.js format."

C++ on the web

• just to be clear, this is compiling C++ to run directly in the browser as javascript (or similar)
• this is separate and distinct from cgi, which is typically native code that runs server-side and emits HTML, etc. to render

My Experience

• So far, not much -- environment setup and a few simple "hello world" adjacent applications
  • input to emscripten is valid C or C++ or already-compiled LLVM (from any language)
  • output is an HTML shell, program memory, and the javascript file with everything
  • The HTML shell lets you define behavior for things like stdin, stdout, stderr, or other callbacks you might create in your C++ application
  • emscripten includes an OpenGL to WebGL shim, which mostly works -- calls that are valid in OpenGL but not in WebGL will cause clearly stated runtime errors

My Experience

• I was surprised at how a lot of it just worked
  • default (if you don't define it otherwise) keyboard input via stdin::getline is weird for terminal programs, I'm currently trying to figure out more user-intuitive behavior
  • true concurrency doesn't work, but it at least round-robins runtime between threads
  • OpenGL really just works
• Portingis a lot easier than I imagined, but I can still see some places where effort may be needed
  • e.g. abstracting away OS-specific libraries (Windows events for keyboard, mouse, frame updates, etc.) and implementing new ones specific to emscripten

Use in the Wild

• QuakeJS - recompiled open-source Quake to run in a browser, using emscripten
• BananaBread - Cube 2 engine recompiled to WebAssembly
• ports of things like Ogg, FFMpeg, zlib, FreeType, QT
• full list here: https://github.com/kripken/emscripten/wiki/Porting-Examples-and-Demos

oh also a live demo

"what about webassembly"

• Good question, and one I don't have an in-depth answer for!
• All I know is that there is an option to compile to WebAssembly instead of asm.js, and WebAssembly strikes me as less horrific than using javascript as a compile target because this is the exact use case WebAssembly is for.