WebAssembly in 2016

https://github.com/webassembly/

What is WebAssembly?

A new standard being developed in a W3C Community Group with Apple, Google, Microsoft and Mozilla which defines:

a compact, portable binary format which is fast to load and runs safely at predictably near-native speed
a 1:1 text format rendered by developer tools when viewing source

Why WebAssembly?

Image/Video processing/editing
Compression, encryption
VR and low latency
WebUsb
Scientific simulation
Platform simulation (QEMU, DOSBox, ...)
Games
Remote desktop, VPN
Existing cpp codebase
... any cpu-bound code

FAQ

Will Wasm replace JS?
No.
No. No.

Wasm binary

Is just a postorder serialization of an AST. They claim its better than Java bytecode.

WebAssembly features

No builtin objects, allocator
No GC
Linear memory
Just like asm.js
Only 32bit for now :(

I want to try it!

Great! Firefox and Chrome has Wasm support under flags. For the compiler follow the step by step.

VM Safety

Callstack and machine code is not in address space.

The only dangerous opcodes are .load and .store operations, as they index into memory. They employ a cool trick in 64bit machines: mmap 4GBs of continouos virtual memory, you can't over-index that with an i32 address. Runtime bound-checks on 32bits.

Target Wasm

Use emscripten
emcc -s BINARYEN=1 -s 'EXPORTED_FUNCTIONS=["_fib"]' fib.c -o fib.js
an llvm backend
produces asm.js
binaryen converts asm.js to webassembly

asm.js example

function module(global, env, buffer) {
  "use asm";
  ...
  function fib(n) {
    n = n|0;
    var a = 0, b = 1, t = 0;
    while ((n|0) >= 0) {
      t = b;
      b = (a + b)|0;
      a = t;
      n = (n - 1)|0;
    }
    return b|0;
  }
  return {fib: fib};
}