(Do we all have to learn C now?)

Dan Callahan / @callahad

Slides derived from a deck by
Luke Wagner / @luke_wagner

(Mozillian, Co-chair WebAssembly W3C Community Group)


  • What is WebAssembly?
  • Why do we need WebAssembly?
  • Little tour of WebAssembly
  • APIs
  • Beyond just C/C++
  • How will WebAssembly be used?
  • Will WebAssembly replace JS?

What is WebAssembly?

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

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

So what is WebAssembly really?

It's a couple things, depending on what you're doing:

1. Compiler Target for the Web

Not a programming language

Compile from programming languages

2. JavaScript Feature

Ability to efficiently load large code +

predictable near-native performance =

Powerful library-building tool

3. Virtual CPU

As close to physical machine instructions as safety/portability allow

WebAssembly x86 ARM
i32.add addl ADD
call call BL
i32.load check + mov check + LDR

4. Evolution of asm.js

(tl;dr - asm.js is an extraordinarily optimizable, low-level subset of JavaScript that can be compiled from languages like C/C++ and shipped on the Web today)

Can think of WebAssembly as a binary encoding of asm.js with tweaks to make it a better compiler target

asm.js WebAssembly
(x+y)|0 i32.add
f()|0 call
HEAP32[i>>2]|0 i32.load

Why WebAssembly?

Why do we need this new target / feature / CPU / asm.js++?

Short History of asm.js

2009 - 2012 Mozilla Research experiments:
Emscripten: C/C++ to JS compiler/toolchain
asm.js: optimize Emscripten-style output
2013 - 2014 Published asm.js subset, shipped optimizations in Firefox, demonstrated on large game engines
2015 - 2016

asm.js optimizations shipping or under development in Edge, Chrome and Safari

Adobe, AutoDesk, Epic, Facebook, Mega, Unity, and more shipping with Emscripten/asm.js

Why WebAssembly?

asm.js use today demonstrates real demand and excitement

asm.js use cases:

  • avoid plugins (deprecation, friction, security)
  • bring existing applications to the Web (too big to rewrite)
  • port high-performance C/C++ libraries for use by JS
  • predictable near-native performance (compared to JS)

But Why WebAssembly?

Because we need to go farther than we can with asm.js.

Based on feedback from developers, we need to:

  • further reduce load time (especially on mobile)
  • reduce over-the-wire size (with and without compression)
  • reduce runtime memory consumption for code
  • new features that don't fit neatly into asm.js
  • keep pushing closer to native code performance

(Also shared memory, but that's coming in JS too.)

WebAssembly is a Work In Progress

Recently reached Firefox/Chrome/Edge interop milestone!

More iteration before stable spec and ship in browsers

Want to ship this year (with usual standardization caveats)

WebAssembly is being specified and shipped iteratively.

The initial release focuses on a Minimum Viable Product: compiling C/C++, better asm.js.

(This means we can use asm.js to write polyfills!)

But there is a lot more we want to do next!

The whole text format is under construction, so I'll use the current temporary S-Expression syntax in this presentation:

Other things that are speculative
or not yet implemented are marked ☃

Let's take a little tour!

Start with some C code:

// demo.c
int add(int lhs, int rhs) {
    return lhs + rhs;
where functions we want to call from JS are exported:

// However DLL/DSO exports are defined in your compiler
#define DLL_EXPORT __attribute__ ((visibility ("default")))

then compile to wasm:

☃ clang -mwasm demo.c -o demo.wasm

Render the binary as text:

☃ wasm2text demo.wasm | less

    (func $add (param $lhs i32) (param $rhs i32) (result i32)
        (i32.add (get_local $lhs) (get_local $rhs))
    (export "add" $add)

Today, we load the wasm via JS API:

fetch('demo.wasm').then(response =>
).then(buffer => {
    let codeBytes = new Uint8Array(buffer);
    let instance = Wasm.instantiateModule(codeBytes);
    alert("1 + 2 = " + instance.exports.add(1, 2));


Experimental support requires Firefox Nightly and setting javascript.options.wasm in about:config

  1. Open the Debugger
  2. Click the wasm source

☃ In the future, streaming, async compilation via Streams

fetch('demo.wasm').then(response =>
).then(instance => {
    alert("1 + 2 = " + instance.exports.add(1, 2));

☃ In the future, with ES6 Module integration:


WebAssembly can call JavaScript too!

// main.c
extern DLL_IMPORT void printInt(int);
int main() {
where JS functions we want to call from wasm are imported:

// However DLL/DSO imports are defined in your compiler
#define DLL_IMPORT __attribute__ ((visibility ("default")))

then compile to wasm:

☃ clang -mwasm main.c -o main.wasm

Rendering the binary as text:

☃ wasm2text main.wasm | less

    (import "imports" "printInt" (param i32))
    (func $main (call_import 0 (i32.const 42)))
    (start $main)

Now write the ES6 module which is called by wasm:

// imports.js
export var printInt = i => console.log(i);

☃ and load the wasm from a <script type='module'> tag:


Imports can also just be functions passed to the JS API:

var codeBytes = ...;
var imports = {printInt:i => console.log(i)};
Wasm.instantiateModule(codeBytes, {imports});

Now let's look at an actual computation:

// accum.c
int accum(int* i, int* end) {
    int sum = 0;
    for (; i != end; i++)
        sum += *i;
    return sum;

This compiles to the function:

(func $accum (param $i i32) (param $end i32) (result i32)
    (local $sum i32)
    (loop $break $top
        (br_if $break (i32.eq (get_local $i) (get_local $end)))
        (set_local $sum (i32.add (get_local $sum)
                                 (i32.load (get_local $i))))
        (set_local $i (i32.add (get_local $i) (i32.const 4)))
        (br $top)
    (get_local $sum)
) \o/

The containg module declares and exports memory

    (memory 1)  ;; declare one page of memory = 64KiB
    (export "memory" memory)
    (func $accum ...)
    (export "accum" $accum)

Which we can then access from JS:

var codeBytes = ...;
var instance = Wasm.instantiateModule(codeBytes);

// Create an array of integers 1..10
var i32 = new Int32Array(instance.exports.memory);
for (var i = 0; i < 10; i++)
    i32[i] = i + 1;

console.log(instance.exports.accum(0, 10 * 4));  // "55"

End of the Tour

What we've seen so far:

  • Calling to and from JavaScript
  • ES6 module integration ☃
  • Computation

But what about APIs?

On a traditional virtual platform:


On a modern Web browser:

The Web is starting to resemble a traditional virtual platform

... with some special "Webby" properties like:

... open standards, multiple implementations, etc

Q: So what are WebAssembly APIs? A: Web APIs!

This is a key difference from plugin architectures

Web API integration

Today, WebAssembly only gets to Web APIs by
"thunking" through JavaScript

☃ In the future, support calling Web APIs directly

Web API integration

Emscripten maps common C/C++ interfaces to Web APIs

For example, using libc and SDL:

#include <SDL/SDL.h>
#include <stdio.h>

int main(int argc, char **argv) {
  SDL_Surface *s = SDL_SetVideoMode(200, 200, 32, SDL_HWSURFACE);
  SDL_Rect rect = { 50, 50, 100, 100 };
  SDL_FillRect(s, &rect, 0xffff0000);
  return 0;

Compiled by Emscripten with:

emcc -O2 test.c -o test.html

Web API integration

Which produces a default HTML harness:

Beyond just C/C++

Currently, WebAssembly only has linear memory

Great if your language has a low-level memory model

But if your language has GC:

  • would need to implement/ship your own GC
  • which misses optimizations in browser GC
  • cannot collect cycles involving browser objects

Beyond just C/C++

To provide first-class support for GC languages, WebAssembly needs direct GC access

☃ Goal: add low-level GC primitives to avoid baking in one language's class model

☃ Goal: share GC heap with JS, allow objects/strings/values to flow back and forth

Beyond just C/C++

Also need some fundamental extensions
to the (shared JS + WebAssembly) GC:

  • Typed Objects
  • Postmortem Notification
  • Weak References

Beyond just C/C++

Need even more features in WebAssembly to
run dynamic languages efficiently:

  • Patching: immediates, branch targets, return address, ...
  • Fine-grained (stub) compilation
  • Good dynamic/any value support

Beyond just C/C++

Also need a good source-maps story

  • debug/profile in source language
  • avoid size problems of today's source maps
  • avoid specifying "WebDWARF"

How will WebAssembly be Used?

Hard to predict, but we can extrapolate from asm.js today:

Will WebAssembly Replace JS?

No. Why?

  • JS remains the privileged high-level language of the Web
  • JS has huge and growing momentum, vibrant ecosystem

In fact, WebAssembly may have quite the opposite effect:

If you're going to target WebAssembly and your app needs a scripting language, JS is a natural choice...


  • Binary and text format
  • Low-level compiler target
  • Many ways and reasons to use
  • Part of the Web platform, sibling to JavaScript
  • Support more languages in the future

Thanks! Questions?

find out more at webassembly.github.io

Dan Callahan / @callahad