Native Speed
on the Web

JavaScript & asm.js

Alon Zakai


The Web

Biggest open and standards-based platform

Great way to reach people

Would be great if it were fast as well ;)

How fast is it?

Speed on the Web

Speed is determined by many things

DOM, graphics, JavaScript

This talk is about JavaScript

How fast is JavaScript?

The Epic Citadel demo is one way to measure

Large C++ codebase compiled to JS using Emscripten

Uses only standard web technologies, JS and WebGL

High-end native apps can run in JS!

Epic Citadel: Progress

Launched March 2013, browsers improved since

back thentoday

only ran in Firefox
runs in Firefox, Chrome

20 sec. startup
10 sec. startup

60fps in Firefox, Chrome

JS engines don't always run at full speed

Motivated asm.js, an easy to optimize subset of JS

  function asmCode(global, env, buffer) {
    'use asm';
    var HEAP = new global.Uint8Array(buffer);
    function fib_like(x) {
      x = x|0;
      if ((x >>> 0) < 2) return HEAP[x]|0;
      return ((fib_like((x-2)|0)|0) + (fib_like((x-1)|0)|0))|0;
    return fib_like;

HEAP cannot be replaced

|0 trick ensures 32-bit ints

Typed arrays

asm.js Performance

Emscripten benchmark suite (VMs and Emscripten from Sep 13 2013, run on 64-bit ubuntu 12.04)

Box2D results

source: @jgw, Box2D Addendum; axis is time slower than native, lower is better

Box2dWeb -Port using "typical" JS
asm.js -Port using asm.js

asm.js is faster than "typical" JavaScript on Internet Explorer, Chrome and Firefox

asm.js - Background

Began as a research project at Mozilla

Every compiler (CoffeeScript, Google Web Toolkit, etc.) generates a particular pattern of JS

Emscripten and Mandreel converged on a pattern for compiled C++ in JS

asm.js is a formal definition of that pattern, with some improvements

Ok, near-native speed on Firefox and Chrome...

...but some of us use Safari or IE?


All browsers optimize JS, but decide which aspects to focus on at which times

source:; x axis is time, y axis is milliseconds (lower is better)


Very close on some, farther on others

Experimenting with using LLVM as a JIT

Internet Explorer

IE11, currently in pre-release, looks promising - I've seen it beat other browsers on some JS tests

Supports WebGL!

So safe to assume typed arrays will be fast, which means asm.js will be fast

Big Picture

JS can run at about half the speed of native code

Why just half - what are the remaining issues?

1. 32-bit Floats

JavaScript numbers are 64-bit doubles

Often take more CPU cycles to compute

Require more memory bandwidth

Preliminary tests on Firefox show 10-20% speed difference on relevant code

32-bit Floats

Sometimes possible to do 32-bit math as an optimization

  var floats = new Float32Array(calc());
  floats[0] = floats[1] + floats[2];
  floats[1] = floats[0] + floats[2];
  floats[2] = floats[1] + floats[2];

Mathematically provable that those additions can be 32-bit

No reason JS engines cannot do this right now (and Firefox is working on it)

32-bit Floats

A "fragile" optimization though

  var floats = new Float32Array(calc());
  floats[0] = floats[1] + floats[2] + 1; // can't
  floats[1] = floats[0] + floats[2] + 1; // be
  floats[2] = floats[1] + floats[2] + 1; // optimized

But ES6 (proposal for the next version of JS) includes Math.fround, which rounds to 32-bit precision

  var floats = new Float32Array(calc());
  floats[0] = Math.fround(floats[1] + floats[2]) + 1;
  floats[1] = Math.fround(floats[0] + floats[2]) + 1;
  floats[2] = Math.fround(floats[1] + floats[2]) + 1;

Now it's optimizable!


Single Instruction, Multiple Data: SSE, NEON, etc.


Large parts of CPUs dedicated to SIMD

Usable from C++, Mono, Dart, etc.


John McCutchan from Google wrote a proposal for SIMD in JS

  var x = float32x4(1, 2, 3.14159, 22.5);
  var y = float32x4(0, 0, 1, 0);
  var z = SIMD.add(x, y);

Immutable 128-bit values, designed for what CPUs provide

Potentially big (e.g. 300% in some cases) speedups on certain types of code

Collaboration is ongoing with Google, Mozilla, Intel and TC39

3. Threads

Web workers allow use of multiple CPU cores

Can transfer typed arrays, no copying

But threads can't read&write to the same data

Good for preventing data races, but bad for some types of projects (for example, modern game engines are heavily multithreaded)


Various proposals, some experiments, but no clear direction yet

Would require all vendors to agree and standardize something new and complex

Part of the challenge is figuring out how it fits in with the rest of the web, which does not currently have data races at all

Already many apps run +- native speed

BananaBread, a port of the Sauerbraten First Person Shooter (FPS) to JS+WebGL

Running inside it: Boon, another FPS

(= Doom code (PrBoom) + Freedoom content)

Demo Tech Details

Boon runs in a web worker

Input events are proxied to it, output of software renderer is sent back

We push those pixels to a WebGL texture

Less than 100 lines of code to integrate the two

Both games run at once at 60fps on a very average laptop


JS is close and getting closer to native speed

Even on missing pieces (SIMD, float32, etc.), progress is happening

That's it! :) Questions?