My experience with asm.js

It all started with Glitch, a tiny algorithmic music composer. I spent lots of time deciding whether I should choose a native approach (e.g. C++, Qt, RtAudio) or an HTML5 way. After all I decided to give HTML5 a try - there is JavaScript which I still like, WebAudio that runs everywhere, UI that can be ported to desktop thanks to Electron, and I’ve heard browsers are really good at performance these days.

My first disappointment was that eval was way too slow (and obviously insecure). So I wrote my own algebraic expression evaluator. The speed was good, so I wrote the rest of Glitch (UI, tiny sound processing library) in JavaScript.

It worked really well, until I started writing multi-part polyphonic songs. Then my browser choked.

If that was Java world, I would rewrite critical parts in C and used JNI. In Go I would use CGo. In Lua I would write a C module. I’ve heard that JavaScript has something similar, too.


Asm.js is annotated JavaScript. It’s perfectly valid JavaScript code that runs in every browser without any special hacks. However, it annotates every arithmetic operation with data types, so some browsers can optimize it a lot.

Yet nobody really writes asm.js manually because it becomes too bloated, most people use Emscripten to translate their C code into asm.js. We’ve all seen those Quake or Doom or Linux in a browser. That’s the result of Emscripten translation.

Installing Emscripten was easy, it’s just a tarball on Linux and Mac:

But it was much harder to figure out how to compile my C library to asm.js and how to use it.


Most of the documentation around asm.js is about how to port pure C app (including the main function) to JS. However, I needed to port a library.

Glitch would be too big for an example, so let’s assume our library deals with Fibonacci numbers - calculates the next fibonacci number and uses Binet’s equation to calculate n-th number:

#include <math.h>

struct fib {
	int now;
	int next;

int fib_next(struct fib *f) {
	int tmp = f->now + f->next;
	f->now = f->next;
	return f->next = tmp;

static double PHI = 1.6180339887;
int fib_binet(int n) {
	return round((pow(PHI, n) - pow(1 - PHI, n))/sqrt(5));

Natually, the first thing to try is to run emcc fib.c (emcc is Emscripten compiler). It produces a.out.js, a 5000 LOC file with some libc functions implemented in JavaScript (malloc, memset, abort etc).

But if we try to find fib string there - no results will be given. It did not include any of our functions to the resulting module.

We must specify exported symbols manually, using the following pleasant syntax (note the underscores):

emcc fib.c -o fib.js -s EXPORTED_FUNCTIONS="['_fib_next','_fib_binet']"

Ok, this actually produces code for our functions as well:

function _fib_next($f) {
 $f = $f|0;
 var $0 = 0, $1 = 0, $10 = 0, $11 = 0, $12 = 0, $13 = 0, $2 = 0, $3 = 0, $4 = 0, $5 = 0, $6 = 0, $7 = 0, $8 = 0, $9 = 0, $tmp = 0, label = 0, sp = 0;
 STACKTOP = STACKTOP + 16|0; if ((STACKTOP|0) >= (STACK_MAX|0)) abort();
 $0 = $f;
 $1 = $0;
 $2 = HEAP32[$1>>2]|0;
 $3 = $0;
 $4 = ((($3)) + 4|0);
 $5 = HEAP32[$4>>2]|0;
 $6 = (($2) + ($5))|0;
 $tmp = $6;
 $7 = $0;
 $8 = ((($7)) + 4|0);
 $9 = HEAP32[$8>>2]|0;
 $10 = $0;
 HEAP32[$10>>2] = $9;
 $11 = $tmp;
 $12 = $0;
 $13 = ((($12)) + 4|0);
 HEAP32[$13>>2] = $11;
 STACKTOP = sp;return ($11|0);

At this point we can try calling Binet approximation from JavaScript (I’m using Node.js here, but asm.js code runs in the browser, too):

var fib = require('./fib.js');

for (var i = 0; i < 20; i++) {
  console.log(fib.ccall('fib_binet', 'number', ['number'], [i]));

ccall() takes function name (without underscore this time), returned value (can be either number or string, pointers are same as numbers), array of parameter types, array of actual parameter values.

There is also cwrap() that wraps asm.js code into a function:

var binet = fib.cwrap('fib_binet', 'number', ['number']);
for (var i = 0; i < 20; i++) {

struct fib

While in C the following code would be common - struct fib f = {1,1}; fib_next(&f);, in asm.js it would be not so easy to write. One can use malloc and free to allocate memory, and maybe even initialize struct fields somehow, but for my structs I ended up writing constructors and descructors:

struct fib *fib_create() {
	struct fib *f = malloc(sizeof(*f));
	if (f != NULL) {
		f->now = f->next = 1;
	return f;
void fib_destroy(struct fib *f) {

After compilation the following code should be working:

var fib = require('./fib.js');

var create = fib.cwrap('fib_create', 'number', []);
var destroy = fib.cwrap('fib_destroy', 'null', ['number']);
var next = fib.cwrap('fib_next', 'number', ['number']);
var binet = fib.cwrap('fib_binet', 'number', ['number']);

var f = create();
for (var i = 0; i < 20; i++) {
  console.log(binet(i+3), next(f));


The size of the compiled asm.js is ~200KB, which sounds too big for a simple library. Let’s pass the -O3 flag to the compiler and enable all the optimizations.

At this point the code is likely to be reduced a lot (~60KB), but… it is also likely to stop working in browsers.

The reason is memory initialization. In debug builds (-O0 or O1) the memory is initialized with javascript arrays. But in release builds (-O2 or -O3) initial memory contents are stored in a separate file, which gets injected using XHR.

It’s an asynchronous process, so your global variables and constants have invalid values at the beginnging. You can register a function to be called when memory is initialized:

Module['onRuntimeInitialized'] = function() {
	// Start using asm.js code here

Another catch is web workers. They have no access to window.location, so memory initializer can’t guess the URL to the memory contents (it only knows the file name, but no host name or port). But you can always specify it explicitly:

Module['memoryInitializerPrefixURL'] = '';

And one last thing, if you’re using Webpack - build times are likely to be increased a lot, so maybe try including asm.js code directly with a script tag.


Asm.js is a great way to improve performance in the modern browsers. Glitch became really fast in Firefox, and got a noticeable boost in Chrome (which does not even support asm.js!). That is explained by the fact that asm.js is a much simpler JavaScript subset, so Chrome’s JIT can optimize it better.

A further step would be to add WebAssembly support, but my Emscripten was crashing when I tried to compile it. WebAssembly will be a binary format of asm.js, smaller and faster. It is also known to be supported in Chrome and Firefox, as well as in Electron. Oh, the future…

Posted on 2016-07-18

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