How to return an array of numbers from a function in asm.js?

Question

I'm sure I could eventually figure this out, but the documentation is a bit verbose and I think this ought to be a common question about asm.js.

Here goes:

Suppose that I have a function that looks like this:

function computeSquares() {
    var i, array = [];
    for(i = 0; i < 100; i++) {
        array.push(i * i);
    }
    return array;
}

The above function computes the square of the integers from 0 to 99 and returns an array containing the results.

I would like to write a function like this in asm.js that returns an object containing the 100 square values. If I've read the documentation, I'm supposed to use the ArrayBuffer object, but I'm a bit confused about how to do it.

Please illuminate me by giving an example of how to code this with asm.js.

Answer 1

You can only return doubles, signed ints and voids from exported functions. If you want to return an array you'll have to write it into the heap.

Answer 2

Instead of returning an array, I write the results in a convenient representation inside asm.js and then extract it with a wrapper function in JS that repeatedly calls into asm.js code to get the next value:

var result = [];
var i = 0;
while (true) {
    i = return_next_array_value_from_asmjs();
    if (i !== 0) {
        result.push(i);
    } else {
         break;
    }
}

The downside is that you need to reserve at least one value as a stop marker. I have not tested the performance because I couldn't get any other method to work.

Answer 3

The module would look like this:

function squaresModule(stdlib, foreign, heap) {
  "use asm";

  var imul = stdlib.Math.imul;
  var array = new stdlib.Uint32Array(heap);

  function compute( max ){
    max = max|0; //max is an integer
    var i = 0;

    for( i = 0;  (i|0) < (max|0);  i = (i+1)|0 ) {

      array[ i <<2>>2 ] = imul(i, i)|0;

    }

    return 0; //asm functions have to return a number
  }

  return {compute:compute};
}

.

Then use execute and log the array:

var array = Uint32Array( 100 );
var module = squareModule(
  {Math:Math,Uint32Array:Uint32Array}, {}, array.buffer
);
module.compute(100);
console.log(array);