Run results for: Inv Fast Sqrt

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.4 Safari/605.1.15

Browser: Safari 15

Operating system: Mac OS X 10.15.7

Device Platform: Desktop

Date tested: 2 years ago

Test name	Executions per second
Reciprocal Math.sqrt	7786.8 Ops/sec
Quakes fast inverse sqrt.	1588.1 Ops/sec

Script Preparation code:

var i;
var r;
var v = 2**16;

function test1() {
  for(i=0; i < 1000; ++i) {
    r = 1.0/Math.sqrt(v);
  }
}

function test2() {
  for(i=0; i < 1000; ++i) {
    r = Q_rsqrt(v);
  }
}

//Based on the fast inverse square root function
// https://en.wikipedia.org/wiki/Fast_inverse_square_root
// Some original comments preserved for humor value
// Designed to try to mimic the original as closely as possible
function Q_rsqrt(number)
{ 
    var i;
    var x2, y;
    const threehalfs = 1.5;
  
    x2 = number * 0.5;
    y = number;
    //evil floating bit level hacking
    var buf = new ArrayBuffer(4);
    (new Float32Array(buf))[0] = number;
    i =  (new Uint32Array(buf))[0];
    i = (0x5f3759df - (i >> 1)); //What the fuck?
    (new Uint32Array(buf))[0] = i;
    y = (new Float32Array(buf))[0];
    y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
//  y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

    return y;
}

Tests:

Reciprocal Math.sqrt
test1();
Quakes fast inverse sqrt.
test2();

Run results for: Inv Fast Sqrt

Compare Quakes fast inverse squareroot with reciprocal of math squareroot

Reciprocal Math.sqrt

Quakes fast inverse sqrt.