Duff's device (v14)

Revision 14 of this benchmark created on

Description

Simple test of variations of loop unrolling/unwinding based on the original Duff's Device.

Result may vary based on the number of iterations, browser, OS and external influences...

Usually Fast Duff's Device #2, #3, #4 runs faster when the number of iterations is bigger or when the test is executed multiple times without browser refresh - bitwise operations have a really strange behavior in JS, sometimes they are really fast and other times they run slower than the Math methods.

Make sure you test each approach using real application code (since results may vary) and just use this kind of optimization if you really need it! Try to combine techniques and avoid property lookups!

PS: It's over 9000 iterations!

Preparation HTML

<script>
  var iterations = 10000000;
</script>

Test runner

Ready to run.

Testing in
TestOps/sec
Regular Loop
var testVal = 0;
var n = iterations;
while (n--) {
 testVal++;
}
ready
Duff's Device
/**
 * Duff's Device 
 * http://home.earthlink.net/~kendrasg/info/js_opt/jsOptMain.html#duffsdevice
 */
var testVal = 0;
var n = iterations / 8;
var caseTest = iterations % 8;

do {
 switch (caseTest) {
 case 0:
  testVal++;
 case 7:
  testVal++;
 case 6:
  testVal++;
 case 5:
  testVal++;
 case 4:
  testVal++;
 case 3:
  testVal++;
 case 2:
  testVal++;
 case 1:
  testVal++;
 }
 caseTest = 0;
}
while (--n > 0);
ready
Fast Duff's Device #1
/**
 * Fast Duff's Device
 * @author Jeff Greenberg
 * http://home.earthlink.net/~kendrasg/info/js_opt/jsOptMain.html#fastDuffsdevice
 */
var testVal = 0;
var n = iterations % 8;
while (n--) {
 testVal++;
}

n = parseInt(iterations / 8);
while (n--) {
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
}
ready
Fast Duff's Device #2
/*
 * Fast Duff's Device
 * @author Miller Medeiros <http://millermedeiros.com> 
 * @version 0.1 (2010/08/25)
 */
var testVal = 0;
var n = iterations % 8;
while (n--) {
 testVal++;
}

n = Math.floor(iterations / 8);
while (n--) {
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
}
ready
Fast Duff's Device #3
/*
 * Fast Duff's Device
 * @author Miller Medeiros <http://millermedeiros.com> 
 * @version 0.2 (2010/08/25)
 */
var testVal = 0;
var n = iterations % 8;
while (n--) {
 testVal++;
}

n = (iterations / 8) ^ 0; //`value ^ 0` is the same as `Math.floor` for positive numbers and `Math.ceil` for negative numbers
while (n--) {
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
}
ready
Fast Duff's Device #4
/*
 * Fast Duff's Device
 * @author Miller Medeiros <http://millermedeiros.com> 
 * @version 0.3 (2010/08/25)
 */
var testVal = 0;
var n = iterations % 8;
while (n--) {
 testVal++;
}

n = (iterations * 0.125) ^ 0; //multiplication is faster than division in some cases
while (n--) {
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
 testVal++;
}
ready

Revisions

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