Math.round vs hack (utility function) (v21)

Revision 21 of this benchmark created by JeffMo on

Description

Math.round() has a function call overhead, so using the ~~ hack (truncate towards 0) and adding 0.5 works quicker, but if you want to handle negative numbers too then you have to check whether to add or subtract 0.5 .... and this wipes out the speed advantage. So ~~(0.5+num) is only worth it if you know your numbers always have the same sign...

This compares Math.round() with the hack methods in http://jsperf.com/math-round-vs-hack/3 wrapped in a utility function.

Preparation HTML

<script>
  var somenum = -500 + (Math.random() * 1000);
  var rounded;
  
  function hack_rounding_utility(value) {
    return ~~ (0.5 + value);
  }
  
  function proper_hack_rounding_utility(value) {
    return ~~ (value + (value > 0 ? .5 : -.5));
  }
  
  function hack_rounding_bitwise_or(value) {
    return (0.5 + value) | 0;
  }
  
  function hack_rounding_bitwise_shift(value) {
    return (0.5 + value) << 0;
  }
</script>

Test runner

Ready to run.

Testing in
TestOps/sec
"proper" rounding
rounded = Math.round(somenum);
ready
Hack rounding
rounded = ~~ (0.5 + somenum);
ready
Proper hack rounding
rounded = ~~ (somenum + (somenum > 0 ? .5 : -.5));
ready
Hack with bitwise OR
rounded = (0.5 + somenum) | 0;
ready
Hack with bitwise shift
rounded = (0.5 + somenum) << 0;
ready
Hack rounding (utility function)
rounded = hack_rounding_utility(somenum);
ready
Proper hack rounding (utility function)
rounded = proper_hack_rounding_utility(somenum);
ready
Hack with bitwise OR (utility function)
rounded = hack_rounding_bitwise_or(somenum);
ready
Hack with bitwise shift (utility function)
rounded = hack_rounding_bitwise_shift(somenum);
ready

Revisions

You can edit these tests or add more tests to this page by appending /edit to the URL.