Math.round vs hack

JavaScript performance comparison

Revision 17 of this test case created by

Preparation code

<script defer="defer" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js"></script>
<script>
  var util = {
      round: Math.round
  };
  
  function hack_round(n) {
    return (n >= 0) ? ( n - 0.5 ) << 0 : ( n + 0.5 ) << 0;
  }
  
  if ( $.browser.mozilla ) {
      util.round = function (n) {
    return (n >= 0) ? ( n - 0.5 ) << 0 : ( n + 0.5 ) << 0;
  };
  alert('blah');
  }
  
  var somenum = -500 + (Math.random() * 1000);
  var rounded;
</script>
    

Preparation code output

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Testing in CCBot 2.0.0 / Other 0.0.0
Test Ops/sec
"proper" rounding
rounded = Math.round(somenum);
pending…
Hack rounding
rounded = ~~ (0.5 + somenum);
pending…
Proper hack rounding
rounded = ~~ (somenum + (somenum > 0 ? .5 : -.5));
pending…
Hack with bitwise OR
rounded = (0.5 + somenum) | 0;
pending…
Hack with bitwise shift
rounded = (0.5 + somenum) << 0;
pending…
Hack rounding with call overhead
rounded = hack_round(somenum);
pending…
util.round
rounded = util.round(somenum);
pending…

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Revisions

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