Fast log base 2 in AS3

programming flash
1

[FONT=Courier New]Is there any really computationally fast way to calculate log base 2 in AS3 other than this:

var x:int=4096;
var x_ld:int=Math.log(x)*Math.LOG2E;

…also, are there any fast methods to determine whether a number is a power of 2?
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