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Thread: Multiply, or shift bits?
 03152011, 06:48 PM #1Member
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Multiply, or shift bits?
Hey, just a quicky!
Let's say I have a program that needs to be fast, and let's say that I want to multiply/divide something with a power of two (2^1, 2^2, ...). Would it be faster to carry out the multiplication, for example n * 2, or would it be faster to shift the bits one step to the left, n << 1? Or makes it no difference?
PLMLast edited by plmpusik; 03152011 at 06:54 PM.
 03152011, 07:18 PM #2Moderator
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I have no idea what the solution is. But I'll post anyway... Because the solution is easy enough to find: compile your program twice using both approaches, test and time.
The JLS just says (15.19 Shift Operators) "The value of n<<s is n leftshifted s bit positions; this is equivalent (even if overflow occurs) to multiplication by two to the power s". And I don't think the JVM specification says anything about a difference in implementation on which you can rely.
In practice I would write the expression in whatever form seemed most natural. This is subjective, of course, and what "natural" means would depend on the result of testing your and similar code.
 03152011, 07:36 PM #3Senior Member
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The difference seems to be negligible. I tested it with this:
Java Code:public class ShiftVsMultiply { public static void main(String[] args) { long t = System.currentTimeMillis(); int j; for (int i = 0; i <= 500000000; i++) { j = i * 2; } System.out.println("Time taken for *2: " + (System.currentTimeMillis()  t) + "ms"); t = System.currentTimeMillis(); for (int i = 0; i <= 500000000; i++) { j = i << 1; } System.out.println("Time taken for <<1: " + (System.currentTimeMillis()  t) + "ms"); } }
Java Code:Time taken for *2: 379ms Time taken for <<1: 373ms
Last edited by Iron Lion; 03152011 at 07:39 PM.
 03152011, 08:05 PM #4
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 03152011, 11:08 PM #5Member
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I didn't know about this, but it makes sense when you think about it.
Wouldn't it be better if the compiler replaced our power of 2 multiplications/divisions with bit shifts and our modulus with the bitwise AND operator? Considering they actually are slightly faster.
PLM
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