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From: Travers Naran <tnaran@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Java left shift and right shift operators.
Date: Tue, 26 Apr 2011 22:40:35 -0700
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On 26/04/2011 9:32 AM, Sanny wrote:
> I have computed and found "if statments" take 20-50 times longer than
> basic arithmetic operations. So I wanted to replace if condition by
> some arithmetics/ Maths.

This is why I think assembly language should be mandatory in schools. :-)

An if statement gets translated into something like (using a 
pseudo-assembly for readability):

;; if (b > 0) return a << b else return a >> -b
  compare b to 0
  jump-less-than to L1
  a := a << b
  jump L2
L1:
  b := -b
  a := a >> b
L2:
  return  ;; result in a

It's the jump that's adding the time.  But on an x86, that kind of 
instruction can be entirely cached and the Intel/AMD processors can 
perform predictive branching to speed up this bit of code.

The only way you should be getting 20-50 times longer is if you aren't 
using any sort of optimizing run-time.  Example: running always as byte 
code without compiling.

But usually this kind of if-checking and bit-twiddling is so fast that 
it's not worth optimizing beyond a single statement.  What are you 
coding that makes this the most expensive calculation in your program?


OFF-TOPIC
=========

Bonus Question:
If branching is "20-50 times longer" than any of the operations 
(negation or shifting), how could you re-write this assembly to use a 
single branch statement?

Extra Bonus:
Using your favorite processor instruction set, come up with a fast 
implementation of: if (b > 0) return a << b; else return a >> -b;