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Date: Wed, 27 Apr 2011 09:23:08 -0700
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: Java left shift and right shift operators.
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On 4/26/2011 6:47 AM, Lew wrote:
> On 04/26/2011 07:34 AM, Eric Sosman wrote:
>> On 4/26/2011 5:04 AM, Sanny wrote:
>>>> Is there any way to avoid the if condition and do right shift and left
>>>> shift using operators depending on shiftby is +ve or -ve?
>>>
>>> I used below technique but it gives error.
>>>
>>> divider=Math.pow(2,shiftby);
>>> then
>>> output = (long) N / divider;
>>
>> Let me get this straight: You're worried about the inefficiency
>> of an `if' that chooses between a left or a right shift, so you plan
>> to eliminate the `if' by evaluating an exponential function?
>>
>> ARE YOU OUT OF YOUR MIND?
>
> Especially given that shift is blazingly efficient on just about any
> computer that supports Java. You're optimizing something that's used
> *as* an optimization, Sanny-Boy.

As far as I can tell, Lew-Boy, he has not asked to optimize out the
shift, except as a means to the end of avoiding a conditional branch.
Whether that is an issue or not depends on the context, the
predictability of the branch, and the performance of the alternatives.

[I had not previously seen appending "-Boy" to the name as a convention
for addressing people in USENET articles, but I'll go along with
whatever mode of address the person I'm responding to seems to prefer.]

Patricia