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NNTP-Posting-Date: Tue, 26 Apr 2011 05:16:50 -0500
Date: Tue, 26 Apr 2011 03:16:45 -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 12:55 AM, Lothar Kimmeringer wrote:
> Sanny wrote:
>
>> I have a problem where I have to do left shift and right shift.
>>
> [...]
>>
>> I have to use use left shift and right shift to shift Number right/
>> left depending on value of shiftby
>>
>> if (shift>0) output = N>>  shiftby; else output = N<<  (-shiftby);//
>> working but inefficient.
>
> In what way is it inefficient? Have you performed measurements
> or do you simply think that it's slow and want to optimize
> it prematurely?

It is very important that the measurement be done in context, using the
pattern of left and right shifts that occur in the real program. The
very worst possible case is the one that many people might use in a test
program, a random, equal probability, mix of left and right shifts. That
makes the hardware branch prediction no better than random.

Patricia