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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: Some questions about FormatableBitSet
Date Tue, 12 Dec 2006 18:13:42 GMT
Given the test run I would be ok with a class that is obviously
in the business of manipulating bits to do "tricks" with shift.
Please when changing the code increase the comments around the
shifts and maybe include your reasoning (ie. perf results) so
that someone else doesn't change it because they can't understand
it.

For me I try to draw the line at where it is used and did a profiler
show that changing it makes a measurable difference to the user.
So in a class
that is trying to hide bit math from outside I think it is ok, but
in a class that mostly is not doing anything with bits I lean toward
the obvious math.

I did expect the compiler to do the obvious transformation, I know
some good C compilers would have done it.

Olav Sandstaa wrote:
> Daniel John Debrunner wrote:
> 
>> Dyre.Tjeldvoll@Sun.COM wrote:
>>
>>> - A minor nit: This class appears to do a lot of division and
>>>   remainder calculations with powers of 2 (frequently 8). Is it not
>>>   generally preferably to do this with shifts and  bitwise ANDs?
>>
>>
>> I prefer the approach that leads to code that is easy to understand.
>> If the purpose of the code is to divide by two I prefer clear code like:
>>
>>   a = b / 2;
>>
>> to
>>
>>   a = b >> 1;
>>
>> Good compilers typically compile multiplication down to shift 
>> instructions, why have the human do it?
> 
> 
> I too prefer code that is easy to read. Still, you are more likely to
> get the compiler to produce more efficient code when you use language
> constructs that you know are easy to map to efficient instructions on
> the underlying hardware.
> 
> As a small experiment I took the two code pieces above and ran them in
> a tight loop to see if the compiler was able to optimize these to the
> same set of machine instructions. The test code looked like this:
> 
>   static int divtest(int a)
>   {
>      int i;
>      for (i=0; i<1000000; ++i) {
>          a = a / 2;
>          /*a = a >> 1;*/
>      }
>      return a;
>   }
> 
> Running this code compiled with two C-compilers and three Java
> compilers/VMs gave the following results:
> 
>                       a = a / 2      a = a >> 1
> GCC                      1.94           0.77
> Sun Studio 11            1.54           0.51
> Sun JDK 5                1.55           0.38
> Sun JDK 6                1.55           0.38
> IBM SDK 5                1.26           0.38
> 
> 
> All numbers are in milliseconds and is the amount if time it took to
> run the above function once (in the test I measured amount if time it
> took to run the function 10.000 times).
> 
> Basically, with these compilers and this example code doing the
> division by two is about 3-4 times as costly as using a shift
> operator. (And since these measurments also account for the code for
> the for-loop, the real difference between the division and the shift
> operation is probably closer to 10x).
> 
> So in this example none of the compilers were able to map the division
> by two to as efficient code as produces by the >> 1 operation. Why?
> Because the compiler did not know what I knew: The number to do the
> operation on was always a positive integer.
> 
> And given that this is code that Dyre refers to is an implementation
> for an operations on bit sets I do not see it as a problem that the
> code is implemented by utilizing "bit manipulation operations",
> particularly if the optimizations are well commented.
> 
> Olav
> 
> 
> 


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