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Thread: Loop unrolling.

  1. #1
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    Default Loop unrolling.

    I learned about loop unrolling a long time ago and got the sudden inspiration this morning to try it in action. So i whipped together this little program.

    String s=""; 
    for (int i = 0; i < 10000 ; i++){
      s = s + i;
    }

    I compared the runtime to this guy here.

    for (int i = 0; i < 10000 ; i = i + 10){
      s = s + i;
      s = s + (i+1);
      s = s + (i+2);
      s = s + (i+3);
      s = s + (i+4);
      s = s + (i+5);
      s = s + (i+6);
      s = s + (i+7);
      s = s + (i+8);
      s = s + (i+9);
    }

    I consistently got a longer run time on the second loop. Is this because the compiler simply did a better job of unrolling the loop than I did or I am missing something much bigger?



    Thanks HelloWorld for fixing code blocks. Was just reading about that =)
    Last edited by PenguinLord; July 18th, 2010 at 02:07 AM.


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Loop unrolling.

    In a short answer, yes (not sure if it's unrolling the loop, all that matters is it optimizes quite well). A lot of the "old optimization tricks" aren't really necessary when using Java (note: this does not include all, there are some tricks that can have an impact)

  3. The Following User Says Thank You to helloworld922 For This Useful Post:

    PenguinLord (July 18th, 2010)

  4. #3
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    Default Re: Loop unrolling.

    Thanks for the reply. I had a hunch the compiler was just smarter than I was but I wasn't sure. It has been a while since I programmed much.

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