Thread: 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 =)

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)