javaoptimizationjitloop-unrolling

Java JIT loop unrolling policy?

What is the loop unrolling policy for JIT? Or if there is no simple answer to that, then is there some way i can check where/when loop unrolling is being performed in a loop? 

GNode child = null;
for(int i=0;i<8;i++){
   child = octree.getNeighbor(nn, i, MethodFlag.NONE);
   if(child==null)
      break;
   RecurseForce(leaf, child, dsq, epssq);
}

Basically, i have a piece of code above that has a static number of iterations (eight), and it does bad when i leave the for loop as it is. But when i manually unroll the loop, it does significantly better. I am interested in finding out if the JIT actually does unroll the loop, and if not, then why.

Solution

  • If the JVM unrolls the loop is probably best answered by actually printing the generated assembly. Note that this requires your code to actually be executed as a hot spot (i.e. the JVM considers it worthy of the expensive optimizations).

    Why the JVM decides one way or another is a much harder question and probably requires in-depth analysis of the JIT code.