llvm dev - May 2011 - [LLVMdev] Loop-Unroll optimization

I am trying loop-unroll optimization but it is not affecting the bit code.
When I looked the loop unroll code it seems to be expecting three
parameter(UnrollThreshold, UnrollCount, UnrollAllowPartial) from the user.
Do I need to pass these parameter to activate loop unroll?

*The loop that I am trying it on is:*
 for(i=0; i< 1000; i++)
    {
      c[i] = a[i] + b[i];
    }
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