Does this loop unrolling make sense?

I am coding a Hanning filter which is a recursive relation:

y[i] = 0.25*(x[i] + 2*x[i-1] + x[i-2])

And here is my code were I attempt to unroll 3 times:

public void HanningFilter(float[] array) {  
        int len = array.length;
        int i;
        for (i = 2; i < len-3; i+=3) {
            array[i] = 0.25f*(array[i] + 2*array[i-1] + array[i-2]);
            array[i+1] = 0.25f*(array[i+1] + 2*array[i] + array[i-1]);
            array[i+2] = 0.25f*(array[i+2] + 2*array[i+1] + array[i]);
        }
        //Clean-up at the end
        for (int j = i; j < len; j++) {
            array[i] = 0.25f*(array[i] + 2*array[i-1] + array[i-2]);
        }
    }

Would this help at all with an OoO superscalar processor? Is 3 the right amount of times to unroll? Thanks

No, rule of thumbs for this case:

• Never guess optimizations, use a profiler

• You're not smarter then the JIT compiler

Loop unrolling only makes sense if you profiled the code and found the unrolled version to be faster then the normal version.

I guess you're coming from a C background, so please don't try to bring C-habits to Java. Java is different, always go for readability first.

public void HanningFilter(float[] array) {

The Java naming conventions dictate that function names are lowerCamelCase.

Also the name array is a bad name, more appropriate would be a name like data or input.

int len = array.length;
int i;
for (i = 2; i < len-3; i+=3) {

What you're doing here is absolutely unnecessary. Java allows declarations of variables inside the for and there's no need to define a variable for the condition.

for (int idx = 2; idx < data.length; idx++) {

Naming loop variables idx or counter is not very famous, I prefer it to i.

StackExchange Code Review Q#30603, answer score: 8