30-Day .NET Challenge - Day 26: Loop Unrolling

The article demonstrates the advantages of loop unrolling by iterating over multiple items per cycle while adjusting the loop counter accordingly.

Learning Objectives

The problem with iterating using traditional loops
Advantages with loop unrolling

Prerequisites for Developers

Basic understanding of C# programming language.
Familiar with for loops

Getting Started

The problem with iterating using traditional loops

Most developers will iterate over array elements and perform a mathematical operation as shown in the following code snippet.

    for (int i = 0; i < array.Length; i++)
    {
        array[i] = i * 2;
    }

Although the above code snippet is quite clean and straightforward, it incurs a cost due to the loop control structure.

Advantages with loop unrolling

Please find below the refactored version of the previous code snippet

    int len = array.Length;
    for (int i = 0; i < len; i += 4)
    {
        array[i] = i * 2;
        if (i + 1 < len) array[i + 1] = (i + 1) * 2;
        if (i + 2 < len) array[i + 2] = (i + 2) * 2;
        if (i + 3 < len) array[i + 3] = (i + 3) * 2;
    }

With refactored code, now we have reduced the loop frequency to 4 which leads to faster execution.

In addition to that, it will:

Reduce loop overheads which in turn decreases any conditional checks inside the loops.
Increase performance as it enhances the execution speed

Complete Code

Create another class named LoopUnrolling and add the following code snippet

    
    public static class LoopUnrolling
    {
        public static void BadWay() {
            const int size = 1024;
            int[] numbers = new int[size];
    
            // Traditional loop
            var watch = System.Diagnostics.Stopwatch.StartNew();
            for (int i = 0; i < numbers.Length; i++)
            {
                numbers[i] = i * 2;
            }
            watch.Stop();
            Console.WriteLine($"Traditional loop time: {watch.ElapsedTicks} ticks");
    
        }
        public static void GoodWay()
        {
            const int size = 1024;
            int[] numbers = new int[size];
            int len = numbers.Length;
            var watch = System.Diagnostics.Stopwatch.StartNew();
            for (int i = 0; i < len; i += 4)
            {
                numbers[i] = i * 2;
                if (i + 1 < len) numbers[i + 1] = (i + 1) * 2;
                if (i + 2 < len) numbers[i + 2] = (i + 2) * 2;
                if (i + 3 < len) numbers[i + 3] = (i + 3) * 2;
            }
            watch.Stop();
            Console.WriteLine($"Unrolled loop time: {watch.ElapsedTicks} ticks");
        }
    }

Execute from the main method as follows

    #region Day 26: Loop Unrolling
    static string ExecuteDay26()
    {
        LoopUnrolling.BadWay();
        LoopUnrolling.GoodWay();
    
        return "Executed Day 26 successfully..!!";
    }
    
    #endregion

Console Output

    Traditional loop time: 20 ticks
    Unrolled loop time: 12 ticks

As the output demonstrates loop unrolling took much less time than the traditional loop approach.

Complete Code here: GitHub — ssukhpinder/30DayChallenge.Net

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