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.

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