Time taken by Loop unrolling vs Normal loop

We have discussed loop unrolling. The idea is to increase performance by grouping loop statements so that there are less number of loop control instruction and loop test instructions

C++

// CPP program to compare normal loops and
// loops with unrolling technique
#include <iostream>
#include <time.h>
using namespace std;

int main() {

  // n is 8 lakhs
  int n = 800000;

  // t to note start time
  clock_t t = clock();

  // to store the sum
  long long int sum = 0;

  // Normal loop
  for (int i = 1; i <= n; i++)
    sum += i;

  // to mark end time
  t = clock() - t;

  cout << "sum is: " << sum << endl;
  cout << "time taken by normal loops:"
      "(float)t / CLOCKS_PER_SEC << " seconds"
       << endl;

  // to mark start time of unrolling
  t = clock();

  // Unrolling technique (assuming that
  // n is a multiple of 8).
  sum = 0;
  for (int i = 1; i <= n; i += 8) {
    sum += i sum += (i + 1);
    sum += (i + 2);
    sum += (i + 3);
    sum += (i + 4);
    sum += (i + 5);
    sum += (i + 6);
    sum += (i + 7);
  }

  // to mark the end of loop
  t = clock() - t;

  cout << "Sum is: " << sum << endl;
  cout << "Time taken by unrolling: "
       "(float)t / CLOCKS_PER_SEC << " seconds";
  return 0;
}

Java

// Java program to compare
// normal loops and loops
// with unrolling technique

class GFG
{

    public static void main(String[] args)
    {

    // n is 8 lakhs
    int n = 800000;

    // t to note start time
    double t = (double)System.nanoTime();

    // to store the sum
    long sum = 0;

    // Normal loop
    for (int i = 1; i <= n; i++)
        sum += i;

    // to mark end time
    t = (double)System.nanoTime() - t;

    System.out.println("sum is: "+
        Double.toString(sum));
    System.out.println("time taken by normal loops:" +
        Double.toString(t / Math.pow(10.0, 9.0)));

    // to mark start time
    // of unrolling
    t = (double)System.nanoTime();

    // Unrolling technique
    // (assuming that n is
    // a multiple of 8).
    sum = 0;
    for (int i = 1; i <= n; i += 8)
    {
        sum += i ;
        sum += (i + 1);
        sum += (i + 2);
        sum += (i + 3);
        sum += (i + 4);
        sum += (i + 5);
        sum += (i + 6);
        sum += (i + 7);
    }

    // to mark the end of loop
    t = (double)System.nanoTime() - t;

    System.out.println("sum is: " +
        Double.toString(sum));
    System.out.println("time taken by normal loops:" +
        Double.toString(t / Math.pow(10.0, 9.0)));
    }
}

// This code is contributed
// by Harshit Saini

Python3

# Python program to compare
# normal loops and loops
# with unrolling technique
from timeit import default_timer as clock

if __name__ == "__main__":

    # n is 8 lakhs
    n = 800000;

    #t to note start time
    t = clock()

    # to store the sum
    sum = 0

    # Normal loop
    for i in range(1, n + 1):
        sum += i

    # to mark end time
    t = clock() - t

    print("sum is: " + str(sum))
    print("time taken by normal " +
                 "loops:" + str(t))

    # to mark start
    # time of unrolling
    t = clock()

    # Unrolling technique
    # (assuming that n is
    # a multiple of 8).
    sum = 0
    for i in range(1, n + 1, 8):
        sum += i
        sum += (i + 1)
        sum += (i + 2)
        sum += (i + 3)
        sum += (i + 4)
        sum += (i + 5)
        sum += (i + 6)
        sum += (i + 7)

    # to mark the
    # end of loop
    t = clock() - t

    print("Sum is: " + str(sum))
    print("Time taken by unrolling: " +
                              str(t))

# This code is contributed
# by Harshit Saini

C#

// C# program to compare
// normal loops and loops
// with unrolling technique
using System;
using System.Diagnostics;

class GFG
{
    private static double nanoTime()
    {
        long nano = 10000L * Stopwatch.GetTimestamp();
        nano /= TimeSpan.TicksPerMillisecond;
        nano *= 100L;
        return nano;
    }
    public static void Main(String[] args)
    {

        // n is 8 lakhs
        int n = 800000;

        // t to note start time
        double t = nanoTime();

        // to store the sum
        long sum = 0;

        // Normal loop
        for (int i = 1; i <= n; i++)
            sum += i;

        // to mark end time
        t = nanoTime() - t;

        Console.WriteLine("sum is: "+ sum);
        Console.WriteLine("time taken by normal loops:" +
                              (t / Math.Pow(10.0, 9.0)));

        // to mark start time
        // of unrolling
        t = nanoTime();

        // Unrolling technique
        // (assuming that n is
        // a multiple of 8).
        sum = 0;
        for (int i = 1; i <= n; i += 8)
        {
            sum += i ;
            sum += (i + 1);
            sum += (i + 2);
            sum += (i + 3);
            sum += (i + 4);
            sum += (i + 5);
            sum += (i + 6);
            sum += (i + 7);
        }

        // to mark the end of loop
        t = nanoTime() - t;

        Console.WriteLine("sum is: " + (sum));
        Console.WriteLine("time taken by normal loops:" +
                              (t / Math.Pow(10.0, 9.0)));
    }
}

// This code is contributed by 29AjayKumar

Javascript

<script>
// javascript program to compare
// normal loops and loops
// with unrolling technique

        // n is 8 lakhs
        var n = 800000;

        // t to note start time
        var t = new Date();

        // to store the sum
        var sum = 0;

        // Normal loop
        for (let i = 1; i <= n; i++)
            sum += i;

        // to mark end time
        t =  new Date() - t;

        document.write("sum is: " +(sum));
        document.write("<br/>time taken by normal loops:" +
        (t / Math.pow(10.0, 9.0)).toFixed(10));

        // to mark start time
        // of unrolling
        t = new Date();

        // Unrolling technique
        // (assuming that n is
        // a multiple of 8).
        sum = 0;
        for (i = 1; i <= n; i += 8) {
            sum += i;
            sum += (i + 1);
            sum += (i + 2);
            sum += (i + 3);
            sum += (i + 4);
            sum += (i + 5);
            sum += (i + 6);
            sum += (i + 7);
        }

        // to mark the end of loop
        t = new Date() - t;

        document.write("<br/>sum is: " + (sum));
        document.write("<br/>time taken by normal loops:" + (t / Math.pow(10.0, 9.0)).toFixed(10));

// This code is contributed by Rajput-Ji
</script>

Output:

Sum is: 320000400000
Time taken: 0.002645 seconds

Sum is: 320000400000
Time taken: 0.001799 seconds

Please refer loop unrolling for comparison of normal loops and loop unrolling.