Time taken by Loop unrolling vs Normal loop

Difficulty Level : Medium
Last Updated : 16 Sep, 2019

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

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.

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