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; } |
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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 |
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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 |
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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 |
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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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