Question

Using the C Programming language, write a program that sums an array of 50 elements. Next, optimize the code using loop unrolling. Loop unrolling is a program transformation that reduces the number of iterations for a loop by increasing the number of elements computed on each iteration. Generate a graph of performance improvement. Tip: Figure 5.17 in the textbook provides an example of a graph depicting performance improvements associated with loop unrolling.

Marking:-

Optimize the code for an array of 50 elements using loop unrolling. Prepare a graph that show the performance improvements. (30)

Answer 1

Loop unrolling is a loop transformation technique that helps to optimize the execution time of a program. We basically remove or reduce iterations. Loop unrolling increases the program’s speed by eliminating loop control instruction and loop test instructions.

c program without loop unrollong:

#include<stdio.h> #include<stdlib.h> int main() { int n,sum=0; n=50; int a[50]; for(int i=0;i<n;i++) { scanf("%d",&a[i]); } for(int i=0;i<n;i++) { sum=sum+a[i]; } printf("sum is %d",sum); return 0; }

now with loop unrolling:

#include<stdio.h> #include<stdlib.h> int main() { int n,sum=0; n=50; int a[50]; for(int i=0;i<n;i++) { scanf("%d",&a[i]); } sum=sum+a[0]; sum=sum+a[1]; sum=sum+a[2]; sum=sum+a[3]; sum=sum+a[4]; sum=sum+a[5]; sum=sum+a[6]; sum=sum+a[7]; sum=sum+a[8]; sum=sum+a[9]; sum=sum+a[10]; sum=sum+a[11]; sum=sum+a[12]; sum=sum+a[13]; sum=sum+a[14]; sum=sum+a[15]; sum=sum+a[16]; sum=sum+a[17]; sum=sum+a[18]; sum=sum+a[19]; sum=sum+a[20]; sum=sum+a[21]; sum=sum+a[22]; sum=sum+a[23]; sum=sum+a[24]; sum=sum+a[25]; sum=sum+a[26]; sum=sum+a[27]; sum=sum+a[28]; sum=sum+a[29]; sum=sum+a[30]; sum=sum+a[31]; sum=sum+a[32]; sum=sum+a[33]; sum=sum+a[34]; sum=sum+a[35]; sum=sum+a[36]; sum=sum+a[37]; sum=sum+a[38]; sum=sum+a[39]; sum=sum+a[40]; sum=sum+a[41]; sum=sum+a[42]; sum=sum+a[43]; sum=sum+a[44]; sum=sum+a[45]; sum=sum+a[46]; sum=sum+a[47]; sum=sum+a[48]; sum=sum+a[49]; printf("sum is %d",sum); return 0; }

Advantage of loop unrolling is that it makes the program fast. However, it makes our program bit lengthy.