Loop optimization
- C course
- Advanced
- Lesson 75/83
- 5 min
Learn loop optimization and unrolling.
Loop optimization
Loop unrolling processes multiple elements per iteration to reduce the loop count. This reduces loop overhead (condition check, increment) and increases instruction-level parallelism.
A 4-way unroll example: for (i = 0; i + 3 < n; i += 4) { sum += a[i] + a[i+1] + a[i+2] + a[i+3]; }. Each iteration handles 4 elements; the loop count is divided by 4. A second loop handles the remaining elements. Modern compilers do this automatically; writing it by hand at advanced level gives further gains.
Remaining elements
4-way unroll misses some elements when the array size is not a multiple of 4. So you add a remainder loop for (; i < n; i++) sum += a[i];.
Example: the 8-element array {1,2,3,4,5,6,7,8} is handled in two unrolled iterations (i=0, i=4) with no remainder; the sum is 36. With 10 elements, 8 would go through the unroll and 2 through the remainder. This pattern is required for both performance and correctness.
Practice 4-way unrolled sum
Write int sum_unrolled(int *a, int n): int sum = 0; int i;, loop for (i = 0; i + 3 < n; i += 4) { sum += a[i] + a[i+1] + a[i+2] + a[i+3]; }, remainder for (; i < n; i++) sum += a[i];, return sum;. In main declare int a[] = {1,2,3,4,5,6,7,8}; and print printf("toplam: %d\n", sum_unrolled(a, 8));.
Starter code
#include <stdio.h>
// Write sum_unrolled: 4-way unroll + remainder loop
// main: declare 8-element array, call sum_unrolled and print
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
int sum_unrolled(int *a, int n) {
int sum = 0;
int i;
for (i = 0; i + 3 < n; i += 4) {
sum += a[i] + a[i+1] + a[i+2] + a[i+3];
}
for (; i < n; i++) sum += a[i];
return sum;
}
int main(void) {
int a[] = {1,2,3,4,5,6,7,8};
printf("toplam: %d\n", sum_unrolled(a, 8));
return 0;
}Expected output
toplam: 36
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 4-way unroll step
for (i = 0; i + 3 < n; i += ____) { sum += a[i] + a[i+1] + a[i+2] + a[i+3]; }for (i = 0; i + 3 < n; i += 4) { sum += a[i] + a[i+1] + a[i+2] + a[i+3]; }Explanation: 4-way unroll uses i += 4 to divide the loop by 4.
2 Match loop concepts
| Term | Meaning |
|---|---|
unroll |
Multiple elements |
i += 4 |
4-way step |
remainder loop |
Leftover elements |
condition check |
Loop overhead |
3 What does loop unrolling reduce?
- Memory usage
- File size
- Loop overhead Correct answer
- Line count
Explanation: Unrolling reduces loop overhead (condition, increment).