What are some examples of loop unrolling in C?

What are some examples of loop unrolling in C?

Loop unrolling is an optimization technique that reduces the overhead of loop control. For example, consider this simple loop:

for (int i = 0; i < 8; i++) {
    array[i] = array[i] * 2;
}

This can be unrolled to:

for (int i = 0; i < 8; i += 2) {
    array[i] = array[i] * 2;
    array[i + 1] = array[i + 1] * 2;
}

This reduces the number of iterations and can improve performance.

What are the benefits of loop unrolling?

The benefits of loop unrolling include reduced loop overhead, improved cache performance, and better instruction-level parallelism. By decreasing the number of iterations, the CPU can execute more instructions in parallel, leading to faster execution.

Are there any downsides to loop unrolling?

Yes, there are downsides. Loop unrolling can increase code size, which may lead to cache misses. It can also make the code less readable and harder to maintain. Additionally, if the loop body is small, the performance gain may not justify the increased complexity.

Can you show me a more complex example of loop unrolling?

Certainly! Here's a more complex example:

for (int i = 0; i < N; i++) {
    array[i] = array[i] + 1;
}

Unrolled:

for (int i = 0; i < N; i += 4) {
    array[i] = array[i] + 1;
    array[i + 1] = array[i + 1] + 1;
    array[i + 2] = array[i + 2] + 1;
    array[i + 3] = array[i + 3] + 1;
}

This example processes four elements per iteration, significantly reducing loop overhead.