What are the differences between loop unrolling and vectorization?

What are the differences between loop unrolling and vectorization?

Loop unrolling is a technique to increase a program's speed by reducing the overhead of loop control. Vectorization, on the other hand, involves converting scalar operations to vector operations, allowing multiple data points to be processed simultaneously. Both aim to optimize performance but do so in different ways.

Can you give me an example of loop unrolling?

Sure! For instance, if you have a loop that adds elements of an array, instead of iterating one element at a time, you could unroll it to process two or more elements in each iteration, like this:

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

What about an example of vectorization?

An example of vectorization can be seen in using SIMD (Single Instruction, Multiple Data) instructions. For instance, instead of adding two arrays element by element in a loop, you can use a vectorized operation:

__m128i a = _mm_loadu_si128((__m128i*)array1);
__m128i b = _mm_loadu_si128((__m128i*)array2);
__m128i result = _mm_add_epi32(a, b);

Which method is generally more effective?

It depends on the context. Loop unrolling can be more effective for small loops with a known number of iterations, while vectorization is often better for large datasets where parallel processing can significantly speed up computations.