Optimization of C Programming Code Loop Unrolling

Assignment Task

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.

We see that the CPE for integer addition improves, achieving the latency bound of 1.00. This result can be attributed to the benefits of reducing loop overhead operations. By reducing the number of overhead operations relative to the number of additions required to compute the vector sum, we can reach the point where the 1-cycle latency of integer addition becomes the performance- limiting factor. On the other hand, none of the other cases improve-they are already at their latency bounds. Figure 5.17 shows CPE measurements when unrolling the loop by up to a factor of 10. We see that the trends we observed for unrolling by 2 and 3 continue-none go below their latency bounds.

To understand why k x 1 unrolling cannot improve performance beyond the latency bound, let us examine the machine-level code for the inner loop of combine5, having k=2. The following code gets generated when type data_t is double, and the operation is multiplication:

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