My interview aims to assess whether the candidate understands that the dependency of each iteration on the previous one prevents effective utilization of a superscalar processor, knows the ways to overcome that, and whether the compiler is able to optimize that automatically, and if so when it absolutely cannot and why.

I generally focus more on sum of arbitrary data, but I used to also ask about a formulaic sum (linear to constant time) as an example of something a compiler is unlikely to do.

My thinking is that I expect good engineers to be able to do those optimizations themselves rather than rely on compilers.