Did you understand the comment? The author used GCC, and GCC is only able to vectorize the loop. But clang on the other hand, essentially turned this O(n) algorithm to calculate a particular sum into an O(1) result.

> the reason the optimizer does what you see is undefined behavior due to signed integer overflow

Yes undefined behavior gives the optimizer the right in this case to transform the code into anything, including a nonsense answer, or a trap instruction. But the optimizer did not; it produced the right answer under 2's complement arithmetic.