access off end of array

Probably the real reason to have this in a benchmark is because it's stupid. Unless your compiler does particularly good flow control analysis, it'll generate a read of d[16], which is a likely cache miss (if the compiler aligns the array, there's a good chance that d[16] will be in a different cache line from d[15] and anything else that's hot). If the compiler can figure out that dd isn't used outside the loop, and that it can therefore be set after the test instead of before, you'll get code that runs faster than if the compiler is less clever. Of course, if you wanted to get a fast result, you'd just write it the obvious way and get the optimal result on any compiler, but they want to have some compilers do better than other compilers.

It's like writing an exam question: it would be easy to write a question that everybody would get right, but you want to write a question that people who know the material will get right more often than people who don't. Obviously, in ordinary life, you want to ask questions which will be more likely to get correct answers, and you want to write code that all compilers will make as fast as possible, but that's not the situation here.