Sure, why wouldn't it be? As far as a cache is concerned, I don't think reverse sequential iteration would be any different than forward sequential. The actual RAM accesses may be less optimal if there's some speculative pre-fetching with assumed forward sequential access, but that's conjecture.

With some exceptions, hardware prefetch works in terms of ascending accesses. To learn if a particular CPU will prefetch for descending access, benchmarking is essential. Best to use soft prefetch calls if performance is critical.

i would suspect that the cache prefetch/prediction could use the "velocity" of the memory access to predict the next access; so if the access pattern was going backwards, the "velocity" would be negative, but prefetching would still work if they just followed the predicted pattern.

It is not, unless your compiler is smart enough to recognize reverse iteration and prefetch appropriately.

If performance matters, you should experiment with __builtin_prefetch, which is available in clang and GCC.