[gnufx]

Yes, as always, it depends. The typical problem with array operations, e.g. in Fortran, is how well they're "scalarized", and the possible cost of not doing copy elimination over subroutine calls, for instance.

Presumably the exact semantics of a for-type loop depends on the language, but surely the C standard doesn't prevent auto-parallelization (including with SIMD vectorization, modulo arithmetic rules). Loops might also have OpenMP or other annotation.