5. Loop may or may not hold enough work to be profitably parallelized

The compiler was not able to determine at compile time whether this loop held enough work to warrant parallelizing. Often loops that are labeled with this hint may also be labeled "parallelized," meaning that the compiler generated two versions of the loop (see Hint 2), and that it will be decided at runtime whether the parallel version or the serial version should be used.

Since all the compiler hints, including the flag that indicates whether or not a loop is parallelized, are generated at compile time, there's no way to be certain that a loop labeled "parallelized" actually executes in parallel.