4.5. Signed Types for Iteration Counters and Limits

In order for automatic vectorization to occur, the iteration counters and iteration limits for loops should have signed types. In other words, use int rather than unsigned int.

The C language standard defines the behavior for unsigned arithmetic overflow, but not for signed arithmetic overflow.

In the unsigned case, an overflowing value will "wrap-around". Therefore, the compiler must assume (in certain cases) that the loop counter may loop around and thus cannot make certain necessary conclusions about the behavior of the loop.

In the signed type case, the compiler can assume the iteration counter will not overflow, because that has undefined behavior according to the C-standard. Thus, the compiler can make certain conclusions about the behavior of the loop and from there may be able to vectorize the loop.