Minimum for loop length for auto vectorization?

  auto-vectorization, c++, clang, llvm, vectorization

I am wondering what should be the minimum for loop length for the auto vectorizer to enable the vectorization. ?

for eg:

       for (int i= 0 ; i< 4 ; i+= 3)
        // Some operations

using Rpass flag, I am getting following remark.

loop control flow is not understood by vectorizer [-Rpass-analysis=loop-vectorize]

can someone help me understand what are the basic requirement for a for loop to get vectorized. ?

Source: Windows Questions C++