Diagnostic 15335: xxxx was not vectorized: vectorization possible but seems inefficient. Use vector always directive or -vec-threshold0 to override

Cause:

This vectorization diagnostic is emitted starting from Intel(R) C++ Compiler 15.0. By default the compiler heuristics decides the vectorization potential of each loop and only vectorizes when the vectorization threshold value is 100%. If the vectorization threshold lesser than 100%, it considers the vectorization as inefficient. This diagnostic message is emitted when the above condition is encountered. 

Examples:

void foo(int *b, int N){
  int i;
  for (i=0; i<5; i++) {
    b[2*i] = b[2*i] + 1;
  }
}