Diagnostic 15331: xxxx was not vectorized: precise FP model implied by the command line or a directive prevents vectorization. Consider using fast FP model

Cause:

This vectorization diagnostic is emitted starting from 15.0 compiler. Re-association on floating point computation can alter the resulting value. Under precise floating point model, such reassociation is disallowed. Vector (or parallel) reduction, by nature, involves re-association. In this example, the for loop does a reduction operation. The compiler doesn't vectorize this loop when -fp-model precise is used since vectorization will change the order of operation. 

Examples:

double foo(double *a, int N){
  double x = 0;
  int i;
  for (i=0;i<N;i++){
    x += a[i];
  }
  return x;
}