# GAP Message (Diagnostic ID 30513)

### Note

This feature is only available for ifort.

## Message

Insert a "%s ivdep" statement right before the loop at line %d to vectorize the loop.

## Advice

Add "!DIR$ IVDEP" before the specified loop. This directive enables the vectorization of the loop at the specified line by ignoring some of the assumed cross-iteration data dependencies.

## Example

Consider the following:

    subroutine foo(a, m, n)
      integer m, n
    		real a(n)
      do i=1,n
        a(i) = a(i+m) + 1
      enddo
    end

In this case, the compiler is unable to vectorize the loop because m could be -1, where each iteration is dependent on the previous iteration. If m is known to be positive, you can vectorize this loop.

If you determine it is safe to do so, you can add the directive as follows:

    subroutine foo(a, m, n)
      integer m, n
    		real a(n)
    !dir$ ivdep
      do i=1,n
        a(i) = a(i+m) + 1
      enddo
    end

## Verify

Confirm that any arrays in the loop do not have unsafe cross-iteration dependencies. A cross-iteration dependency exists if a memory location is modified in an iteration of the loop and accessed by a fetch or store operation in another iteration of the loop. Make sure that there are no such dependencies, or that any cross-iteration dependencies can be safely ignored.

Parent topic: Guided Auto Parallelism Messages