# GAP Message (Diagnostic ID 30513)

## Message

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

## Advice

Add "#pragma ivdep" before the specified loop. This pragma enables the vectorization of the loop at the specified line by ignoring some of the assumed cross-iteration data dependencies.

## Example

Consider the following:

    void foo(float *a, int x, int n) {
      int i;
      for (i=0; i<n; i++) {
        a[i] = a[i+x]+1;
      }
      return;
    }

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

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

    void foo(float *a, int x, int n) {
      int i;
    #pragma ivdep
      for (i=0; i<n; i++) {
        a[i] = a[i+x]+1;
      }
      return;
    }

## 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 read or a write statement 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