Diagnostic 15541: Loop was not vectorized: outer loop was not auto-vectorized: consider using SIMD directive.

Product Version: Intel(R) Visual Fortran Compiler XE 15.0.0.070

Cause:

The vectorization report generated when using Visual Fortran Compiler's optimization options ( -O2 -assume:dummy_aliases -Qopt-report:2 ) states that loop was not vectorized due vector dependence, outer loop depends on inner loop.

Example:

An example below will generate the following remark in optimization report:

subroutine foo(a, n1, n)
        implicit none
        integer, intent(in) :: n, n1
        real, intent(inout) :: a(n,n1)
        integer :: i, j
        do i=1,n
            do j=1,n
                a(j,i) = a(j-1,i)+1
            end do
        end do
end subroutine foo

Report from: Loop nest, Vector & Auto-parallelization optimizations [loop, vec, par]

  LOOP BEGIN  
      remark #15541: outer loop was not auto-vectorized: consider using SIMD directive.
 
  LOOP END

Resolution:

Using !DIR$ SIMD directive results in outer loop being vectorized, as shown:

subroutine foo(a, n1, n)
        implicit none
        integer, intent(in) :: n, n1
        real, intent(inout) :: a(n,n1)
        integer :: i, j

         !DIR$ SIMD
        do i=1,n
            do j=1,n
                a(j,i) = a(j-1,i)+1
            end do
        end do

end subroutine foo

LOOP BEGIN  
      remark #15301: SIMD LOOP WAS VECTORIZED.

LOOP END

See also:

Requirements for Vectorizable Loops

Vectorization Essentials

Vectorization and Optimization Reports

Back to the list of vectorization diagnostics for Intel Fortran