Product Version: Intel® Fortran Compiler 15.0 and above
The vectorization report generated using Visual Fortran Compiler's optimization and vectorization report options -O2 -Qvec-report2 -Qopt-report:2 states that loop was not vectorized: vectorization possible but seems inefficient.
An example below will generate the following remark in optimization report:
subroutine foo(a, b, n) implicit none integer, intent(in) :: n real, intent(inout) :: a(n) real, intent(out) :: b real :: x = 0 integer :: i do i=1,n x = x + a(2*i) end do b = x end subroutine foo
ifort -c /O2 /Qopt-report:2 /Qopt-report-phase:vec /Qopt-report-file:stdout f15335.f90
Report from: Vector optimizations [vec]
LOOP BEGIN at f15335.f90(9,5)
remark #15335: loop was not vectorized: vectorization possible but seems inefficient. Use vector always directive or /Qvec-threshold0 to override
Using !DEC$ VECTOR ALWAYS directive in the code will vectorize the loop by overriding efficiency heuristics of the vectorizer.
VECTOR and NOVECTOR