How to trick the compiler to unroll loops for you without manually unrolling the loop.

loop_unrolling.F90

! Modern compilers with -O3 usually unroll loops when the start and stop bounds of the loop are known
! at compile time. Here is an example where I use a new secondary loop with fixed bounds to unroll by 
! the amount specified in the parameter nunroll. This allows the routine to be general with only a change
! to nunroll (and a recompile) to unroll by a different amount.

program loop_unrolling
    implicit none
    integer :: i, ii, iend, istart
    integer, parameter :: nunroll=2
    
    iend = 11
    
    print *, "Hello World!"
    
    do istart = 1, iend, nunroll
        print *, "istart = ", istart    
        do ii = 1, nunroll  ! compilers usually unroll loops with compile time known bounds.    
            i = istart + ii-1
            if(i .le. iend) then
              print *, i
            endif
        enddo
    enddo
    
    
end program loop_unrolling