Why a simple code, Intel Fortran -O3 -O2 gives wrong results?

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module tests
    implicit none
contains
    subroutine test01
        integer :: i
        real  :: x(0:4)
        x(0) = 1.0
        do i = 1, 4
            call rk4_ti_step_mod3 ( x(i-1), x(i) ) ! check n=10 bug.
        end do
        write (*,*)x
    end subroutine test01
    subroutine rk4_ti_step_mod3 ( x, xstar )
        real :: x, xstar
        xstar = x
        xstar = xstar + x
        !xstar = x + x
    end subroutine rk4_ti_step_mod3
end module tests

program main
    use tests
    implicit none
    call test01
end program main

I have changed the code like this one.