xl(i).eq.xl(i) what does this sentence do？

[oswardlx]

src/jni/fortran/slsqp.f90

subroutine bound(n, x, xl, xu)
integer n, i
double precision x(n), xl(n), xu(n)
do i = 1, n
! Note that xl(i) and xu(i) may be NaN to indicate no bound
if(xl(i).eq.xl(i).and.x(i) < xl(i))then
x(i) = xl(i)
else if(xu(i).eq.xu(i).and.x(i) > xu(i))then
x(i) = xu(i)
end if
end do
end subroutine bound

what does this sentence do？
xl(i).eq.xl(i)