Compute the division via shift instruction

I write the code as following to evaluate the expression n = n / 2

asrs r0, r0, #1

But, I found the GCC will translate the expression n = n / 2 into the following instruction

lsrs r1, r0, #31                                                                             

adds r0, r1, r0

asrs r0, r0, #1

Why does it need to add the sign bit?