Function opnd_get_reg_used 

pub unsafe extern "C" fn opnd_get_reg_used(
    opnd: opnd_t,
    index: c_int,
) -> reg_id_t

Used in conjunction with opnd_num_regs_used(), this routine can be used to iterate through all registers used by \p opnd. The index values begin with 0 and proceed through opnd_num_regs_used(opnd)-1.