The safest way to gain access to SFRs in assembly code is to have symbols defined in your assembly code that equate to the corresponding SFR address. Header files are provided with the compiler so that you do not need to define these yourselves (detailed in Accessing Registers From Assembly Code).
There is no guarantee that you will be able to access symbols generated by the compilation of C code, even the code that accesses the SFR that you require.