Compiler/MSP430FR5949: _MPU_ENABLE leads to link error

Part Number:MSP430FR5949

Tool/software: TI C/C++ Compiler

firstly, my project codes compiled/linked successfully. the following link error reported when add another pieces of code into project.

undefined first referenced

symbol in file
--------- ----------------
MPUSEG <whole-program>
__mpusam <whole-program>
__mpuseg <whole-program>

it is weird that when I remove the new added code, the above error still exist.

when I remove _MPU_ENABLE definition in linker option, the above error disappear.

 So, I wonder what's the purpose of this _MPU_ENABLE define? is there any other problem if I undefine it?

Thanks,

Falong