How to Read a General Purpose Register (in C)

The data sheet for my LM3S1968 says that core registers, such as the general purpose registers, are not memory mapped and are accessed by register name. In this case, I want to read General Purpose Register 5, and the data sheet says that the name of that register is "R5." When I write the name R5 in my code, the compiler does not recognize it. I'm including the header file for the board, but it is no help.

Will somebody please be nice enough to write a code example that shows how to read General Purpose Register 5 by using its name (not a mapped address) using C?