[LLVMdev] Virtual register??

[Seung Jae Lee]

On the document about writing an LLVM backend, I became to wonder what the 'virtual register' is in the following statement.

"You also need to define register classes to contain these registers, such as the integer register class and floating-point register class, so that you can allocate virtual registers to instructions from these sets, and let the target-independent register allocator automatically choose the actual architected registers. (Writing an LLVM backend @ llvm.org)"

Would you mind telling me the difference between 'virtual register' and just 'register'?
Thank you so much.