Scratch register not for use.

There is one "pure" global scratch register which compiler doesn't use and you can not use for computation, for Blackfin architecture.
That is USP.
Yes. If your code always in the supervisor mode, nobody need to keep USP. I hesitate to use it (No, thank you) because someday the Blackfin architecture may be changed unpredictably. But "one time" program may use it conveniently...