https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63521
--- Comment #2 from Richard Earnshaw <rearnsha at gcc dot gnu.org> --- Ideally, a port should not need to define reg_alloc_order; it's rather a blunt instrument. Better would be for the register allocator to have a better understanding of which registers are being used for parameter passing in the current routine so that it can pick from the remaining call-clobbered list.
