[tiarkrompf]

(author here) Yes, one of our points is that 'register' is much too low-level, because one can't abstract over it. The same holds e.g. for OpenMP directives. In the paper we propose (among other things) to attach such directives to dynamic scopes. This makes it possible to e.g. unroll all innermost loops, including those in function calls, without putting unrolling directives on individual loops.