This is not applicable to most programming scenarios since the cache gets trashed unpredictably during context switches (including the user-level task switches involved in cooperative async patterns). It's not a true scratchpad storage, and turning it into one would slow down context switches a lot since the scratchpad would be processor state. Maybe this can be revisited once even low-end computers have so many hardware cores/threads that context switches become so rare that the overhead is not a big deal. But we are very far from anything of the sort.