Mill Computing, Inc. Forums The Mill Architecture Grab bag of questions Reply To: Grab bag of questions

Ivan Godard
Keymaster
Post count: 656

– 18) Any clever ideas you’ve had lately that didn’t pan out?
Lots 🙁

An example in my own code: the specializer must turn the program into something that can execute on the target member, and the generated code must not exceed the physical limitations of the target. Example limitations include the number of simultaneously live objects (limited by the belt size), the number of concurrent active loads (limited by the number of retire stations), and others. Straightforward scheduling of pending instructions (an NP-complete bin-packing problem) can produce a binary that exceeds these limits. The initial specializer implementation did a schedule and then examined the resulting code for physicality violations. If it found some, it modified the program by inserting suitable spill/fill and rescue operations and rescheduled. This iterated, called annealing, until a valid schedule was produced.

Unfortunately we found examples where the annealing did not converge, or converged only after an impractical number of iterations or inserted operation counts. After a great deal of work, the scheduling part of the specializer was abandoned and rewritten (the predictive scheduler) to anticipate situations in which resource exhaustion was possible and avoid them. The problem is that the predictive scheduler will avoid problems that might not actually happen in the actual schedule, producing poorer code than annealing does when annealing works. Someday someone will get a sheepskin for coming up with a better scheduler, but for now we live with it.