Efficient Backtracking Instruction Schedulers

Current schedulers for acyclic regions schedule operations in dependence order and never undo a scheduling decision. In contrast, backtracking schedulers may unschedule operations and can often generate better schedules. In this paper, we describe a conventional cycle scheduler followed by two novel backtracking schedulers, OperBT and ListBT. The full-backtracking OperBT scheduler enables backtracking for all operations and unschedules operations to make space for the current operation. The OperBT scheduler increases the percentage of super-blocks scheduled optimally over a conventional non-backtracking scheduler from an average of 66.9% to 81.4%, an increase of 21.7%. The selective backtracking ListBT scheduler enables backtracking only when scheduling operations for which backtracking is likely to be advantageous. This hybrid scheduler is almost as good as the OperBT backtracking scheduler in terms of generated code quality, but backtracks about four times less often.