Balance scheduling: Weighting branch tradeoffs in superblocks

Since there is generally insufficient instruction level parallelism within a single basic block higher performance is achieved by speculatively scheduling operations in superblocks. This is difficult in general because each branch competes for the processor's limited resources. Previous work manages the performance tradeoffs that exist between branches only indirectly. We show here that dependence and resource constraints can be wed to gather explicit knowledge about scheduling tradeoffs between branches. The first contribution of this paper is a set of new, tighter lower bounds on the execution times of superblocks that specifically accounts for the dependence and resource conflicts between pairs of branches. The second contribution of this paper is a novel superblock scheduling heuristic that finds high performance schedules by determining the operations that each branch needs to be scheduled early and selecting branches with compatible needs that favor beneficial branch tradeoffs. Performance evaluations for superblocks from SPECint95 indicate that our bounds are very tight and that our scheduling heuristic outperforms well known superblock scheduling algorithms.