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Meld Scheduling: A Technique for Relaxing Scheduling Constraints

Abraham, Santosh G.; Kathail, Vinod; Deitrich, Brian L.

HPL-97-39

Keyword(s):instruction scheduling; global scheduling; meld scheduling; latency constraint propagation; instruction-level parallel processors; compiler optimization

Abstract: Meld scheduling melds the schedules of neighboring scheduling regions to respect latencies of operations issued in one region but completing after control transfers to the other. In contrast, conventional schedulers ignore latency constraints from other regions leading to potentially avoidable stalls in an interlocked (superscalar) machine or incorrect schedules for non-interlocked (VLIW) machines. Alternatively, schedulers that conservatively require all operations to complete before the branch takes effect produce inefficient schedules. In this paper, we present general data structures for maintaining latency constraint information at region boundaries. We present a meld scheduling algorithm for non-interlocked processors that generates latency constraints at the boundaries of scheduled regions and utilizes this information during the scheduling of other regions. We present a range of design options and describe the reasons behind our particular choices. We cover certain pitfalls and discuss how to develop an algorithm that address these issues. We extend the alogorithm to take advantage of interlocked processors by selectively propagating latencies across region boundaries. We evaluate the performance of meld scheduling on a range of machine models on a set of SPEC 92 and Unix benchmarks. We investigate the sensitivity of the performance improvements due to changes in issue width and instruction latencies.

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