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A computational study of heuristic and exact techniques for superblock instruction scheduling

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Abstract

Compilers perform instruction scheduling to improve the performance of code on modern computer architectures. Superblocks—a straight-line sequence of code with a single entry point and multiple possible exit points—are a commonly used scheduling region within compilers. Superblock scheduling is NP-complete, and is done suboptimally in production compilers using a greedy algorithm coupled with a heuristic. Recently, exact schedulers have also been proposed. In this paper, we perform an extensive computational study of heuristic and exact techniques for scheduling superblocks. Our study extends previous work in using a more realistic architectural model, in not assuming perfect profile information, and in systematically investigating the case where profile information is not available. Our experimental results show that heuristics can be brittle and what looks promising under idealized (but unrealistic) conditions may not be robust in practice. As well, for the case where profile information is not available, some methods clearly dominate. Notably, a much inferior method is deployed in at least one existing compiler.

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Notes

  1. See Sect. 2 for detailed definitions and explanations of terms in computer architecture and instruction scheduling.

  2. See Sect. 3 for a detailed comparison to previous experimental studies.

  3. This is especially true in the case of instruction scheduling which rarely or never leads to decreased performance. Of course, in isolation a small improvement of 2–3 % would not be worthwhile. However, instruction scheduling is just one of many optimizations that a compiler performs. Individually, most or all such optimizations will each lead to small improvements—inconsequential in themselves, but cumulatively offering a significant impact on performance.

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Acknowledgements

This research was supported by an IBM Center for Advanced Studies (CAS) Fellowship, an NSERC Postgraduate Scholarship, and an NSERC CRD Grant. The experimental results were obtained using the facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET). The authors are grateful to the anonymous referees for their comments, which prompted us to re-examine some of our assumptions and to improve the paper.

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Authors and Affiliations

  1. Cheriton School of Computer Science, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada

    Michael Chase, Abid M. Malik, Tyrel Russell & Peter van Beek

  2. Department of Statistics & Actuarial Science, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

    R. Wayne Oldford

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  1. Michael Chase
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  2. Abid M. Malik
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  3. Tyrel Russell
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  5. Peter van Beek
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Correspondence to Peter van Beek.

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Chase, M., Malik, A.M., Russell, T. et al. A computational study of heuristic and exact techniques for superblock instruction scheduling. J Sched 15, 743–756 (2012). https://doi.org/10.1007/s10951-012-0276-y

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