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A Proposal for Supporting Speculation in the OpenMP taskloop Construct

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OpenMP: Conquering the Full Hardware Spectrum (IWOMP 2019)

Part of the book series: Lecture Notes in Computer Science ((LNPSE,volume 11718))

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Abstract

Parallelization constructs in OpenMP, such as parallel for or taskloop, are typically restricted to loops that have no loop-carried dependencies (DOALL) or that contain well-known structured dependence patterns (e.g. reduction). These restrictions prevent the parallelization of many computational intensive may DOACROSS loops. In such loops, the compiler cannot prove that the loop is free of loop-carried dependencies, although they may not exist at runtime. This paper proposes a new clause for taskloop that enables speculative parallelization of may DOACROSS loops: the tls clause. We also present an initial evaluation that reveals that: (a) for certain loops, slowdowns using DOACROSS techniques can be transformed in speed-ups of up to \(2.14\times \) by applying speculative parallelization of tasks; and (b) the scheduling of tasks implemented in the Intel OpenMP runtime exacerbates the ratio of order inversion aborts after applying the taskloop-tls parallelization to a loop.

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Notes

  1. 1.

    Small \(\%lc\). ordered results in performance degradation respect to serial execution for these loops [10].

  2. 2.

    Clang 4.0 was adapted to generate AST to support the new clause as explained in Sect. 3.

  3. 3.

    Speculative privatizations described in [19] were implemented manually.

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Acknowledgments

The authors would like to thank the anonymous reviewers for the insightful comments. This work is supported by FAPESP (grants 18/07446-8 and 18/15519-5).

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Authors and Affiliations

  1. São Paulo State University, São Paulo, Brazil

    Juan Salamanca & Alexandro Baldassin

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  1. Juan Salamanca
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  2. Alexandro Baldassin
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Correspondence to Juan Salamanca .

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Editors and Affiliations

  1. University of Auckland, Auckland, New Zealand

    Xing Fan

  2. Lawrence Livermore National Laboratory, Livermore, CA, USA

    Bronis R. de Supinski

  3. University of Auckland, Auckland, New Zealand

    Oliver Sinnen

  4. University of Auckland, Auckland, New Zealand

    Nasser Giacaman

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Salamanca, J., Baldassin, A. (2019). A Proposal for Supporting Speculation in the OpenMP taskloop Construct. In: Fan, X., de Supinski, B., Sinnen, O., Giacaman, N. (eds) OpenMP: Conquering the Full Hardware Spectrum. IWOMP 2019. Lecture Notes in Computer Science(), vol 11718. Springer, Cham. https://doi.org/10.1007/978-3-030-28596-8_17

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  • DOI: https://doi.org/10.1007/978-3-030-28596-8_17

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  • Publisher Name: Springer, Cham

  • Print ISBN: 978-3-030-28595-1

  • Online ISBN: 978-3-030-28596-8

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