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Verifying Pipeline Implementations in OpenMP

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Model Checking Software (SPIN 2021)

Part of the book series: Lecture Notes in Computer Science ((LNTCS,volume 12864))

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Abstract

OpenMP is a popular API for the development of parallel, shared memory programs and allows programmers to easily ready their programs to utilize modern multi-core processors. However, OpenMP-compliant programs do not guarantee that the OpenMP parallelization is functionally equivalent to a sequential execution of the program. Therefore, several approaches analyze OpenMP programs. While some approaches check functional equivalence, they are either general purpose approaches, which ignore the structure of the program and the design pattern applied for parallelization, or they focus on parallelized for-loops. In this paper, we propose a verification approach that aims at pipeline parallelism. To show functional equivalence, our approach mainly computes the dependencies that a sequential execution imposes on the pipeline stages and checks whether these dependencies are incorporated in the OpenMP parallelzation. We implemented our verification approach in a prototype tool and evaluated it on some examples. Our evaluation shows that our approach soundly detects incorrect pipeline implementations.

This work was funded by the Hessian LOEWE initiative within the Software-Factory 4.0 project.

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Notes

  1. 1.

    In iteration i, the second task reads memory location b[i] after it is written by the first task. Similarly, the first task reads memory location a[i-1] in iteration i after the second task writes to it in iteration \(i-1\).

  2. 2.

    Note that we currently do not support taskwait directives with depend clauses.

  3. 3.

    In general, the constraints apply to sibling tasks only. Due to the construction of tasks in parallel pipeline implementations that we support, all tasks are siblings.

  4. 4.

    The OpenMP standard also allows other variants of the depend clause but we stick to these because they are the main ones used when realizing the pipeline pattern. Ignoring other types is sound but can lead to false positives.

  5. 5.

    Currently, we support for and while loops.

  6. 6.

    Note that Algorithm 1 assumes, but does not check that the checked code segment follows the pipeline structure described in the previous section. Therefore, its result is only reliable for those segments.

  7. 7.

    In contrast, leaving out some of those edges only makes our approach imprecise.

  8. 8.

    Note that Fig. 2b does not contain dependencies between tasks T2 and T1 and tasks T2 and T3 because T3 and T1 are generated before T1.

  9. 9.

    The same holds for pairs of tasks and statements.

  10. 10.

    While one can reflect RAW and WAR dependencies with OpenMP depend clauses, a RAW depend specification can prevent a WAR dependency and vice versa.

  11. 11.

    Although read-only variables are excluded, this is sufficient because Algorithm 4 only checks live and modified variables.

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Authors and Affiliations

  1. Department of Computer Science, Technical University of Darmstadt, Darmstadt, Germany

    Maik Wiesner & Marie-Christine Jakobs

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  1. Maik Wiesner
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Correspondence to Maik Wiesner or Marie-Christine Jakobs .

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Editors and Affiliations

  1. Leiden University, Leiden, The Netherlands

    Alfons Laarman

  2. University of Salzburg, Salzburg, Austria

    Ana Sokolova

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Wiesner, M., Jakobs, MC. (2021). Verifying Pipeline Implementations in OpenMP. In: Laarman, A., Sokolova, A. (eds) Model Checking Software. SPIN 2021. Lecture Notes in Computer Science(), vol 12864. Springer, Cham. https://doi.org/10.1007/978-3-030-84629-9_5

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  • DOI: https://doi.org/10.1007/978-3-030-84629-9_5

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