pocl: A Performance-Portable OpenCL Implementation

Abstract

OpenCL is a standard for parallel programming of heterogeneous systems. The benefits of a common programming standard are clear; multiple vendors can provide support for application descriptions written according to the standard, thus reducing the program porting effort. While the standard brings the obvious benefits of platform portability, the performance portability aspects are largely left to the programmer. The situation is made worse due to multiple proprietary vendor implementations with different characteristics, and, thus, required optimization strategies. In this paper, we propose an OpenCL implementation that is both portable and performance portable. At its core is a kernel compiler that can be used to exploit the data parallelism of OpenCL programs on multiple platforms with different parallel hardware styles. The kernel compiler is modularized to perform target-independent parallel region formation separately from the target-specific parallel mapping of the regions to enable support for various styles of fine-grained parallel resources such as subword SIMD extensions, SIMD datapaths and static multi-issue. Unlike previous similar techniques that work on the source level, the parallel region formation retains the information of the data parallelism using the LLVM IR and its metadata infrastructure. This data can be exploited by the later generic compiler passes for efficient parallelization. The proposed open source implementation of OpenCL is also platform portable, enabling OpenCL on a wide range of architectures, both already commercialized and on those that are still under research. The paper describes how the portability of the implementation is achieved. We test the two aspects to portability by utilizing the kernel compiler and the OpenCL implementation to run OpenCL applications in various platforms with different style of parallel resources. The results show that most of the benchmarked applications when compiled using pocl were faster or close to as fast as the best proprietary OpenCL implementation for the platform at hand.

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Notes

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    At the time of this writing, pocl does not yet support popular commercial GPU targets. However, the SPMD/GPU path of the kernel compiler has been tested by using research targets to ensure GPU-like devices can be supported using pocl.

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Acknowledgments

The work has been financially supported by the Academy of Finland (funding decision 253087), Finnish Funding Agency for Technology and Innovation (Project “Parallel Acceleration”, funding decision 40115/13), ARTEMIS joint undertaking under Grant Agreement No. 641439 (ALMARVI), by NSF awards 0905046, 0941653, and 1212401, as well as NSERC grant 2012-RGPIN-1505. In addition to the financial supporters, the authors would also like to thank the constructive comments and references pointed out by the reviewers.

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Correspondence to Pekka Jääskeläinen.

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Jääskeläinen, P., de La Lama, C.S., Schnetter, E. et al. pocl: A Performance-Portable OpenCL Implementation. Int J Parallel Prog 43, 752–785 (2015). https://doi.org/10.1007/s10766-014-0320-y

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Keywords

  • OpenCL
  • LLVM
  • GPGPU
  • VLIW
  • SIMD
  • Parallel programming
  • Heterogeneous platforms
  • Performance portability