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A Compiler Approach for Exploiting Partial SIMD Parallelism

Published: 28 March 2016 Publication History

Abstract

Existing vectorization techniques are ineffective for loops that exhibit little loop-level parallelism but some limited superword-level parallelism (SLP). We show that effectively vectorizing such loops requires partial vector operations to be executed correctly and efficiently, where the degree of partial SIMD parallelism is smaller than the SIMD datapath width. We present a simple yet effective SLP compiler technique called Paver (PArtial VEctorizeR), formulated and implemented in LLVM as a generalization of the traditional SLP algorithm, to optimize such partially vectorizable loops. The key idea is to maximize SIMD utilization by widening vector instructions used while minimizing the overheads caused by memory access, packing/unpacking, and/or masking operations, without introducing new memory errors or new numeric exceptions. For a set of 9 C/C++/Fortran applications with partial SIMD parallelism, Paver achieves significantly better kernel and whole-program speedups than LLVM on both Intel’s AVX and ARM’s NEON.

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    cover image ACM Transactions on Architecture and Code Optimization
    ACM Transactions on Architecture and Code Optimization  Volume 13, Issue 1
    April 2016
    347 pages
    ISSN:1544-3566
    EISSN:1544-3973
    DOI:10.1145/2899032
    Issue’s Table of Contents
    Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]

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    Publication History

    Published: 28 March 2016
    Accepted: 01 January 2016
    Revised: 01 November 2015
    Received: 01 August 2015
    Published in TACO Volume 13, Issue 1

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    Author Tags

    1. Basic block vectorization
    2. SLP vectorization
    3. loop vectorization
    4. partial SIMD parallelism
    5. partial vectorization

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