Divergence Analysis

Diogo Sampaio, Rafael Martins De Souza, Sylvain Collange, Fernando Magno Quintao Pereira
Universidade Federal de Minas Gerais
hal-00909072, (26 November 2013)


   title={Divergence Analysis},

   author={Sampaio, Diogo and De Souza, Rafael Martins and Collange, Sylvain and Pereira, Fernando Magno Quint{~a}o and others},



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The growing interest in graphics processing units has brought renewed attention to the Single Instruction Multiple Data (SIMD) execution model. SIMD machines give application developers tremendous computational power; however, programming them is still challenging. In particular, developers must deal with memory and control flow divergences. These phenomena stem from a condition that we call data divergence, which occurs whenever two processing elements (PEs) see the same variable name holding different values. This paper introduces divergence analysis, a static analysis that discovers data divergences. This analysis, currently deployed in an industrial quality compiler, is useful in several ways: it improves the translation of SIMD code to non-SIMD CPUs, it helps developers to manually improve their SIMD applications, and it also guides the automatic optimization of SIMD programs. We demonstrate this last point by introducing the notion of a divergence aware register allocator. This allocator uses information from our analysis to either rematerialize or share common data between PEs. As a testimony of its effectiveness, we have tested it on a suite of 395 CUDA kernels from well-known benchmarks. The divergence aware allocator produces GPU code that is 26.21% faster than the code produced by the allocator used in the baseline compiler.
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