Whole-function vectorization
Ralf Karrenberg, Sebastian Hack · 2011
Abstract—Data-parallel programming languages are an impor-tant component in today’s parallel computing landscape. Among those are domain-specific languages like shading languages in graphics (HLSL, GLSL, RenderMan, etc.) and “general-purpose” languages like CUDA or OpenCL. Current implementations of those languages on CPUs solely rely on multi-threading to imple-ment parallelism and ignore the additional intra-core parallelism provided by the SIMD instruction set of those processors (like Intel’s SSE and the upcoming AVX or Larrabee instruction sets). In this paper, we discuss several aspects of implementing data-parallel languages on machines with SIMD instruction sets. Our main contribution is a language- and platform-independent code transformation that performs whole-function vectorization on low-level intermediate code given by a control flow graph in SSA form. We evaluate our technique in two scenarios: First, incorpo-rated in a compiler for a domain-specific language used in real-time ray tracing. Second, in a stand-alone OpenCL driver. We observe average speedup factors of 3.9 for the ray tracer and factors between 0.6 and 5.2 for different OpenCL kernels. I.