PostSLP: Cross-Region Vectorization of Fully or Partially Vectorized Code

Modern optimizing compilers rely on auto-vectorization algorithms for generating high-performance code. Both loop and straight-line code vectorization algorithms generate SIMD vector instructions out of scalar code, with no intervention from the programmer. In this work, we show that the existing auto-vectorization algorithms operate on restricted code regions and therefore are missing out vectorization opportunities by either generating narrower vectors than those possible for the target architecture or are completely failing and leaving some of the code in scalar form. We show the need for a specialized post-processing re-vectorization pass, called PostSLP, that has the ability to span across multiple regions, and to generate more effective vector code. PostSLP is designed to convert already vectorized, or partially vectorized code into wider forms that perform better on the target architecture. We implemented PostSLP in LLVM and our evaluation shows significant performance improvements in SPEC CPU2006.