Runtime Vectorization Transformations of Binary Code

Abstract

In many cases, applications are not optimized for the hardware on which they run. Several reasons contribute to this unsatisfying situation, such as legacy code, commercial code distributed in binary form, or deployment on compute farms. In fact, backward compatibility of ISA guarantees only the functionality, not the best exploitation of the hardware. In this work, we focus on maximizing the CPU efficiency for the SIMD extensions. The first contribution was originally published in the International Conference on Embedded Computer Systems: Architectures, Modeling and Simulation, SAMOS XV, July 2015, Agios Konstantinos, Greece. It is a binary-to-binary optimization framework where loops vectorized for an older version of the processor SIMD extension are automatically converted to a newer one. It is a lightweight mechanism that does not include a vectorizer, but instead leverages what a static vectorizer previously did. We show that many loops compiled for x86 SSE can be dynamically converted to the more recent and more powerful AVX; as well as, how correctness is maintained with regards to challenges such as data dependencies and reductions. We obtain speedups in line with those of a native compiler targeting AVX. The second contribution is the runtime vectorization of loops in binary codes that were not originally vectorized. For this purpose, we use open source frameworks that we have tuned and integrated to (1) dynamically lift the x86 binary into the Intermediate Representation form of the LLVM compiler, (2) abstract hot loops in the polyhedral model, (3) use the power of this mathematical framework to vectorize them, and (4) finally compile them back into executable form using the LLVM Just-In-Time compiler. In most cases, the obtained speedups are close to the number of elements that can be simultaneously processed by the SIMD unit. The re-vectorizer and auto-vectorizer are implemented inside a dynamic optimization platform; it is completely transparent to the user, does not require any rewriting of the binaries, and operates during program execution.