Using LLVM for Optimized Lightweight Binary Re-Writing at Runtime Alexis Engelke, Josef Weidendorfer Department of Informatics Technical University of Munich Munich, Germany EMail:
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[email protected] Abstract—Providing new parallel programming models/ab- helpful in reducing any overhead of provided abstractions. stractions as a set of library functions has the huge advantage But with new languages, porting of existing application that it allows for an relatively easy incremental porting path for code is required, being a high burden for adoption with legacy HPC applications, in contrast to the huge effort needed when novel concepts are only provided in new programming old code bases. Therefore, to provide abstractions such as languages or language extensions. However, performance issues PGAS for legacy codes, APIs implemented as libraries are are to be expected with fine granular usage of library functions. proposed [5], [6]. Libraries have the benefit that they easily In previous work, we argued that binary rewriting can bridge can be composed and can stay small and focused. However, the gap by tightly coupling application and library functions at libraries come with the caveat that fine granular usage of runtime. We showed that runtime specialization at the binary level, starting from a compiled, generic stencil code can help library functions in inner kernels will severely limit compiler in approaching performance of manually written, statically optimizations such as vectorization and thus, may heavily compiled version. reduce performance. In this paper, we analyze the benefits of post-processing the To this end, in previous work [7], we proposed a technique re-written binary code using standard compiler optimizations for lightweight code generation by re-combining existing as provided by LLVM.