[Engineering Paper] RECKA and RPromF: Two Frama-C Plug-ins for Optimizing Registers Usage in CUDA, OpenACC and OpenMP Programs

Pointer aliasing still hinders compiler optimizations. The ISO C standard 99 has added the restrict keyword that allows programmer to specify non-aliasing as an aid to the compiler's optimizer. The task of annotating pointers with the restrict keyword is still left to the programmer and this task is, in general, tedious and prone to errors. Scalar replacement is an optimization widely used by compilers. In this paper, we present two new Frama-C plug-ins, RECKA for automatic annotation of CUDA kernels arguments with the restrict keyword, and RPromF for scalar replacement in OpenACC and OpenMP 4.0/4.5 codes for GPU. More specifically, RECKA works as follows: (i) an alias analysis is performed on CUDA kernels and their callers; (ii) if not found any alias then CUDA kernels are cloned, the clones are renamed and their arguments are annotated with the restrict qualifier; and (iii) instructions are added to kernels call sites to perform at runtime a less-than check analysis on kernel actuals parameters and determine if the clone must be called or the original one. RPromF includes five main steps: (i) OpenACC/OpenMP offloading regions are identified; (ii) functions containing these offloading codes and their callers are analyzed to check that there is no alias; (iii) if there is no alias then the offloading codes are cloned; (iv) clone's instructions are analyzed to retrieve data reuse information and perform scalar replacement; and instructions are added to be able to use the optimized clone whenever possible. We have evaluated the two plug-ins on PolyBench benchmark suite. The results show that both scalar replacement and the usage of restrict keyword are effective for improving the overall performance of OpenACC, OpenMP 4.0/4.5 and CUDA codes.