Fine-tuning Vectorization and Memory Traffic on Intel Xeon Phi Coprocessors: Lu Decomposition of Small Matrices

Common techniques for fine-tuning the performance of automatically vectorized loops in applications for Intel Xeon Phi coprocessors are discussed. These techniques include strength reduction, regularizing the vectorization pattern, data alignment and aligned data hint, and pointer disambiguation. In addition, the loop tiling technique of memory traffic tuning is shown. The optimization methods are illustrated on an example of single-threaded LU decomposition of a single precision matrix of size 128× 128. Benchmarks show that the discussed optimizations improve the application performance on the coprocessor by a factor of 2.8 compared to the unoptimized code, and by a factor of 1.7 on the multi-core host system, achieving roughly the same performance on the host and on the coprocessor. The code discussed in the paper can be freely downloaded from the Colfax Research Web site. Table of