AAlign: A SIMD Framework for Pairwise Sequence Alignment on x86-Based Multi-and Many-Core Processors

Pairwise sequence alignment algorithms, e.g., Smith-Waterman and Needleman-Wunsch, with adjustable gap penalty systems are widely used in bioinformatics. The strong data dependencies in these algorithms, however, prevents compilers from effectively auto-vectorizing them. When programmers manually vectorize them on multi-and many-core processors, two vectorizing strategies are usually considered, both of which initially ignore data dependencies and then appropriately correct in a subsequent stage: (1) iterate, which vectorizes and then compensates the scoring results with multiple rounds of corrections and (2) scan, which vectorizes and then corrects the scoring results primarily via one round of parallel scan. However, manually writing such vectorizing code efficiently is non-trivial, even for experts, and the code may not be portable across ISAs. In addition, even highly vectorized and optimized codes may not achieve optimal performance because selecting the best vectorizing strategy depends on the algorithms, configurations (gap systems), and input sequences. Therefore, we propose a framework called AAlign to automatically vectorize pairwise sequence alignment algorithms across ISAs. AAlign ingests a sequential code (which follows our generalized paradigm for pairwise sequence alignment) and automatically generates efficient vector code for iterate and scan. To reap the benefits of both vectorization strategies, we propose a hybrid mechanism where AAlign automatically selects the best vectorizing strategy at runtime no matter which algorithms, configurations, and input sequences are specified. On Intel Haswell and MIC, the generated codes for Smith-Waterman and Needleman-Wunsch achieve up to a 26-fold speedup over their sequential counterparts. Compared to the highly optimized and multi-threaded sequence alignment tools, e.g., SWPS3 and SWAPHI, our codes can deliver up to 2.5-fold and 1.6-fold speedups, respectively.