Explainable Port Mapping Inference with Sparse Performance Counters for AMD's Zen Architectures

Abstract

Performance models are instrumental for optimizing performance-sensitive code. When modeling the use of functional units of out-of-order x86-64 CPUs, data availability varies by the manufacturer: Instruction-to-port mappings for Intel's processors are available, whereas information for AMD's designs are lacking. The reason for this disparity is that standard techniques to infer exact port mappings require hardware performance counters that AMD does not provide.In this work, we modify the port mapping inference algorithm of the widely used uops.info project to not rely on Intel's performance counters. The modifications are based on a formal port mapping model with a counter-exampleguided algorithm powered by an SMT solver. We investigate in how far AMD's processors comply with this model and where unexpected performance characteristics prevent an accurate port mapping. Our results provide valuable insights for creators of CPU performance models as well as for software developers who want to achieve peak performance on recent AMD CPUs.