Correct Wrong Path

TL;DR

This paper introduces a new infrastructure for trace-driven simulation that accurately models wrong path execution effects in modern CPUs, significantly improving simulation fidelity for micro-architectural research.

Contribution

It develops a method to incorporate wrong path effects into trace-driven simulators and provides open traces and tools to facilitate industry-academic collaboration.

Findings

Wrong path execution impacts cache and performance significantly.

Ignoring wrong path leads to errors of up to 20.9% in simulation results.

Open traces enable more accurate and industry-relevant simulation studies.

Abstract

Modern OOO CPUs have very deep pipelines with large branch misprediction recovery penalties. Speculatively executed instructions on the wrong path can significantly change cache state, depending on speculation levels. Architects often employ trace-driven simulation models in the design exploration stage, which sacrifice precision for speed. Trace-driven simulators are orders of magnitude faster than execution-driven models, reducing the often hundreds of thousands of simulation hours needed to explore new micro-architectural ideas. Despite this strong benefit of trace-driven simulation, these often fail to adequately model the consequences of wrong path because obtaining them is nontrivial. Prior works consider either a positive or negative impact of wrong path but not both. Here, we examine wrong path execution in simulation results and design a set of infrastructure for enabling wrong-path execution in a trace driven simulator. Our analysis shows the wrong path affects structures on both the instruction and data sides extensively, resulting in performance variations ranging from $-3.05$\% to $20.9$\% when ignoring wrong path. To benefit the research community and enhance the accuracy of simulators, we opened our traces and tracing utility in the hopes that industry can provide wrong-path traces generated by their internal simulators, enabling academic simulation without exposing industry IP.