Correct Wrong Path

• Published in: IEEE computer architecture letters Vol. 24; no. 2; pp. 221 - 224
• Main Authors: Godala, Bhargav Reddy, Ramesh, Sankara Prasad, Tibrewala, Krishnam, Pepi, Chrysanthos, Chacon, Gino, Kanev, Svilen, Pokam, Gilles A., Ros, Alberto, Jimenez, Daniel A., Gratz, Paul V., August, David I.
• Format: Journal Article
• Language: English
• Published: IEEE 01.07.2025
• Subjects: Accuracy; Benchmark testing; CPU microarchitecture; Industries; IP networks; Microarchitecture; Out of order; out of order execution; Pipelines; Pollution; Prefetching; Simulation
• ISSN: 1556-6056; 1556-6064
• DOI: 10.1109/LCA.2025.3542809

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 the strong benefits of trace-driven simulation, it often fails to adequately model the consequences of wrong-path execution because obtaining such traces from real systems is nontrivial. Prior works exclusively consider either pollution or prefetching in the instruction stream/L1-I cache and often ignore the impact on the data stream. 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 <inline-formula><tex-math notation="LaTeX">-3.05</tex-math> <mml:math><mml:mrow><mml:mo>-</mml:mo><mml:mn>3</mml:mn><mml:mo>.</mml:mo><mml:mn>05</mml:mn></mml:mrow></mml:math><inline-graphic xlink:href="godala-ieq1-3542809.gif"/> </inline-formula>% to 20.9% versus 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.