Evaluating two-bit branch predictors for user/kernel code

Branch prediction problem is one of the most significant problems in modern processor design. This paper examines the prediction accuracy of various two-bit finite state machines (FSMs), as a part of a wider research, that investigates possible improvements due to separate treatment of user and kernel branches. Nair [1995] simulated all possible FSMs, for SPEC89 set of benchmarks, considering only user code branches. We base our research on IBS traces, including both user and kernel branch references. Twenty best FSMs from open literature were simulated, for user, kernel, and full code, as the basic unit of 2bc, GAs, gshare, and PAs predictors. The results justify the use of two-bit saturation counter.