Accurately modeling speculative instruction fetching in trace-driven simulation

Performance evaluation of modern, highly speculative, out-of-order microprocessors and the corresponding production of detailed, valid accurate results have become serious challenges. A popular evaluation methodology is trace-driven simulation which provides the advantage of a highly portable simulator that is independent of the constraints of the trace generation system. While developing and maintaining a trace-driven simulator is relatively easier than other alternatives, a primary drawback is the inability to accurately simulate speculative instruction fetching and subsequent execution. Fetching from an incorrect path occurs often in a speculative processor, however it a's difficult to capture this information in a trace. This paper investigates a scheme to accurately model instruction fetching within a trace-driven framework. This is accomplished by recreating an, approximate copy of the object code segment, which we call resurrected code, using a preliminary pass through the trace. We discuss a fast and memory-efficient method for implementing this resurrected code. In addition, we characterize UltraSPARC traces of C, C++, and Fortran programs generated using Shade to determine the potential of this method. Using these traces, and a modest branch predicting scheme, we find that in 14 of 16 cases more than 99% of all branches will find their target instruction in the resurrected code. furthermore, on these occasions, a large amount of consecutive instructions are available along the mispredicted path. These results indicate that the inaccuracies associated with speculative fetching ist trace-driven simulation can be significantly reduced using this resurrected code.