Evaluating Trace Cache Energy Efficiency

Authors:Co, Michele, Department of Computer ScienceUniversity of Virginia Skadron, Kevin, Department of Computer ScienceUniversity of Virginia

Future fetch engines need to be energy-efficient. Therefore, a thorough evaluation and comparison of fetch engine design is necessary for futuristic processors.
Our work compares the energy-efficiency of concurrent trace caches (CTCs), sequential trace caches (STCs), block-based trace caches (BBTCs), and instruction caches (ICs). We compare: CTCs and STCs with path-based next trace predictor (NTP), ICs with branch pre- dictor (IC-BPRED), and BBTCs with trace table (BBTC-TT). To separate out predictor organi- zation and prediction effects we also evaluate ICs with NTP (IC-NTP) and BBTCs with NTP (BBTC-NTP). In our experiments, we first evaluate the fetch engines with no area budget re- strictions. Then, to consider higher clock rates we evaluate the fetch engines when restricting the area budget for each component. To consider future process technologies, we also evaluate the effect of increased leakage.
We find that branch prediction (whether explicit or implicit) is a key component in the energy-efficiency of the fetch engine designs evaluated. Branch prediction effects are elim- inated by artificially equalizing the effective branch prediction accuracy for the fetch engine designs and the results are evaluated.
We find that access delay limits the theoretical performance of the fetch engines evaluated. We propose a novel ahead pipelined NTP that performs nearly as well as the single-cycle access NTP.

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Source Citation:

Co, Michele, and Kevin Skadron. "Evaluating Trace Cache Energy Efficiency." University of Virginia Dept. of Computer Science Tech Report (2004).

University of Virginia, Department of Computer Science
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