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L1 instruction fetch misses remain a critical performance bottleneck, accounting for up to 40% slowdowns in server applications. Whereas instruction footprints typically fit within last-level caches, they overwhelm L1 caches, whose capacity is limited by latency constraints. Past work has shown that server application instruction miss sequences are highly repetitive. By recording, indexing, and prefetching according to these sequences, nearly all L1 instruction misses can be eliminated. However, existing schemes require impractical storage and considerable complexity to correct for minor control-flow variations that disrupt sequences. In this work, we simplify and reduce the energy requirements of accurate instruction prefetching via two observations: (1) program context as captured in the call stack correlates strongly with L1 instruction misses, and (2) the return address stack (RAS), already present in all high performance processors, succinctly summarizes program context. We propose RAS-Directed Instruction Prefetching (RDIP), which associates prefetch operations with signatures formed from the contents of the RAS. RDIP achieves 70% of the potential speedup of an ideal L1 cache, outperforms a prefetcher-less baseline by 11.5% and reduces energy and complexity relative to sequence-based prefetching. RDIP's performance is within 2% of the state-of-the-art Proactive Instruction Fetch, with nearly 3X reduction in storage and 1.9X reduction in energy overheads. |
