Compiler managed micro-cache bypassing for high performance EPIC processors

Advanced microprocessors have been increasing clock rates, well beyond the Gigahertz boundary. For such high performance microprocessors, a small and fast data micro cache (ucache) is important to overall performance, and proper management of it via load bypassing has a significant performance impact. In this paper, we propose and evaluate a hardware-software collaborative technique to manage ucache bypassing for EPIC processors. The hardware supports the ucache bypassing with a flag in the load instruction format, and the compiler employs static analysis and profiling to identify loads that should bypass the ucache. The collaborative method achieves a significant improvement in performance for the SpecInt2000 benchmarks. On average, about 40%, 30%, 24%, and 22% of load references are identified to bypass 256B, 1K, 4K, and 8K sized ucaches, respectively. This reduces the ucache miss rates by 39%, 32%, 28%, and 26%. The number of pipeline stalls from loads to their uses is reduced by 13%, 9%, 6%, and 5%. Meanwhile, the L1 and L2 cache misses remain largely unchanged. For the 256B ucache, bypassing improves overall performance on average by 5%.