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Fetch-Directed Instruction Prefetching Revisited

Prior work has observed that fetch-directed prefetching (FDIP) is highly effective at covering instruction cache misses. The key to FDIP's effectiveness is having a sufficiently large BTB to accommodate the application's branch working set. In this work, we introduce several optimizations that significantly extend the reach of the BTB within the available storage budget. Our optimizations target nearly every source of storage overhead in each BTB entry; namely, the tag, target address, and size fields. We observe that while most dynamic branch instances have short offsets, a large number of branches has longer offsets or requires the use of full target addresses. Based on this insight, we break-up the BTB into multiple smaller BTBs, each storing offsets of different length. This enables a dramatic reduction in storage for target addresses. We further compress tags to 16 bits and avoid the use of the basic-block-oriented BTB advocated in prior FDIP variants. The latter optimization eliminates the need to store the basic block size in each BTB entry. Our final design, called FDIP-X, uses an ensemble of 4 BTBs and always outperforms conventional FDIP with a unified basic-block-oriented BTB for equal storage budgets.