AVM-BTB: Adaptive and Virtualized Multi-level Branch Target Buffer

Branch Target Buffer (BTB) plays an important role in modern processors. It is used to identify branches in the instruction stream and predict branch targets. The accuracy of BTB is highly impacted by BTB capacity. However, expanding BTB capacity using traditional methods requires valuable on-chip SRAM. Both timing and area restriction make these approaches unsustainable. Moreover, these methods overlook the different demands of various applications, leading to increased power consumption and resource waste in some cases. To address this problem, we propose AVM-BTB. The key observations behind AVM-BTB come from three aspects: 1) BTB requirements vary over different applications and even over different running stages of the same application. 2) Micro-operation Cache (Uop Cache) and ICache exhibit inefficiency when confronted with instruction footprints that greatly exceed their capacity. 3) In specific scenarios of frontend overload, reducing cache capacity and increasing BTB size can effectively mitigate expensive branch prediction errors. Simultaneously, the implementation of Fetch Directed Instruction Prefetching (FDIP) can offset the limitations in cache capacity to some extent. These observations reveal the feasibility of dynamically borrowing cache capacity as temporary BTB and returning these BTB to cache when they are not needed, further resulting in an adaptive and virtualized multi-level BTB scheme. However, such a BTB structure is non-trivial. In this work, from the perspective of instructions, the cache hierarchy stores instruction data, while the BTB stores metadata used for branch prediction and instruction prefetch. Targeting high performance, AVM-BTB maintains a dynamic balance between data and metadata by monitoring the BTB error rate and effective accesses. Evaluation with 1253 traces shows that AVM-BTB is suitable for both frontend-bound and frontend-friendly scenarios, without consuming additional SRAM and with reasonable implementation efforts. Compared to baseline, AVM-BTB delivers an average performance boost of $18.22\%$ and a power consumption reduction of $2.77\%$. It also outperforms the five state-of-the-art solutions by $6.26 \%-18.26 \%$ on average in terms of IPC.