Chainsaw: Von-neumann accelerators to leverage fused instruction chains.

A central tenet behind accelerators is to partition a program execution into regions with different behavior (e.g., SIMD, Irregular, Compute-Intensive) and then use behavior-specialized architectures [1] for each region. It is unclear whether the gains in efficiency arise from recognizing that a simpler microarchitecture is sufficient for the acceleratable code region or the actual microarchitecture, or a combination of both. Many proposals [2], [3] seem to choose dataflow-based accelerators which encounters challenges with fabric utilization and static power when the available instruction parallelism is below the peak operation parallelism available [4]. In this paper, we develop, Chainsaw, a Von-Neumann based accelerator and demonstrate that many of the fundamental overheads (e.g., fetch-decode) can be amortized by adopting the appropriate instruction abstraction. The key insight is the notion of chains, which are compiler fused sequences of instructions. chains adapt to different acceleration behaviors by varying the length of the chains and the types of instructions that are fused into a chain. Chains convey the producer-consumer locality between dependent instructions, which the Chainsaw architecture then captures by temporally scheduling such operations on the same execution unit and uses pipeline registers to forward the values between dependent operations. Chainsaw is a generic multi-lane architecture (4-stage pipeline per lane) and does not require any specialized compound function units; it can be reloaded enabling it to accelerate multiple program paths. We have developed a complete LLVM-based compiler prototype and simulation infrastructure and demonstrated that a 8-lane Chainsaw is within 73% of the performance of an ideal dataflow architecture, while reducing the energy consumption by 45% compared to a 4-way OOO processor.