Automatically Retargeting Hardware and Code Generation for RISC-V Custom Instructions

Custom instruction (CI) set extensions are beneficial for increasing performance and energy efficiency in a set of target applications. For rapid prototyping of these types of application-specific processors, designers leverage hardware (HW)/software (SW) co-design to create hardware implementations and retarget the compiler using a high-level description of the instruction set extension. Ideally, the architecture description should be flexible enough to support both hardware generation and compiler retargeting from the same description format. The challenge with these methods lies in coupling hardware extensions with the processor core, because using microarchitecture-specific interfaces leads to low design reuse and increased verification effort. To mitigate these challenges, we introduce a HW/SW co-design toolset capable of adapting to a user-defined architecture description that captures the instruction set extension semantics. Based on the architecture description, the toolset can both retarget the compiler and generate co-processors interfacing with the Core-V eXtension interface (CV-X-IF) and Rocket custom co-processor interface (RoCC) protocols that are widely used standard interfaces for RISC-V processors. To demonstrate our methods, we integrate the co-processors with two different variations of CVA6 and Rocket core. The resulting execution time reduction is up to 40% on average, with an area overhead of 8% for the CVA6. For the Rocket core, the execution time reduction is 27% with a 6% area overhead.