Seal5: Semi-Automated LLVM Support for RISC-V ISA Extensions Including Autovectorization

Philipp van Kempen, Mathis Salmen, Daniel Mueller-Gritschneder, Ulf Schlichtmann

Abstract: The RISC-V instruction set architecture (ISA) is popular for its extensibility, allowing easy integration of custom vendor-defined instructions tailored to specific applications. However, a quick exploration of instruction candidates fails due to the lack of tools to auto-generate embedded software toolchain support. In particular, exploiting SIMD instructions to accelerate typical DSP and machine learning workloads needs specialized integration. This work establishes a semi-automated flow to generate LLVM compiler support for custom instructions based on a C-style ISA description language. The implemented Seal5 tool is capable of generating support for functionalities ranging from baseline assembler-level support, over builtin functions to compiler code generation patterns for scalar as well as vector instructions, while requiring no deeper compiler know-how. This paper focuses primarily on a novel pattern generator approach for the optimized code generation for SIMD instructions, including support for autovectorization. The auto generated LLVM toolchain reduces development times drastically while performing similarly or better compared to the existing, manually implemented Core-V reference LLVM toolchain on a wide variety of benchmarks. Seal5 further allows the addition of compiler code generation support for the Core-VSIMD instructions, which is not yet available in the reference toolchain. Additionally, Seal5 facilitates a quick exploration of custom instruction candidates as demonstrated for a cryptography extension.