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GraalVM Native Image on RISC-V

The fifth generation of the Reduced Instruction Set Computer (RISC) architecture, called RISC-V, was introduced in 2010. RISC is an open standard instruction set for processors, available for free, under open source licenses. GraalVM may now be used to compile and run native images on RISC-V machines.

GraalVM, by default, uses the Graal compiler to create machine code. The hardest part of supporting new architectures is implementing the Graal compiler for those new architectures. Alternatively, the LLVM backend may be used, which allows users to support CPU architectures, currently unsupported by the GraalVM Native Image. LLVM contains several compiler and toolchain technologies which may be used as a backend for any instruction set architecture. LLVM is written in C++ and supports various instruction sets such as x86-64, ARM, WebAssembly and RISC-V.

The LLVM backend for GraalVM is a separate component for native images and may be installed with the GraalVM updater:

gu install native-image-llvm-backend

The LLVM backend may be enabled by supplying the -H:CompilerBackend=llvm argument to the native-image command:

$JAVA_HOME/bin/native-image -H:CompilerBackend=llvm

Now the LLVM backend uses the LLVM compiler together with the LLVM intermediate representation to create native executables.

Application binaries may be built directly on a RISC-V machine:

$JAVA_HOME/bin/native-image MyFirstRISCVApplication -H:CompilerBackend=llvm

Alternatively, the binary may be cross-built on another architecture as Linux runs relatively slow on RISC-V machines. Sacha Coppey, working on GraalVM for Oracle Labs, provided instructions on how to set up the environment with the RISC-V Toolchain, CAPCaches and Static Libraries, in order to cross build the application on another architecture.

An example Micronaut application was used to measure the performance difference between running on a JVM and running as a Native Image on RISC-V. The startup time on the JVM was about 9300 milliseconds, compared to about 300 milliseconds for a Native Image.

The Native Image LLVM backend for RISC-V based on Linux may be found in the GraalVM CE dev builds. The LLVM backend supports most features of the other backends, but lacks support for macOS and other operating systems. There's also some room for future optimizations by using the RISC-V specific features. The implementation for RISC-V took just six months, which led the team to consider evaluating other environments such as WebAssembly.

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