So you want to build software for the RISC-V architecture (I’m playing with the Open ISA Vega RV32M1 development board)? Great, you’ll need a compiler. Although GCC officially supports you, with full upstream support as of GCC 7.1 and binutils 2.28, if you’re interested in hacking the compiler before building software, you’ve probably chosen LLVM. In that case, you’ll need a bit more effort to get LLVM to build as a cross-compiler.

Prerequisites

This process assumes that you begin in the directory where your RISC-V GNU build tools and LLVM project directories will reside. You will need to determine the following locations, and expand them into a fully qualified path (i.e. /home/user/... not ~/...) wherever they appear in a command.

Location Description Example Value
$TOP The location where you want your cross-compiler export TOP=$(pwd)
$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT The location where the GNU toolchain installs itself. /$TOP/riscv-gnu-toolchain/install
$LLVM_INSTALL_ROOT The location where LLVM installs itself. /$TOP/llvm-project/install

You’ll also need some packages. This tutorial presumes Ubuntu 18.04.2 LTS, and thus gets the packages from Aptitude. These instructions will probably also work for other operating systems as well, you’re just on your own for package management.

sudo apt-get update
sudo apt-get install autoconf automake autotools-dev bison build-essential \
  cmake curl flex gawk gcc gcc-multilib gperf libmpc-dev libmpfr-dev \
  libgmp-dev libncurses5-dev make ninja-build openjdk-8-jre python python3 \
  texinfo u-boot-tools zlib1g-dev

Installation

Change to the directory at which you want your cross-compiler source tree and set $TOP to this directory.

export TOP=$(pwd)

Get the RISC-V GNU Toolchain

Briefly, the RISC-V foundation maintains a RISC-V GNU toolchain for x86 cross-compilation, which you will need to cross-compile. The build tooling handles this for you, more information here.

cd $TOP
git clone --recursive https://github.com/riscv/riscv-gnu-toolchain
cd riscv-gnu-toolchain
mkdir install && export RISCV_GNU_TOOLCHAIN_INSTALL_ROOT=$(pwd)/install
./configure --prefix=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT --with-arch=rv32imc --with-abi=ilp32
make -j

Get the LLVM Project Monorepo

The LLVM project monorepo is the easiest way to get all of the sources and put them in the correct location. Previously, one had to acquire all of the individual LLVM projects and extract them to the correct subdirectories in order for CMake to detect that you wanted to build these subprojects. Now, we simply specify these in the CMake invocation (see Build LLVM).

cd $TOP
git clone https://github.com/llvm/llvm-project.git

If you’d like to set your repository to a particular project release, reset the repository to the release’s commit SHA (e.g. d0d8eb2) or its tag (e.g llvmorg-7.0.1), for example:

cd $TOP/llvm-project
git reset --hard llvmorg-7.0.1`.

Build LLVM

cd $TOP/llvm-project
mkdir build install && export LLVM_INSTALL_ROOT=$(pwd)/install
cd build
cmake -G Ninja \
  -DCMAKE_BUILD_TYPE="Debug" \
  -DCMAKE_INSTALL_PREFIX="$LLVM_INSTALL_ROOT" \
  -DBUILD_SHARED_LIBS=True \
  -DLLVM_USE_SPLIT_DWARF=True \
  -DLLVM_OPTIMIZED_TABLEGEN=True \
  -DLLVM_BUILD_TESTS=True \
  -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="RISCV" \
  -DLLVM_ENABLE_PROJECTS="clang;lld;debuginfo-tests" \
  -DDEFAULT_SYSROOT="$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT/riscv32-unknown-elf" \
  -DGCC_INSTALL_PREFIX="$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT" \
  -DLLVM_DEFAULT_TARGET_TRIPLE="riscv32-unknown-elf" \
  ../
cmake --build . --target install
cmake --build . --target check-all

If you’d prefer a simple go/no-go test on the build, instead of running the time-expensive check-all target in the last step, inspect the return value (echo $?) of the first cmake --build command for a value of 0.

Invoking the Compiler

If you set the default sysroot (-DDEFAULT_SYSROOT), GCC install prefix (-DGCC_INSTALL_PREFIX) and default target triple (-DLLVM_DEFAULT_TARGET_TRIPLE) when building using CMake, you don’t need to pass these arguments explicitly when invoking Clang:

$LLVM_INSTALL_ROOT/bin/clang [options...] infile

Otherwise, you do have to pass these arguments when invoking Clang:

$LLVM_INSTALL_ROOT/bin/clang \
  --sysroot=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT/riscv32-unknown-elf \
  --gcc-toolchain=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT \
  --target=riscv32-unknown-elf \
  [options...] infile

Since passing these arguments can be tedious, you can alias these commands:

alias riscv-clang=$LLVM_INSTALL_ROOT/bin/clang --sysroot=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT/riscv32-unknown-elf --gcc-toolchain=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT --target=riscv32-unknown-elf
alias riscv-clang++=$LLVM_INSTALL_ROOT/bin/clang++ --sysroot=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT/riscv32-unknown-elf --gcc-toolchain=$RISCV_GNU_TOOLCHAIN_INSTALL_ROOT --target=riscv32-unknown-elf
alias | grep riscv-clang