Building LLVM from Sources

Using the Microsoft C++ Library Manager is the simplest way to install LLVM on Windows (x64). There are, however, some drawbacks. First, vcpkg will compile and install both the release and the debug version of LLVM, which requires a significant amount of free harddisk space during installation. Second, it typically takes a while before a new release of LLVM is integrated into the vcpkg distribution. Should any of these two factors present a problem, please refer to the instructions given below on how to build LLVM from the sources.

If the latest version of LLVM is not yet available as a vcpkg package or if you only want to compile the release or the debug version, you can build LLVM from the official project sources. As a prerequisite, make sure that you have a recent version of CMake for Windows and Python installed on your system. Then build LLVM by following the steps described below.

1. Clone the LLVM project repository from GitHub and create a separate build directory to avoid interfering with the source tree.

   git clone https://github.com/llvm/llvm-project.git
   

   cd llvm-project
   

   mkdir build
   

   cd build
   

2. Create the Visual Studio 2019 projects for the build targets using CMake. It is recommended to build LLVM in Debug mode in order to be later able to also build the Oberon LLVM frontend in Debug mode. If you prefer to build LLVM in Release mode, set the CMAKE_BUILD_TYPE accordingly.

   cmake.exe ..\llvm -Thost=x64 -G "Visual Studio 16 2019" -A x64 -DCMAKE_BUILD_TYPE=Debug -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_BUILD_TOOLS=ON -DLLVM_BUILD_EXAMPLES=OFF -DLLVM_BUILD_TESTS=OFF -DLLVM_BUILD_BENCHMARKS=OFF -DLLVM_BUILD_LLVM_DYLIB=OFF -DLLVM_LINK_LLVM_DYLIB=OFF -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON
   

   Note that this command builds the LLVM libraries and tools, but does not build the examples, tests, and benchmarks. The option LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON is currently required to enable the build using Visual Studio 2019 (Version 16.4), which is known to miscompile parts of LLVM. Please refer to the LLVM documentation for detailed explanations of the build options given above.

3. Once the build targets have been created by CMake, LLVM can be built and installed using the following command. Note that the given configuration option needs to match the build type specified in the previous command, i.e., Debug in the example given here.

   msbuild /m -p:Configuration=Debug INSTALL.vcxproj
   

   As a default, this command will install the LLVM Compiler Infrastructure into the C:\Program Files (x86)\LLVM directory. As a consequence, the command prompt from which the command is run needs elevated privilege as, otherwise, it cannot write to that directory. If you would, therefore, like to install LLVM to a different directory, consider specifying the CMAKE_INSTALL_PREFIX option in Step 2.