Overview¶

Welcome to LLVM on Windows! This document only covers LLVM on Windows usingVisual Studio, not WSL, mingw or cygwin. In order to get started, you first needto know some basic information.

There are many different projects that compose LLVM. The first piece is theLLVM suite. This contains all of the tools, libraries, and header files neededto use LLVM. It contains an assembler, disassembler, bitcode analyzer andbitcode optimizer. It also contains basic regression tests that can be used totest the LLVM tools and the Clang front end.

The second piece is theClang front end. Thiscomponent compiles C, C++, Objective C, and Objective C++ code into LLVMbitcode. Clang typically uses LLVM libraries to optimize the bitcode and emitmachine code. LLVM fully supports the COFF object file format, which iscompatible with all other existing Windows toolchains.

There are more LLVM projects which this document does not discuss.

Requirements¶

Before you begin to use the LLVM system, review the requirements givenbelow. This may save you some trouble by knowing ahead of time what hardwareand software you will need.

Getting Started¶

Here’s the short story for getting up and running quickly with LLVM.These instruction were tested with Visual Studio 2019 and Python 3.9.6:

1. Download and installVisual Studio.

2. In the Visual Studio installer, Workloads tab, select theDesktop development with C++ workload. Under Individual components tab,selectGit for Windows.

3. Complete the Visual Studio installation.

4. Download and install the latestPython 3 release.

5. In the first install screen, select bothInstall launcher for all usersandAdd Python to the PATH. This will allow installing psutil for allusers for the regression tests and make Python available from the commandline.

6. In the second install screen, select (again)Install for all users andif you want to developlldb, selectingDownload debug binaries is useful.

7. Complete the Python installation.

8. Run a “Developer Command Prompt for VS 2019”as administrator. This command

   prompt provides correct path and environment variables to Visual Studio andthe installed tools.

9. In the terminal window, type the commands:

   c:cd \

You may install the llvm sources in other location thanc:\llvm but do notinstall into a path containing spaces (e.g.c:\DocumentsandSettings\...)as it will fail.

10. Register the Microsoft Debug Interface Access (DIA) DLLs

    regsvr32"%VSINSTALLDIR%\DIA SDK\bin\msdia140.dll"regsvr32"%VSINSTALLDIR%\DIA SDK\bin\amd64\msdia140.dll"

The DIA library is required for LLVM PDB tests andLLDB development.

11. Install psutil and obtain LLVM source code:

    pip install psutilgit clone https://github.com/llvm/llvm-project.git llvm

Instead ofgitclone you may download a compressed source distributionfrom thereleases page.Select the last link:Sourcecode(zip) and unpack the downloaded file usingWindows Explorer built-in zip support or any other unzip tool.

12. Finally, configure LLVM using CMake:

    cmake -S llvm\llvm -B build -DLLVM_ENABLE_PROJECTS=clang -DLLVM_TARGETS_TO_BUILD=X86 -Thost=x64exit

LLVM_ENABLE_PROJECTS specifies any additional LLVM projects you want tobuild whileLLVM_TARGETS_TO_BUILD selects the compiler targets. IfLLVM_TARGETS_TO_BUILD is omitted by default all targets are builtslowing compilation and using more disk space.See theLLVM CMake guide for detailed information abouthow to configure the LLVM build.

Thecmake command line tool is bundled with Visual Studio but its GUI isnot. You may installCMake to use its GUI to changeCMake variables or modify the above command line.

• Once CMake is installed then the simplest way is to just start theCMake GUI, select the directory where you have LLVM extracted to, andthe default options should all be fine. One option you may reallywant to change, regardless of anything else, might be theCMAKE_INSTALL_PREFIX setting to select a directory to INSTALL toonce compiling is complete, although installation is not mandatory forusing LLVM. Another important option isLLVM_TARGETS_TO_BUILD,which controls the LLVM target architectures that are included on thebuild.

• CMake generates project files for all build types. To select a specificbuild type, use the Configuration manager from the VS IDE or the/property:Configuration command line option when using MSBuild.

• By default, the Visual Studio project files generated by CMake use the32-bit toolset. If you are developing on a 64-bit version of Windows andwant to use the 64-bit toolset, pass the-Thost=x64 flag whengenerating the Visual Studio solution. This requires CMake 3.8.0 or later.

13. Start Visual Studio and select configuration:

In the directory you created the project files will have anllvm.slnfile, just double-click on that to open Visual Studio. The default VisualStudio configuration isDebug which is slow and generates a huge amountof debug information on disk. For now, we recommend selectingReleaseconfiguration for the LLVM project which will build the fastest orRelWithDebInfo which is also several time larger than Release.Another technique is to build all of LLVM in Release mode and changecompiler flags, disabling optimization and enabling debug information, onlyfor specific libraries or source files you actually need to debug.

14. Test LLVM in Visual Studio:

You can run LLVM tests by merely building the project “check-all”. The testresults will be shown in the VS output window. Once the build succeeds, youhave verified a working LLVM development environment!

You should not see any unexpected failures, but will see many unsupportedtests and expected failures:

114>Testing Time: 1124.66s114>  Skipped          :    39114>  Unsupported      : 21649114>  Passed           : 51615114>  Expectedly Failed:    93========== Build: 114 succeeded, 0 failed, 321 up-to-date, 0 skipped ==========``

Alternatives to manual installation¶

Instead of the steps above, to simplify the installation procedure you can useChocolatey as package manager.After theinstallation of Chocolatey,run these commands in an admin shell to install the required tools:

choco install -y git cmake python3pip3 install psutil

There is also a WindowsDockerfilewith the entire build tool chain. This can be used to test the build with atool chain different from your host installation or to create build servers.

Next steps¶

1. Read the documentation.

2. Seriously, read the documentation.

3. Remember that you were warned twice about reading the documentation.

c:\llvm> python ..\build\Release\bin\llvm-lit.py llvm\test

c:\llvm> python ..\build\Release\bin\llvm-lit.py llvm\test\Transforms\Util

Links¶

This document is just anintroduction to how to use LLVM to do some simplethings… there are many more interesting and complicated things that you cando that aren’t documented here (but we’ll gladly accept a patch if you want towrite something up!). For more information about LLVM, check out:

• LLVM homepage

• LLVM doxygen tree

• Additional information about the LLVM directory structure and tool chaincan be found on the mainGetting Started with the LLVM System page.

• If you are having problems building or using LLVM, or if you have any othergeneral questions about LLVM, please consult theFrequently Asked Questions page.