# Getting Started with the LLVM System using Microsoft Visual Studio

## Overview

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

LLVM is composed of many different projects. The first piece is the LLVM suite. This contains all of the tools, libraries, and header files needed to use LLVM. It contains an assembler, disassembler, bitcode analyzer, and bitcode optimizer. It also contains basic regression tests that can be used to test the LLVM tools and the Clang front end.

The second piece is the Clang front end. This component compiles C, C++, Objective C, and Objective C++ code into LLVM bitcode. Clang typically uses LLVM libraries to optimize the bitcode and emit machine code. LLVM fully supports the COFF object file format, which is compatible with all other existing Windows toolchains.

This document does not discuss other LLVM projects.

## Requirements

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

### Hardware

Any system that can adequately run Visual Studio 2019 is fine. The LLVM source tree, including the git index, consumes approximately 3GB. Object files, libraries, and executables consume approximately 5GB in Release mode and much more in Debug mode. An SSD drive and >16GB RAM are recommended.

### Software

You will need Visual Studio 2019 or later, with the latest Update installed. Visual Studio Community Edition suffices.

You will also need the CMake build system since it generates the project files you will use to build with. CMake is bundled with Visual Studio 2019 so a separate installation is not required. If you do install CMake separately, Visual Studio 2022 will require CMake Version 3.21 or later.

If you would like to run the LLVM tests you will need Python. Version 3.8 and newer are known to work. You can install Python with Visual Studio 2019, from the Microsoft store or from the Python web site. We recommend the latter since it allows you to adjust installation options.

You will need Git for Windows with bash tools, too. Git for Windows is also bundled with Visual Studio 2019.

## Getting Started

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

1. Download and install Visual Studio.

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

3. Complete the Visual Studio installation.

4. Download and install the latest Python 3 release.

5. In the first install screen, select both Install launcher for all users and Add Python to the PATH. This will allow installing psutil for all users for the regression tests and make Python available from the command line.

6. In the second install screen, select (again) Install for all users and if you want to develop lldb, selecting Download debug binaries is useful.

7. Complete the Python installation.

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

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

9. In the terminal window, type the commands:

       c:
       cd \

> You may install the llvm sources in a location other than `c:\llvm` but do not install into a path containing spaces (e.g., `c:\Documents and Settings\...`) 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 and LLDB development.

11. Install psutil and obtain the LLVM source code:

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

> Instead of `git clone` you may download a compressed source distribution from the releases page. Select the last link: `Source code (zip)` and unpack the downloaded file using Windows 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=x64
        exit

> Note

> By default, the Visual Studio project files generated by CMake use the 32-bit toolset. If you are developing on a 64-bit version of Windows and want to use the 64-bit toolset, pass the `-Thost=x64` flag when generating the Visual Studio solution. This requires CMake 3.8.0 or later.

> For Windows on Arm the equivalent is `-Thost=ARM64`, but this the default for those hosts, so you do not have to use this option.

> `LLVM_ENABLE_PROJECTS` specifies any additional LLVM projects you want to build while `LLVM_TARGETS_TO_BUILD` selects the compiler targets. If `LLVM_TARGETS_TO_BUILD` is omitted by default all targets are built slowing compilation and using more disk space. See the LLVM CMake guide for detailed information about how to configure the LLVM build.

> The `cmake` command line tool is bundled with Visual Studio but its GUI is not. You may install CMake to use its GUI to change CMake variables or modify the above command line.

> - Once CMake is installed then the simplest way is to just start the CMake GUI, select the directory where you have LLVM extracted to, and the default options should all be fine. One option you may really want to change, regardless of anything else, might be the `CMAKE_INSTALL_PREFIX` setting to select a directory to INSTALL to once compiling is complete, although installation is not mandatory for using LLVM. Another important option is `LLVM_TARGETS_TO_BUILD`, which controls the LLVM target architectures that are included on the build.

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

13. Start Visual Studio and select configuration:

> In the directory you created the project files will have an `llvm.sln` file, just double-click on that to open Visual Studio. The default Visual Studio configuration is Debug which is slow and generates a huge amount of debug information on disk. For now, we recommend selecting Release configuration for the LLVM project which will build the fastest or RelWithDebInfo which is also several time larger than Release. Another technique is to build all of LLVM in Release mode and change compiler flags, disabling optimization and enabling debug information, only for 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 test results will be shown in the VS output window. Once the build succeeds, you have verified a working LLVM development environment!

> You should not see any unexpected failures, but you will see many unsupported tests and expected failures:

>     114>Testing Time: 1124.66s
>     114>  Skipped          :    39
>     114>  Unsupported      : 21649
>     114>  Passed           : 51615
>     114>  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 use Chocolatey as package manager. After the installation of Chocolatey, run these commands in an admin shell to install the required tools:

    choco install -y git cmake python3
    pip3 install psutil

There is also a Windows Dockerfile with the entire build toolchain. This can be used to test the build with a toolchain 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.

### Test LLVM on the command line:

The LLVM tests can be run by changing directory to the llvm source directory and running:

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

This example assumes that Python is in your `PATH` variable, which would be after Add Python to the PATH was selected during Python installation. If you had opened a command window prior to Python installation, you would have to close and reopen it to get the updated `PATH`.

A specific test or test directory can be run with:

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

### Build the LLVM Suite:

- The projects may still be built individually, but to build them all do not just select all of them in batch build (as some are meant as configuration projects), but rather select and build just the `ALL_BUILD` project to build everything, or the `INSTALL` project, which first builds the `ALL_BUILD` project, then installs the LLVM headers, libs, and other useful things to the directory set by the `CMAKE_INSTALL_PREFIX` setting when you first configured CMake.

- The Fibonacci project is a sample program that uses the JIT. Modify the project’s debugging properties to provide a numeric command-line argument or run it from the command line. The program will print the corresponding Fibonacci value.

## Links

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

- LLVM homepage

- LLVM doxygen tree

- Additional information about the LLVM directory structure and tool chain can be found on the main Getting Started with the LLVM System page.

- If you are having problems building or using LLVM, or if you have any other general questions about LLVM, please consult the Frequently Asked Questions page.