# Getting Started with the LLVM System using Microsoft Visual Studio

- Overview
- Requirements
  - Hardware
  - Software
- Getting Started
- An Example Using the LLVM Tool Chain
- Common Problems
- Links

## Overview

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

There are many different projects that compose LLVM. 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.

The last major part of LLVM, the execution Test Suite, does not run on Windows, and this document does not discuss it.

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

## 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 2017 is fine. The LLVM source tree and object files, libraries and executables will consume approximately 3GB.

### Software

You will need Visual Studio 2017 or higher, with the latest Update installed.

You will also need the CMake build system since it generates the project files you will use to build with.

If you would like to run the LLVM tests you will need Python. Version 2.7 and newer are known to work. You will need GnuWin32 tools, too.

Do not install the LLVM directory tree into a path containing spaces (e.g. `C:\Documents and Settings\...`) as the configure step will fail.

## Getting Started

Here’s the short story for getting up and running quickly with LLVM:

1. Read the documentation.

2. Seriously, read the documentation.

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

4. Get the Source Code

   - With the distributed files:

     > 1. `cd <where-you-want-llvm-to-live>`
     > 2. `gunzip --stdout llvm-VERSION.tar.gz | tar -xvf -` (or use WinZip)
     > 3. `cd llvm`

   - With git access:

     Note: some regression tests require Unix-style line ending (`\n`).

     > 1. `cd <where-you-want-llvm-to-live>`
     > 2. `git clone https://github.com/llvm/llvm-project.git llvm`
     > 3. `cd llvm`

5. Use CMake to generate up-to-date project files:

   - 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.
   - If CMake complains that it cannot find the compiler, make sure that you have the Visual Studio C++ Tools installed, not just Visual Studio itself (trying to create a C++ project in Visual Studio will generally download the C++ tools if they haven’t already been).
   - See the LLVM CMake guide for detailed information about how to configure the LLVM 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.
   - 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.

6. Start Visual Studio

   - In the directory you created the project files will have an `llvm.sln` file, just double-click on that to open Visual Studio.

7. 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.

8. Test LLVM in Visual Studio:

   - If `%PATH%` does not contain GnuWin32, you may specify `LLVM_LIT_TOOLS_DIR` on CMake for the path to GnuWin32.
   - You can run LLVM tests by merely building the project “check”. The test results will be shown in the VS output window.

9. 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\bin\llvm-lit --param build_config=Win32 --param build_mode=Debug --param llvm_site_config=../build/test/lit.site.cfg test

     This example assumes that Python is in your PATH variable, you have built a Win32 Debug version of llvm with a standard out of line build. You should not see any unexpected failures, but will see many unsupported tests and expected failures.

     A specific test or test directory can be run with:

         C:\..\llvm> python ..\build\bin\llvm-lit --param build_config=Win32 --param build_mode=Debug --param llvm_site_config=../build/test/lit.site.cfg test/path/to/test

## An Example Using the LLVM Tool Chain

1. First, create a simple C file, name it ‘`hello.c`’:

       #include <stdio.h>
       int main() {
         printf("hello world\n");
         return 0;
       }

2. Next, compile the C file into an LLVM bitcode file:

       C:\..> clang -c hello.c -emit-llvm -o hello.bc

   This will create the result file `hello.bc` which is the LLVM bitcode that corresponds the compiled program and the library facilities that it required. You can execute this file directly using `lli` tool, compile it to native assembly with the `llc`, optimize or analyze it further with the `opt` tool, etc.

   Alternatively you can directly output an executable with clang with:

       C:\..> clang hello.c -o hello.exe

   The `-o hello.exe` is required because clang currently outputs `a.out` when neither `-o` nor `-c` are given.

3. Run the program using the just-in-time compiler:

       C:\..> lli hello.bc

4. Use the `llvm-dis` utility to take a look at the LLVM assembly code:

       C:\..> llvm-dis < hello.bc | more

5. Compile the program to object code using the LLC code generator:

       C:\..> llc -filetype=obj hello.bc

6. Link to binary using Microsoft link:

       C:\..> link hello.obj -defaultlib:libcmt

7. Execute the native code program:

       C:\..> hello.exe

## Common Problems

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.

## 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