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# 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 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 a test suite that can be used to test the LLVM tools.

Another useful project on Windows is Clang. Clang is a C family ([Objective]C/C++) compiler. Clang mostly works on Windows, but does not currently understand all of the Microsoft extensions to C and C++. Because of this, clang cannot parse the C++ standard library included with Visual Studio, nor parts of the Windows Platform SDK. However, most standard C programs do compile. Clang can be used to emit bitcode, directly emit object files or even linked executables using Visual Studio’s `link.exe`.

The large LLVM test suite cannot be run on the Visual Studio port at this time.

Most of the tools build and work. `bugpoint` does build, but does not work.

Additional information about the LLVM directory structure and tool chain can be found on the main Getting Started 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 2008 is fine. The LLVM source tree and object files, libraries and executables will consume approximately 3GB.

### Software

You will need Visual Studio 2008 or higher. Earlier versions of Visual Studio have bugs, are not completely compatible, or do not support the C++ standard well enough.

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. Versions 2.4-2.7 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 anonymous Subversion access:

     > 1. `cd <where-you-want-llvm-to-live>`
     > 2. `svn co http://llvm.org/svn/llvm-project/llvm/trunk 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.
   - See the LLVM CMake guide for detailed information about how to configure the LLVM build.

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 on 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:

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

         C:\..\llvm> llvm-lit test

     Note that quite a few of these test will fail.

     A specific test or test directory can be run with:

         C:\..\llvm> llvm-lit 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 a 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

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