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 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 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 2012 is fine. The LLVM source tree and object files, libraries and executables will consume approximately 3GB.

Software

You will need Visual Studio 2012 or higher.

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 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.