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Getting Started with the LLVM System

Overview

Welcome to LLVM! In order to get started, you first need to know some basic information.

First, LLVM comes in three pieces. 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. Once compiled into LLVM bitcode, a program can be manipulated with the LLVM tools from the LLVM suite.

There is a third, optional piece called Test Suite. It is a suite of programs with a testing harness that can be used to further test LLVM's functionality and performance.

Getting Started Quickly (A Summary)

The LLVM Getting Started documentation may be out of date. So, the Clang Getting Started page might also be a good place to start.

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

  1. Read the documentation.

  2. Read the documentation.

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

    • In particular, the relative paths specified are important.

  4. Checkout LLVM:

    • cd where-you-want-llvm-to-live
    • svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm

  5. Checkout Clang:

    • cd where-you-want-llvm-to-live
    • cd llvm/tools
    • svn co http://llvm.org/svn/llvm-project/cfe/trunk clang

  6. Checkout Compiler-RT (required to build the sanitizers) [Optional]:

    • cd where-you-want-llvm-to-live
    • cd llvm/projects
    • svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt

  7. Checkout Libomp (required for OpenMP support) [Optional]:

    • cd where-you-want-llvm-to-live
    • cd llvm/projects
    • svn co http://llvm.org/svn/llvm-project/openmp/trunk openmp

  8. Checkout libcxx and libcxxabi [Optional]:

    • cd where-you-want-llvm-to-live
    • cd llvm/projects
    • svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx
    • svn co http://llvm.org/svn/llvm-project/libcxxabi/trunk libcxxabi

  9. Get the Test Suite Source Code [Optional]

    • cd where-you-want-llvm-to-live
    • cd llvm/projects
    • svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite

  10. Configure and build LLVM and Clang:

    Warning: Make sure you've checked out all of the source code before trying to configure with cmake. cmake does not pickup newly added source directories in incremental builds.

    The build uses CMake. LLVM requires CMake 3.4.3 to build. It is generally recommended to use a recent CMake, especially if you're generating Ninja build files. This is because the CMake project is constantly improving the quality of the generators, and the Ninja generator gets a lot of attention.

    • cd where you want to build llvm

    • mkdir build

    • cd build

    • cmake -G <generator> [options] <path to llvm sources>

      Some common generators are:

      • Unix Makefiles --- for generating make-compatible parallel makefiles.
      • Ninja --- for generating Ninja build files. Most llvm developers use Ninja.
      • Visual Studio --- for generating Visual Studio projects and solutions.
      • Xcode --- for generating Xcode projects.

      Some Common options:

      • -DCMAKE_INSTALL_PREFIX=directory --- Specify for directory the full pathname of where you want the LLVM tools and libraries to be installed (default /usr/local).
      • -DCMAKE_BUILD_TYPE=type --- Valid options for type are Debug, Release, RelWithDebInfo, and MinSizeRel. Default is Debug.
      • -DLLVM_ENABLE_ASSERTIONS=On --- Compile with assertion checks enabled (default is Yes for Debug builds, No for all other build types).

    • Run your build tool of choice!

      • The default target (i.e. make) will build all of LLVM
      • The check-all target (i.e. make check-all) will run the regression tests to ensure everything is in working order.
      • CMake will generate build targets for each tool and library, and most LLVM sub-projects generate their own check-<project> target.
      • Running a serial build will be slow. Make sure you run a parallel build; for make, use make -j.

    • For more information see CMake

    • If you get an "internal compiler error (ICE)" or test failures, see below.

Consult the Getting Started with LLVM section for detailed information on configuring and compiling LLVM. Go to Directory Layout to learn about the layout of the source code tree.

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

LLVM is known to work on the following host platforms:

OS Arch Compilers
Linux x861 GCC, Clang
Linux amd64 GCC, Clang
Linux ARM4 GCC, Clang
Linux PowerPC GCC, Clang
Solaris V9 (Ultrasparc) GCC
FreeBSD x861 GCC, Clang
FreeBSD amd64 GCC, Clang
MacOS X2 PowerPC GCC
MacOS X x86 GCC, Clang
Cygwin/Win32 x861, 3 GCC
Windows x861 Visual Studio
Windows x64 x86-64 Visual Studio

Note

  1. Code generation supported for Pentium processors and up
  2. Code generation supported for 32-bit ABI only
  3. To use LLVM modules on Win32-based system, you may configure LLVM with -DBUILD_SHARED_LIBS=On.
  4. MCJIT not working well pre-v7, old JIT engine not supported any more.

Note that Debug builds require a lot of time and disk space. An LLVM-only build will need about 1-3 GB of space. A full build of LLVM and Clang will need around 15-20 GB of disk space. The exact space requirements will vary by system. (It is so large because of all the debugging information and the fact that the libraries are statically linked into multiple tools).

If you you are space-constrained, you can build only selected tools or only selected targets. The Release build requires considerably less space.

The LLVM suite may compile on other platforms, but it is not guaranteed to do so. If compilation is successful, the LLVM utilities should be able to assemble, disassemble, analyze, and optimize LLVM bitcode. Code generation should work as well, although the generated native code may not work on your platform.

Software

Compiling LLVM requires that you have several software packages installed. The table below lists those required packages. The Package column is the usual name for the software package that LLVM depends on. The Version column provides "known to work" versions of the package. The Notes column describes how LLVM uses the package and provides other details.

Package Version Notes
GNU Make 3.79, 3.79.1 Makefile/build processor
GCC >=4.7.0 C/C++ compiler1
python >=2.7 Automated test suite2
zlib >=1.2.3.4 Compression library3

Note

  1. Only the C and C++ languages are needed so there's no need to build the other languages for LLVM's purposes. See below for specific version info.
  2. Only needed if you want to run the automated test suite in the llvm/test directory.
  3. Optional, adds compression / uncompression capabilities to selected LLVM tools.

Additionally, your compilation host is expected to have the usual plethora of Unix utilities. Specifically:

  • ar --- archive library builder
  • bzip2 --- bzip2 command for distribution generation
  • bunzip2 --- bunzip2 command for distribution checking
  • chmod --- change permissions on a file
  • cat --- output concatenation utility
  • cp --- copy files
  • date --- print the current date/time
  • echo --- print to standard output
  • egrep --- extended regular expression search utility
  • find --- find files/dirs in a file system
  • grep --- regular expression search utility
  • gzip --- gzip command for distribution generation
  • gunzip --- gunzip command for distribution checking
  • install --- install directories/files
  • mkdir --- create a directory
  • mv --- move (rename) files
  • ranlib --- symbol table builder for archive libraries
  • rm --- remove (delete) files and directories
  • sed --- stream editor for transforming output
  • sh --- Bourne shell for make build scripts
  • tar --- tape archive for distribution generation
  • test --- test things in file system
  • unzip --- unzip command for distribution checking
  • zip --- zip command for distribution generation

Host C++ Toolchain, both Compiler and Standard Library

LLVM is very demanding of the host C++ compiler, and as such tends to expose bugs in the compiler. We are also planning to follow improvements and developments in the C++ language and library reasonably closely. As such, we require a modern host C++ toolchain, both compiler and standard library, in order to build LLVM.

For the most popular host toolchains we check for specific minimum versions in our build systems:

  • Clang 3.1
  • GCC 4.7
  • Visual Studio 2013

Anything older than these toolchains may work, but will require forcing the build system with a special option and is not really a supported host platform. Also note that older versions of these compilers have often crashed or miscompiled LLVM.

For less widely used host toolchains such as ICC or xlC, be aware that a very recent version may be required to support all of the C++ features used in LLVM.

We track certain versions of software that are known to fail when used as part of the host toolchain. These even include linkers at times.

GCC 4.6.3 on ARM: Miscompiles llvm-readobj at -O3. A test failure in test/Object/readobj-shared-object.test is one symptom of the problem.

GNU ld 2.16.X. Some 2.16.X versions of the ld linker will produce very long warning messages complaining that some ".gnu.linkonce.t.*" symbol was defined in a discarded section. You can safely ignore these messages as they are erroneous and the linkage is correct. These messages disappear using ld 2.17.

GNU binutils 2.17: Binutils 2.17 contains a bug which causes huge link times (minutes instead of seconds) when building LLVM. We recommend upgrading to a newer version (2.17.50.0.4 or later).

GNU Binutils 2.19.1 Gold: This version of Gold contained a bug which causes intermittent failures when building LLVM with position independent code. The symptom is an error about cyclic dependencies. We recommend upgrading to a newer version of Gold.

Clang 3.0 with libstdc++ 4.7.x: a few Linux distributions (Ubuntu 12.10, Fedora 17) have both Clang 3.0 and libstdc++ 4.7 in their repositories. Clang 3.0 does not implement a few builtins that are used in this library. We recommend using the system GCC to compile LLVM and Clang in this case.

Clang 3.0 on Mageia 2. There's a packaging issue: Clang can not find at least some (cxxabi.h) libstdc++ headers.

Clang in C++11 mode and libstdc++ 4.7.2. This version of libstdc++ contained a bug which causes Clang to refuse to compile condition_variable header file. At the time of writing, this breaks LLD build.

Getting a Modern Host C++ Toolchain

This section mostly applies to Linux and older BSDs. On Mac OS X, you should have a sufficiently modern Xcode, or you will likely need to upgrade until you do. On Windows, just use Visual Studio 2013 as the host compiler, it is explicitly supported and widely available. FreeBSD 10.0 and newer have a modern Clang as the system compiler.

However, some Linux distributions and some other or older BSDs sometimes have extremely old versions of GCC. These steps attempt to help you upgrade you compiler even on such a system. However, if at all possible, we encourage you to use a recent version of a distribution with a modern system compiler that meets these requirements. Note that it is tempting to to install a prior version of Clang and libc++ to be the host compiler, however libc++ was not well tested or set up to build on Linux until relatively recently. As a consequence, this guide suggests just using libstdc++ and a modern GCC as the initial host in a bootstrap, and then using Clang (and potentially libc++).

The first step is to get a recent GCC toolchain installed. The most common distribution on which users have struggled with the version requirements is Ubuntu Precise, 12.04 LTS. For this distribution, one easy option is to install the toolchain testing PPA and use it to install a modern GCC. There is a really nice discussions of this on the ask ubuntu stack exchange. However, not all users can use PPAs and there are many other distributions, so it may be necessary (or just useful, if you're here you are doing compiler development after all) to build and install GCC from source. It is also quite easy to do these days.

Easy steps for installing GCC 4.8.2:

% wget https://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.bz2
% wget https://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.bz2.sig
% wget https://ftp.gnu.org/gnu/gnu-keyring.gpg
% signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-4.8.2.tar.bz2.sig`
% if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi
% tar -xvjf gcc-4.8.2.tar.bz2
% cd gcc-4.8.2
% ./contrib/download_prerequisites
% cd ..
% mkdir gcc-4.8.2-build
% cd gcc-4.8.2-build
% $PWD/../gcc-4.8.2/configure --prefix=$HOME/toolchains --enable-languages=c,c++
% make -j$(nproc)
% make install

For more details, check out the excellent GCC wiki entry, where I got most of this information from.

Once you have a GCC toolchain, configure your build of LLVM to use the new toolchain for your host compiler and C++ standard library. Because the new version of libstdc++ is not on the system library search path, you need to pass extra linker flags so that it can be found at link time (-L) and at runtime (-rpath). If you are using CMake, this invocation should produce working binaries:

% mkdir build
% cd build
% CC=$HOME/toolchains/bin/gcc CXX=$HOME/toolchains/bin/g++ \
  cmake .. -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$HOME/toolchains/lib64 -L$HOME/toolchains/lib64"

If you fail to set rpath, most LLVM binaries will fail on startup with a message from the loader similar to libstdc++.so.6: version `GLIBCXX_3.4.20' not found. This means you need to tweak the -rpath linker flag.

When you build Clang, you will need to give it access to modern C++11 standard library in order to use it as your new host in part of a bootstrap. There are two easy ways to do this, either build (and install) libc++ along with Clang and then use it with the -stdlib=libc++ compile and link flag, or install Clang into the same prefix ($HOME/toolchains above) as GCC. Clang will look within its own prefix for libstdc++ and use it if found. You can also add an explicit prefix for Clang to look in for a GCC toolchain with the --gcc-toolchain=/opt/my/gcc/prefix flag, passing it to both compile and link commands when using your just-built-Clang to bootstrap.

Getting Started with LLVM

The remainder of this guide is meant to get you up and running with LLVM and to give you some basic information about the LLVM environment.

The later sections of this guide describe the general layout of the LLVM source tree, a simple example using the LLVM tool chain, and links to find more information about LLVM or to get help via e-mail.

Terminology and Notation

Throughout this manual, the following names are used to denote paths specific to the local system and working environment. These are not environment variables you need to set but just strings used in the rest of this document below. In any of the examples below, simply replace each of these names with the appropriate pathname on your local system. All these paths are absolute:

SRC_ROOT

This is the top level directory of the LLVM source tree.

OBJ_ROOT

This is the top level directory of the LLVM object tree (i.e. the tree where object files and compiled programs will be placed. It can be the same as SRC_ROOT).

Unpacking the LLVM Archives

If you have the LLVM distribution, you will need to unpack it before you can begin to compile it. LLVM is distributed as a set of two files: the LLVM suite and the LLVM GCC front end compiled for your platform. There is an additional test suite that is optional. Each file is a TAR archive that is compressed with the gzip program.

The files are as follows, with x.y marking the version number:

llvm-x.y.tar.gz

Source release for the LLVM libraries and tools.

llvm-test-x.y.tar.gz

Source release for the LLVM test-suite.

Checkout LLVM from Subversion

If you have access to our Subversion repository, you can get a fresh copy of the entire source code. All you need to do is check it out from Subversion as follows:

  • cd where-you-want-llvm-to-live
  • Read-Only: svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
  • Read-Write: svn co https://[email protected]/svn/llvm-project/llvm/trunk llvm

This will create an 'llvm' directory in the current directory and fully populate it with the LLVM source code, Makefiles, test directories, and local copies of documentation files.

If you want to get a specific release (as opposed to the most recent revision), you can checkout it from the 'tags' directory (instead of 'trunk'). The following releases are located in the following subdirectories of the 'tags' directory:

  • Release 3.4: RELEASE_34/final
  • Release 3.3: RELEASE_33/final
  • Release 3.2: RELEASE_32/final
  • Release 3.1: RELEASE_31/final
  • Release 3.0: RELEASE_30/final
  • Release 2.9: RELEASE_29/final
  • Release 2.8: RELEASE_28
  • Release 2.7: RELEASE_27
  • Release 2.6: RELEASE_26
  • Release 2.5: RELEASE_25
  • Release 2.4: RELEASE_24
  • Release 2.3: RELEASE_23
  • Release 2.2: RELEASE_22
  • Release 2.1: RELEASE_21
  • Release 2.0: RELEASE_20
  • Release 1.9: RELEASE_19
  • Release 1.8: RELEASE_18
  • Release 1.7: RELEASE_17
  • Release 1.6: RELEASE_16
  • Release 1.5: RELEASE_15
  • Release 1.4: RELEASE_14
  • Release 1.3: RELEASE_13
  • Release 1.2: RELEASE_12
  • Release 1.1: RELEASE_11
  • Release 1.0: RELEASE_1

If you would like to get the LLVM test suite (a separate package as of 1.4), you get it from the Subversion repository:

% cd llvm/projects
% svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite

By placing it in the llvm/projects, it will be automatically configured by the LLVM cmake configuration.

Git Mirror

Git mirrors are available for a number of LLVM subprojects. These mirrors sync automatically with each Subversion commit and contain all necessary git-svn marks (so, you can recreate git-svn metadata locally). Note that right now mirrors reflect only trunk for each project. You can do the read-only Git clone of LLVM via:

% git clone http://llvm.org/git/llvm.git

If you want to check out clang too, run:

% cd llvm/tools
% git clone http://llvm.org/git/clang.git

If you want to check out compiler-rt (required to build the sanitizers), run:

% cd llvm/projects
% git clone http://llvm.org/git/compiler-rt.git

If you want to check out libomp (required for OpenMP support), run:

% cd llvm/projects
% git clone http://llvm.org/git/openmp.git

If you want to check out libcxx and libcxxabi (optional), run:

% cd llvm/projects
% git clone http://llvm.org/git/libcxx.git
% git clone http://llvm.org/git/libcxxabi.git

If you want to check out the Test Suite Source Code (optional), run:

% cd llvm/projects
% git clone http://llvm.org/git/test-suite.git

Since the upstream repository is in Subversion, you should use git pull --rebase instead of git pull to avoid generating a non-linear history in your clone. To configure git pull to pass --rebase by default on the master branch, run the following command:

% git config branch.master.rebase true

Sending patches with Git

Please read Developer Policy, too.

Assume master points the upstream and mybranch points your working branch, and mybranch is rebased onto master. At first you may check sanity of whitespaces:

% git diff --check master..mybranch

The easiest way to generate a patch is as below:

% git diff master..mybranch > /path/to/mybranch.diff

It is a little different from svn-generated diff. git-diff-generated diff has prefixes like a/ and b/. Don't worry, most developers might know it could be accepted with patch -p1 -N.

But you may generate patchset with git-format-patch. It generates by-each-commit patchset. To generate patch files to attach to your article:

% git format-patch --no-attach master..mybranch -o /path/to/your/patchset

If you would like to send patches directly, you may use git-send-email or git-imap-send. Here is an example to generate the patchset in Gmail's [Drafts].

% git format-patch --attach master..mybranch --stdout | git imap-send

Then, your .git/config should have [imap] sections.

[imap]
      host = imaps://imap.gmail.com
      user = [email protected]
      pass = himitsu!
      port = 993
      sslverify = false
; in English
      folder = "[Gmail]/Drafts"
; example for Japanese, "Modified UTF-7" encoded.
      folder = "[Gmail]/&Tgtm+DBN-"
; example for Traditional Chinese
      folder = "[Gmail]/&g0l6Pw-"

For developers to work with git-svn

To set up clone from which you can submit code using git-svn, run:

% git clone http://llvm.org/git/llvm.git
% cd llvm
% git svn init https://llvm.org/svn/llvm-project/llvm/trunk --username=<username>
% git config svn-remote.svn.fetch :refs/remotes/origin/master
% git svn rebase -l  # -l avoids fetching ahead of the git mirror.

# If you have clang too:
% cd tools
% git clone http://llvm.org/git/clang.git
% cd clang
% git svn init https://llvm.org/svn/llvm-project/cfe/trunk --username=<username>
% git config svn-remote.svn.fetch :refs/remotes/origin/master
% git svn rebase -l

Likewise for compiler-rt, libomp and test-suite.

To update this clone without generating git-svn tags that conflict with the upstream Git repo, run:

% git fetch && (cd tools/clang && git fetch)  # Get matching revisions of both trees.
% git checkout master
% git svn rebase -l
% (cd tools/clang &&
   git checkout master &&
   git svn rebase -l)

Likewise for compiler-rt, libomp and test-suite.

This leaves your working directories on their master branches, so you'll need to checkout each working branch individually and rebase it on top of its parent branch.

For those who wish to be able to update an llvm repo/revert patches easily using git-svn, please look in the directory for the scripts git-svnup and git-svnrevert.

To perform the aforementioned update steps go into your source directory and just type git-svnup or git svnup and everything will just work.

If one wishes to revert a commit with git-svn, but do not want the git hash to escape into the commit message, one can use the script git-svnrevert or git svnrevert which will take in the git hash for the commit you want to revert, look up the appropriate svn revision, and output a message where all references to the git hash have been replaced with the svn revision.

To commit back changes via git-svn, use git svn dcommit:

% git svn dcommit

Note that git-svn will create one SVN commit for each Git commit you have pending, so squash and edit each commit before executing dcommit to make sure they all conform to the coding standards and the developers' policy.

On success, dcommit will rebase against the HEAD of SVN, so to avoid conflict, please make sure your current branch is up-to-date (via fetch/rebase) before proceeding.

The git-svn metadata can get out of sync after you mess around with branches and dcommit. When that happens, git svn dcommit stops working, complaining about files with uncommitted changes. The fix is to rebuild the metadata:

% rm -rf .git/svn
% git svn rebase -l

Please, refer to the Git-SVN manual (man git-svn) for more information.

Local LLVM Configuration

Once checked out from the Subversion repository, the LLVM suite source code must be configured before being built. This process uses CMake. Unlinke the normal configure script, CMake generates the build files in whatever format you request as well as various *.inc files, and llvm/include/Config/config.h.

Variables are passed to cmake on the command line using the format -D<variable name>=<value>. The following variables are some common options used by people developing LLVM.

Variable Purpose
CMAKE_C_COMPILER Tells cmake which C compiler to use. By default, this will be /usr/bin/cc.
CMAKE_CXX_COMPILER Tells cmake which C++ compiler to use. By default, this will be /usr/bin/c++.
CMAKE_BUILD_TYPE Tells cmake what type of build you are trying to generate files for. Valid options are Debug, Release, RelWithDebInfo, and MinSizeRel. Default is Debug.
CMAKE_INSTALL_PREFIX Specifies the install directory to target when running the install action of the build files.
LLVM_TARGETS_TO_BUILD A semicolon delimited list controlling which targets will be built and linked into llc. This is equivalent to the --enable-targets option in the configure script. The default list is defined as LLVM_ALL_TARGETS, and can be set to include out-of-tree targets. The default value includes: AArch64, AMDGPU, ARM, BPF, Hexagon, Mips, MSP430, NVPTX, PowerPC, Sparc, SystemZ, X86, XCore.
LLVM_ENABLE_DOXYGEN Build doxygen-based documentation from the source code This is disabled by default because it is slow and generates a lot of output.
LLVM_ENABLE_SPHINX Build sphinx-based documentation from the source code. This is disabled by default because it is slow and generates a lot of output.
LLVM_BUILD_LLVM_DYLIB Generate libLLVM.so. This library contains a default set of LLVM components that can be overridden with LLVM_DYLIB_COMPONENTS. The default contains most of LLVM and is defined in tools/llvm-shlib/CMakelists.txt.
LLVM_OPTIMIZED_TABLEGEN Builds a release tablegen that gets used during the LLVM build. This can dramatically speed up debug builds.

To configure LLVM, follow these steps:

  1. Change directory into the object root directory:

    % cd OBJ_ROOT

  2. Run the cmake:

    % cmake -G "Unix Makefiles" -DCMAKE_INSTALL_PREFIX=prefix=/install/path
  [other options] SRC_ROOT

Compiling the LLVM Suite Source Code

Unlike with autotools, with CMake your build type is defined at configuration. If you want to change your build type, you can re-run cmake with the following invocation:

% cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=type SRC_ROOT

Between runs, CMake preserves the values set for all options. CMake has the following build types defined:

Debug

These builds are the default. The build system will compile the tools and libraries unoptimized, with debugging information, and asserts enabled.

Release

For these builds, the build system will compile the tools and libraries with optimizations enabled and not generate debug info. CMakes default optimization level is -O3. This can be configured by setting the CMAKE_CXX_FLAGS_RELEASE variable on the CMake command line.

RelWithDebInfo

These builds are useful when debugging. They generate optimized binaries with debug information. CMakes default optimization level is -O2. This can be configured by setting the CMAKE_CXX_FLAGS_RELWITHDEBINFO variable on the CMake command line.

Once you have LLVM configured, you can build it by entering the OBJ_ROOT directory and issuing the following command:

% make

If the build fails, please check here to see if you are using a version of GCC that is known not to compile LLVM.

If you have multiple processors in your machine, you may wish to use some of the parallel build options provided by GNU Make. For example, you could use the command:

% make -j2

There are several special targets which are useful when working with the LLVM source code:

make clean

Removes all files generated by the build. This includes object files, generated C/C++ files, libraries, and executables.

make install

Installs LLVM header files, libraries, tools, and documentation in a hierarchy under $PREFIX, specified with CMAKE_INSTALL_PREFIX, which defaults to /usr/local.

make docs-llvm-html

If configured with -DLLVM_ENABLE_SPHINX=On, this will generate a directory at OBJ_ROOT/docs/html which contains the HTML formatted documentation.

Cross-Compiling LLVM

It is possible to cross-compile LLVM itself. That is, you can create LLVM executables and libraries to be hosted on a platform different from the platform where they are built (a Canadian Cross build). To generate build files for cross-compiling CMake provides a variable CMAKE_TOOLCHAIN_FILE which can define compiler flags and variables used during the CMake test operations.

The result of such a build is executables that are not runnable on on the build host but can be executed on the target. As an example the following CMake invocation can generate build files targeting iOS. This will work on Mac OS X with the latest Xcode:

% cmake -G "Ninja" -DCMAKE_OSX_ARCHITECTURES="armv7;armv7s;arm64"
  -DCMAKE_TOOLCHAIN_FILE=<PATH_TO_LLVM>/cmake/platforms/iOS.cmake
  -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_RUNTIME=Off -DLLVM_INCLUDE_TESTS=Off
  -DLLVM_INCLUDE_EXAMPLES=Off -DLLVM_ENABLE_BACKTRACES=Off [options]
  <PATH_TO_LLVM>

Note: There are some additional flags that need to be passed when building for iOS due to limitations in the iOS SDK.

Check :doc:`HowToCrossCompileLLVM` and Clang docs on how to cross-compile in general for more information about cross-compiling.

The Location of LLVM Object Files

The LLVM build system is capable of sharing a single LLVM source tree among several LLVM builds. Hence, it is possible to build LLVM for several different platforms or configurations using the same source tree.

  • Change directory to where the LLVM object files should live:

    % cd OBJ_ROOT

  • Run cmake:

    % cmake -G "Unix Makefiles" SRC_ROOT

The LLVM build will create a structure underneath OBJ_ROOT that matches the LLVM source tree. At each level where source files are present in the source tree there will be a corresponding CMakeFiles directory in the OBJ_ROOT. Underneath that directory there is another directory with a name ending in .dir under which you'll find object files for each source.

For example:

% cd llvm_build_dir
% find lib/Support/ -name APFloat*
lib/Support/CMakeFiles/LLVMSupport.dir/APFloat.cpp.o

Optional Configuration Items

If you're running on a Linux system that supports the binfmt_misc module, and you have root access on the system, you can set your system up to execute LLVM bitcode files directly. To do this, use commands like this (the first command may not be required if you are already using the module):

% mount -t binfmt_misc none /proc/sys/fs/binfmt_misc
% echo ':llvm:M::BC::/path/to/lli:' > /proc/sys/fs/binfmt_misc/register
% chmod u+x hello.bc   (if needed)
% ./hello.bc

This allows you to execute LLVM bitcode files directly. On Debian, you can also use this command instead of the 'echo' command above:

% sudo update-binfmts --install llvm /path/to/lli --magic 'BC'

Directory Layout

One useful source of information about the LLVM source base is the LLVM doxygen documentation available at http://llvm.org/doxygen/. The following is a brief introduction to code layout:

llvm/examples

Simple examples using the LLVM IR and JIT.

llvm/include

Public header files exported from the LLVM library. The three main subdirectories:

llvm/include/llvm

All LLVM-specific header files, and subdirectories for different portions of LLVM: Analysis, CodeGen, Target, Transforms, etc...

llvm/include/llvm/Support

Generic support libraries provided with LLVM but not necessarily specific to LLVM. For example, some C++ STL utilities and a Command Line option processing library store header files here.

llvm/include/llvm/Config

Header files configured by the configure script. They wrap "standard" UNIX and C header files. Source code can include these header files which automatically take care of the conditional #includes that the configure script generates.

llvm/lib

Most source files are here. By putting code in libraries, LLVM makes it easy to share code among the tools.

llvm/lib/IR/

Core LLVM source files that implement core classes like Instruction and BasicBlock.

llvm/lib/AsmParser/

Source code for the LLVM assembly language parser library.

llvm/lib/Bitcode/

Code for reading and writing bitcode.

llvm/lib/Analysis/

A variety of program analyses, such as Call Graphs, Induction Variables, Natural Loop Identification, etc.

llvm/lib/Transforms/

IR-to-IR program transformations, such as Aggressive Dead Code Elimination, Sparse Conditional Constant Propagation, Inlining, Loop Invariant Code Motion, Dead Global Elimination, and many others.

llvm/lib/Target/

Files describing target architectures for code generation. For example, llvm/lib/Target/X86 holds the X86 machine description.

llvm/lib/CodeGen/

The major parts of the code generator: Instruction Selector, Instruction Scheduling, and Register Allocation.

llvm/lib/MC/

(FIXME: T.B.D.) ....?

llvm/lib/ExecutionEngine/

Libraries for directly executing bitcode at runtime in interpreted and JIT-compiled scenarios.

llvm/lib/Support/

Source code that corresponding to the header files in llvm/include/ADT/ and llvm/include/Support/.

llvm/projects

Projects not strictly part of LLVM but shipped with LLVM. This is also the directory for creating your own LLVM-based projects which leverage the LLVM build system.

llvm/test

Feature and regression tests and other sanity checks on LLVM infrastructure. These are intended to run quickly and cover a lot of territory without being exhaustive.

test-suite

A comprehensive correctness, performance, and benchmarking test suite for LLVM. Comes in a separate Subversion module because not every LLVM user is interested in such a comprehensive suite. For details see the :doc:`Testing Guide <TestingGuide>` document.

llvm/tools

Executables built out of the libraries above, which form the main part of the user interface. You can always get help for a tool by typing tool_name -help. The following is a brief introduction to the most important tools. More detailed information is in the Command Guide.

bugpoint

bugpoint is used to debug optimization passes or code generation backends by narrowing down the given test case to the minimum number of passes and/or instructions that still cause a problem, whether it is a crash or miscompilation. See HowToSubmitABug.html for more information on using bugpoint.

llvm-ar

The archiver produces an archive containing the given LLVM bitcode files, optionally with an index for faster lookup.

llvm-as

The assembler transforms the human readable LLVM assembly to LLVM bitcode.

llvm-dis

The disassembler transforms the LLVM bitcode to human readable LLVM assembly.

llvm-link

llvm-link, not surprisingly, links multiple LLVM modules into a single program.

lli

lli is the LLVM interpreter, which can directly execute LLVM bitcode (although very slowly...). For architectures that support it (currently x86, Sparc, and PowerPC), by default, lli will function as a Just-In-Time compiler (if the functionality was compiled in), and will execute the code much faster than the interpreter.

llc

llc is the LLVM backend compiler, which translates LLVM bitcode to a native code assembly file or to C code (with the -march=c option).

opt

opt reads LLVM bitcode, applies a series of LLVM to LLVM transformations (which are specified on the command line), and outputs the resultant bitcode. 'opt -help' is a good way to get a list of the program transformations available in LLVM.

opt can also run a specific analysis on an input LLVM bitcode file and print the results. Primarily useful for debugging analyses, or familiarizing yourself with what an analysis does.

llvm/utils

Utilities for working with LLVM source code; some are part of the build process because they are code generators for parts of the infrastructure.

codegen-diff

codegen-diff finds differences between code that LLC generates and code that LLI generates. This is useful if you are debugging one of them, assuming that the other generates correct output. For the full user manual, run `perldoc codegen-diff'.

emacs/

Emacs and XEmacs syntax highlighting for LLVM assembly files and TableGen description files. See the README for information on using them.

getsrcs.sh

Finds and outputs all non-generated source files, useful if one wishes to do a lot of development across directories and does not want to find each file. One way to use it is to run, for example: xemacs `utils/getsources.sh` from the top of the LLVM source tree.

llvmgrep

Performs an egrep -H -n on each source file in LLVM and passes to it a regular expression provided on llvmgrep's command line. This is an efficient way of searching the source base for a particular regular expression.

makellvm

Compiles all files in the current directory, then compiles and links the tool that is the first argument. For example, assuming you are in llvm/lib/Target/Sparc, if makellvm is in your path, running makellvm llc will make a build of the current directory, switch to directory llvm/tools/llc and build it, causing a re-linking of LLC.

TableGen/

Contains the tool used to generate register descriptions, instruction set descriptions, and even assemblers from common TableGen description files.

vim/

vim syntax-highlighting for LLVM assembly files and TableGen description files. See the README for how to use them.

An Example Using the LLVM Tool Chain

This section gives an example of using LLVM with the Clang front end.

Example with clang

  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 native executable:

    % clang hello.c -o hello

    Note

    Clang works just like GCC by default. The standard -S and -c arguments work as usual (producing a native .s or .o file, respectively).

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

    % clang -O3 -emit-llvm hello.c -c -o hello.bc

    The -emit-llvm option can be used with the -S or -c options to emit an LLVM .ll or .bc file (respectively) for the code. This allows you to use the standard LLVM tools on the bitcode file.

  4. Run the program in both forms. To run the program, use:

    % ./hello

    and

    % lli hello.bc

    The second examples shows how to invoke the LLVM JIT, :doc:`lli <CommandGuide/lli>`.

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

    % llvm-dis < hello.bc | less

  6. Compile the program to native assembly using the LLC code generator:

    % llc hello.bc -o hello.s

  7. Assemble the native assembly language file into a program:

    % /opt/SUNWspro/bin/cc -xarch=v9 hello.s -o hello.native   # On Solaris

% gcc hello.s -o hello.native                              # On others

  8. Execute the native code program:

    % ./hello.native

    Note that using clang to compile directly to native code (i.e. when the -emit-llvm option is not present) does steps 6/7/8 for you.

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 on 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: