cmake -G Ninja -S path/to/llvm-project/llvm -B $builddir \
      -DLLVM_INSTALL_UTILS=ON \
      -DCMAKE_INSTALL_PREFIX=/path/to/llvm/install/prefix \
      < other options >
ninja -C $builddir install
Once llvm is installed, to configure a project for a stand-alone build, invoke CMake like this:




    

cmake -G Ninja -S path/to/llvm-project/$subproj \
      -B $buildir_subproj \
      -DLLVM_EXTERNAL_LIT=/path/to/lit \
      -DLLVM_ROOT=/path/to/llvm/install/prefix
Notice that:
The stand-alone build needs to happen in a folder that is not the
original folder where LLVMN was built
($builddir!=$builddir_subproj).
LLVM_ROOT should point to the prefix of your llvm installation,
so for example, if llvm is installed into /usr/bin and
/usr/lib64, then you should pass -DLLVM_ROOT=/usr/.
Both the LLVM_ROOT and LLVM_EXTERNAL_LIT options are
required to do stand-alone builds for all sub-projects.  Additional
required options for each sub-project can be found in the table
below.
The check-$subproj and install build targets are supported for the
sub-projects listed in the table below.
cmake -G Ninja -S $llvm/llvm -B $build_llvm \
      -DLLVM_INSTALL_UTILS=ON \
      -DCMAKE_INSTALL_PREFIX=$installprefix \
      -DCMAKE_BUILD_TYPE=Release
ninja -C $build_llvm install
cmake -G Ninja -S $llvm/clang -B $build_clang \
      -DLLVM_EXTERNAL_LIT=$build_llvm/utils/lit \
      -DLLVM_ROOT=$installprefix
ninja -C $build_clang
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:
Code generation supported for Pentium processors and up
Code generation supported for 32-bit ABI only
To use LLVM modules on Win32-based system, you may configure LLVM
with -DBUILD_SHARED_LIBS=On.
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 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.
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.
Only needed if you want to run the automated test suite in the
llvm/test directory.
Optional, adds compression / uncompression capabilities to selected LLVM
tools.
Optional, you can use any other build tool supported by CMake.
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 also attempt 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.
LLVM is written using the subset of C++ documented in coding
standards. To enforce this language version, we check the most
popular host toolchains for specific minimum versions in our build systems:
Clang 5.0
Apple Clang 10.0
GCC 7.1
Visual Studio 2019 16.7
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.
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.
Getting a Modern Host C++ Toolchain¶
This section mostly applies to Linux and older BSDs. On macOS, you should
have a sufficiently modern Xcode, or you will likely need to upgrade until you
do. Windows does not have a “system compiler”, so you must install either Visual
Studio 2019 (or later), or a recent version of mingw64. 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 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 and a
github gist with updated commands. 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 7.1.0:
% gcc_version=7.1.0
% wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2
% wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2.sig
% wget https://ftp.gnu.org/gnu/gnu-keyring.gpg
% signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-${gcc_version}.tar.bz2.sig`
% if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi
% tar -xvjf gcc-${gcc_version}.tar.bz2
% cd gcc-${gcc_version}
% ./contrib/download_prerequisites
% cd ..
% mkdir gcc-${gcc_version}-build
% cd gcc-${gcc_version}-build
% $PWD/../gcc-${gcc_version}/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.
This method will add an absolute path to the rpath of all executables. That’s
fine for local development. If you want to distribute the binaries you build
so that they can run on older systems, copy libstdc++.so.6 into the
lib/ directory.  All of LLVM’s shipping binaries have an rpath pointing at
$ORIGIN/../lib, so they will find libstdc++.so.6 there.  Non-distributed
binaries don’t have an rpath set and won’t find libstdc++.so.6. Pass
-DLLVM_LOCAL_RPATH="$HOME/toolchains/lib64" to cmake to add an absolute
path to libstdc++.so.6 as above. Since these binaries are not distributed,
having an absolute local path is fine for them.
When you build Clang, you will need to give it access to modern C++
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 number of different
subprojects. Each one has its own download which 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.
cfe-x.y.tar.gz
Source release for the Clang frontend.
Checkout LLVM from Git¶
You can also checkout the source code for LLVM from Git.
Passing --config core.autocrlf=false should not be required in
the future after we adjust the .gitattribute settings correctly, but
is required for Windows users at the time of this writing.
Simply run:
% git clone https://github.com/llvm/llvm-project.git
or on Windows,
% git clone --config core.autocrlf=false https://github.com/llvm/llvm-project.git
This will create an ‘llvm-project’ directory in the current directory and
fully populate it with all of the source code, test directories, and local
copies of documentation files for LLVM and all the related subprojects. Note
that unlike the tarballs, which contain each subproject in a separate file, the
git repository contains all of the projects together.
If you want to get a specific release (as opposed to the most recent revision),
you can check out a tag after cloning the repository. E.g., git checkout
llvmorg-6.0.1 inside the llvm-project directory created by the above
command.  Use git tag -l to list all of them.
Sending patches¶
See Contributing.
Bisecting commits¶
See Bisecting LLVM code for how to use git bisect
on LLVM.
Reverting a change¶
When reverting changes using git, the default message will say “This reverts
commit XYZ”. Leave this at the end of the commit message, but add some details
before it as to why the commit is being reverted. A brief explanation and/or
links to bots that demonstrate the problem are sufficient.
Local LLVM Configuration¶
Once checked out 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/llvm/Config/config.h.cmake.
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.
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.
Python3_EXECUTABLE
Forces CMake to use a specific Python version by
passing a path to a Python interpreter. By default
the Python version of the interpreter in your PATH
is used.
LLVM_TARGETS_TO_BUILD
A semicolon delimited list controlling which
targets will be built and linked into llvm.
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, AVR, BPF, Hexagon, Lanai,
Mips, MSP430, NVPTX, PowerPC, RISCV, Sparc,
SystemZ, WebAssembly, X86, XCore. Setting this
to "host" will only compile the host
architecture (e.g. equivalent to specifying X86
on an x86 host machine) can
significantly speed up compile and test times.
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_PROJECTS
A semicolon-delimited list selecting which of the
other LLVM subprojects to additionally build. (Only
effective when using a side-by-side project layout
e.g. via git). The default list is empty. Can
include: clang, clang-tools-extra,
cross-project-tests, flang, libc, libclc, lld,
lldb, mlir, openmp, polly, or pstl.
LLVM_ENABLE_RUNTIMES
A semicolon-delimited list selecting which of the
runtimes to build. (Only effective when using the
full monorepo layout). The default list is empty.
Can include: compiler-rt, libc, libcxx, libcxxabi,
libunwind, or openmp.
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. Sphinx version
1.5 or later recommended.
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. This option is
not available on Windows.
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:
Change directory into the object root directory:
% cd OBJ_ROOT
Run the cmake:
% cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=<type> -DCMAKE_INSTALL_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 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 macOS
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 How To Cross-Compile Clang/LLVM using Clang/LLVM 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" -DCMAKE_BUILD_TYPE=Release 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
https://llvm.org/doxygen/.  The following is a brief introduction to code
layout:
llvm/cmake¶
Generates system build files.
llvm/cmake/modules
Build configuration for llvm user defined options. Checks compiler version and
linker flags.
llvm/cmake/platforms
Toolchain configuration for Android NDK, iOS systems and non-Windows hosts to
target MSVC.
llvm/examples¶
Some simple examples showing how to use LLVM as a compiler for a custom
language - including lowering, optimization, and code generation.
Kaleidoscope Tutorial: Kaleidoscope language tutorial run through the
implementation of a nice little compiler for a non-trivial language
including a hand-written lexer, parser, AST, as well as code generation
support using LLVM- both static (ahead of time) and various approaches to
Just In Time (JIT) compilation.
Kaleidoscope Tutorial for complete beginner.
BuildingAJIT: Examples of the BuildingAJIT tutorial that shows how LLVM’s
ORC JIT APIs interact with other parts of LLVM. It also, teaches how to
recombine them to build a custom JIT that is suited to your use-case.
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 cmake.  They wrap “standard” UNIX and
C header files.  Source code can include these header files which
automatically take care of the conditional #includes that cmake
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/
The libraries represent and process code at machine code level. Handles
assembly and object-file emission.
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/bindings¶
Contains bindings for the LLVM compiler infrastructure to allow
programs written in languages other than C or C++ to take advantage of the LLVM
infrastructure.
LLVM project provides language bindings for OCaml and Python.
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.  This comes in a separate git repository
<https://github.com/llvm/llvm-test-suite>, because it contains a
large amount of third-party code under a variety of licenses. For
details see the Testing Guide 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 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 is the LLVM backend compiler, which translates LLVM bitcode to a
native code assembly file.
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.
TableGen/
Contains the tool used to generate register
descriptions, instruction set descriptions, and even assemblers from common
TableGen description files.
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¶
First, create a simple C file, name it ‘hello.c’:
#include <stdio.h>
int main() {
  printf("hello world\n");
  return 0;
Next, compile the C file into a native executable:
% clang hello.c -o hello
Clang works just like GCC by default.  The standard -S and -c arguments
work as usual (producing a native .s or .o file, respectively).
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.
Run the program in both forms. To run the program, use:
% ./hello
% lli hello.bc
The second examples shows how to invoke the LLVM JIT, lli.
Use the llvm-dis utility to take a look at the LLVM assembly code:
% llvm-dis < hello.bc | less
Compile the program to native assembly using the LLC code generator:
% llc hello.bc -o hello.s
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
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.
If you are having problems with limited memory and build time, please try
building with ninja instead of make. Please consider configuring the
following options with cmake:
-G Ninja
Setting this option will allow you to build with ninja instead of make.
Building with ninja significantly improves your build time, especially with
incremental builds, and improves your memory usage.
-DLLVM_USE_LINKER
Setting this option to lld will significantly reduce linking time for LLVM
executables on ELF-based platforms, such as Linux. If you are building LLVM
for the first time and lld is not available to you as a binary package, then
you may want to use the gold linker as a faster alternative to GNU ld.
-DCMAKE_BUILD_TYPE
Controls optimization level and debug information of the build.  This setting
can affect RAM and disk usage, see CMAKE_BUILD_TYPE
for more information.
-DLLVM_ENABLE_ASSERTIONS
This option defaults to ON for Debug builds and defaults to OFF for Release
builds. As mentioned in the previous option, using the Release build type and
enabling assertions may be a good alternative to using the Debug build type.
-DLLVM_PARALLEL_LINK_JOBS
Set this equal to number of jobs you wish to run simultaneously. This is
similar to the -j option used with make, but only for link jobs. This option
can only be used with ninja. You may wish to use a very low number of jobs,
as this will greatly reduce the amount of memory used during the build
process. If you have limited memory, you may wish to set this to 1.
-DLLVM_TARGETS_TO_BUILD
Set this equal to the target you wish to build. You may wish to set this to
X86; however, you will find a full list of targets within the
llvm-project/llvm/lib/Target directory.
-DLLVM_OPTIMIZED_TABLEGEN
Set this to ON to generate a fully optimized tablegen during your build. This
will significantly improve your build time. This is only useful if you are
using the Debug build type.
-DLLVM_ENABLE_PROJECTS
Set this equal to the projects you wish to compile (e.g. clang, lld, etc.) If
compiling more than one project, separate the items with a semicolon. Should
you run into issues with the semicolon, try surrounding it with single quotes.
-DLLVM_ENABLE_RUNTIMES
Set this equal to the runtimes you wish to compile (e.g. libcxx, libcxxabi, etc.)
If compiling more than one runtime, separate the items with a semicolon. Should
you run into issues with the semicolon, try surrounding it with single quotes.
-DCLANG_ENABLE_STATIC_ANALYZER
Set this option to OFF if you do not require the clang static analyzer. This
should improve your build time slightly.
-DLLVM_USE_SPLIT_DWARF
Consider setting this to ON if you require a debug build, as this will ease
memory pressure on the linker. This will make linking much faster, as the
binaries will not contain any of the debug information; however, this will
generate the debug information in the form of a DWARF object file (with the
extension .dwo). This only applies to host platforms using ELF, such as Linux.
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:
LLVM Homepage
LLVM Doxygen Tree
Starting a Project that Uses LLVM