Getting Started with the LLVM System — LLVM 23.0.0git documentation

Getting Started with the LLVM System — LLVM 23.0.0git documentation
index
next
previous
LLVM Home
Documentation
Getting Started/Tutorials
Getting Started with the LLVM System
Documentation
Getting Started/Tutorials
User Guides
Reference
Getting Involved
Contributing to LLVM
Submitting Bug Reports
Mailing Lists
Discord
Meetups and Social Events
Additional Links
FAQ
Glossary
Publications
Github Repository
This Page
Show Source
Quick search
Getting Started with the LLVM System
Overview
Welcome to the LLVM project!
The LLVM project has multiple components. The core of the project is
itself called “LLVM”. This contains all of the tools, libraries, and header
files needed to process intermediate representations and convert them into
object files. Tools include an assembler, disassembler, bitcode analyzer, and
bitcode optimizer. It also contains basic regression tests.
C-like languages use the
Clang
front end. This
component compiles C, C++, Objective-C, and Objective-C++ code into LLVM bitcode
– and from there into object files, using LLVM.
Other components include:
the
libc++ C++ standard library
the
LLD linker
, and more.
Getting the Source Code and Building LLVM
Check out LLVM (including subprojects like Clang):
git
clone
Or, on Windows:
git
clone
--config
core.autocrlf=false
To save storage and speed up the checkout time, you may want to do a
shallow clone
For example, to get the latest revision of the LLVM project, use
git
clone
--depth
You are likely not interested in the user branches in the repo (used for
stacked pull requests and reverts), you can filter them from your
git fetch
(or
git pull
) with this configuration:
git config --add remote.origin.fetch '^refs/heads/users/*'
git config --add remote.origin.fetch '^refs/heads/revert-*'
Configure and build LLVM and Clang:
cd
llvm-project
cmake
-S
llvm
-B
build
-G

[options]
Some common build system generators are:
Ninja
— for generating
Ninja
build files. Most llvm developers use Ninja.
Unix
Makefiles
— for generating make-compatible parallel makefiles.
Visual
Studio
— for generating Visual Studio projects and
solutions.
Xcode
— for generating Xcode projects.
See the
CMake docs
for a more comprehensive list.
Some common options:
-DLLVM_ENABLE_PROJECTS='...'
— A semicolon-separated list of the LLVM
subprojects you’d like to additionally build. Can include any of: clang,
clang-tools-extra, lldb, lld, polly, or cross-project-tests.
For example, to build LLVM, Clang, and LLD, use
-DLLVM_ENABLE_PROJECTS="clang;lld"
-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
— Controls the optimization level and debug
information of the build. Valid options for
type
are
Debug
Release
RelWithDebInfo
, and
MinSizeRel
. For more detailed
information, see
CMAKE_BUILD_TYPE
-DLLVM_ENABLE_ASSERTIONS=ON
— Compile with assertion checks enabled
(default is ON for Debug builds, OFF for all other build types).
-DLLVM_USE_LINKER=lld
— Link with the
lld linker
, assuming it
is installed on your system. This can dramatically speed up link times
if the default linker is slow.
-DLLVM_PARALLEL_{COMPILE,LINK,TABLEGEN}_JOBS=N
— Limit the number of
compile/link/tablegen jobs running in parallel at the same time. This is
especially important for linking since linking can use lots of memory. If
you run into memory issues building LLVM, try setting this to limit the
maximum number of compile/link/tablegen jobs running at the same time.
cmake
--build
build
[--target
]
or the build system specified
above directly.
The default target (i.e.
cmake
--build
build
or
make
-C
build
will build all of LLVM.
The
check-all
target (i.e.
ninja
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-
target.
Running a serial build will be
slow
. To improve speed, try running a
parallel build. That’s done by default in Ninja; for
make
, use the
option
-j
NN
, where
NN
is the number of parallel jobs, e.g. the
number of available CPUs.
A basic CMake and build/test invocation which only builds LLVM and no other
subprojects:
cmake
-S
llvm
-B
build
-G
Ninja
-DCMAKE_BUILD_TYPE=Debug
ninja
-C
build
check-llvm
This will set up an LLVM build with debugging info, then compile LLVM and
run LLVM tests.
For more detailed information on CMake options, see
CMake
If you get build 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.
Stand-alone Builds
Stand-alone builds allow you to build a sub-project against a pre-built
version of the clang or llvm libraries that is already present on your
system.
You can use the source code from a standard checkout of the llvm-project
(as described above) to do stand-alone builds, but you may also build
from a
sparse checkout
or from the
tarballs available on the
releases
page.
For stand-alone builds, you must have an llvm install that is configured
properly to be consumable by stand-alone builds of the other projects.
This could be a distro-provided LLVM install, or you can build it yourself,
like this:
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 LLVM 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.
Sub-Project
Required Sub-Directories
Required CMake Options
llvm
llvm, cmake, third-party
LLVM_INSTALL_UTILS=ON
clang
clang, cmake
CLANG_INCLUDE_TESTS=ON (Required for check-clang only)
lld
lld, cmake
Example of building stand-alone
clang
!/bin/sh
build_llvm=`pwd`/build-llvm
build_clang=`pwd`/build-clang
installprefix=`pwd`/install
llvm=`pwd`/llvm-project
mkdir -p $build_llvm
mkdir -p $installprefix
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 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
x86
GCC, Clang
Linux
amd64
GCC, Clang
Linux
ARM
GCC, Clang
Linux
AArch64
GCC, Clang
Linux
LoongArch
GCC, Clang
Linux
Mips
GCC, Clang
Linux
PowerPC
GCC, Clang
Linux
RISC-V
GCC, Clang
Linux
SystemZ
GCC, Clang
Solaris
V9 (Ultrasparc)
GCC
DragonFlyBSD
amd64
GCC, Clang
FreeBSD
x86
GCC, Clang
FreeBSD
amd64
GCC, Clang
FreeBSD
AArch64
GCC, Clang
NetBSD
x86
GCC, Clang
NetBSD
amd64
GCC, Clang
OpenBSD
x86
GCC, Clang
OpenBSD
amd64
GCC, Clang
macOS
PowerPC
GCC
macOS
x86
GCC, Clang
macOS
arm64
Clang
Cygwin/Win32
x86
1, 3
GCC
Windows
x86
Visual Studio
Windows x64
x86-64
Visual Studio, Clang
Windows on Arm
ARM64
Visual Studio, Clang
Note
Code generation supported for Pentium processors and up
Code generation supported for 32-bit ABI only
To use LLVM modules on a Win32-based system, you may configure LLVM
with
-DBUILD_SHARED_LIBS=On
Visual Studio alone can compile LLVM. When using Clang, you
must also have Visual Studio installed.
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.
Package
Version
Notes
CMake
>=3.20.0
Makefile/workspace generator
python
>=3.8
Automated test suite
zlib
>=1.2.3.4
Compression library
GNU Make
3.79, 3.79.1
Makefile/build processor
PyYAML
>=5.1
Header generator
Note
Only needed if you want to run the automated test suite in the
llvm/test
directory, or if you plan to utilize any Python libraries,
utilities, or bindings.
Optional, adds compression/uncompression capabilities to selected LLVM
tools.
Optional, you can use any other build tool supported by CMake.
Only needed when building libc with New Headergen. Mainly used by libc.
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 7.4
Visual Studio 2019 16.8
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 your
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 discussion 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 a specific version of GCC:
gcc_version
.4.0
wget
${
gcc_version
/gcc-
${
gcc_version
.tar.bz2
wget
${
gcc_version
/gcc-
${
gcc_version
.tar.bz2.sig
wget
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
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 a 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 toolchain, 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).
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. Unlike 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
-Dname>=
. The following variables are some common options
used by people developing LLVM.
CMAKE_C_COMPILER
CMAKE_CXX_COMPILER
CMAKE_BUILD_TYPE
CMAKE_INSTALL_PREFIX
Python3_EXECUTABLE
LLVM_TARGETS_TO_BUILD
LLVM_ENABLE_PROJECTS
LLVM_ENABLE_RUNTIMES
LLVM_ENABLE_DOXYGEN
LLVM_ENABLE_SPHINX
LLVM_BUILD_LLVM_DYLIB
LLVM_LINK_LLVM_DYLIB
LLVM_PARALLEL_LINK_JOBS
LLVM_OPTIMIZED_TABLEGEN
See
the list of frequently-used CMake variables
for more information.
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

-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

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=/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]

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
. 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 runs 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 corresponds 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.
The 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

, 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
llvm-reduce
llvm-reduce
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
llvm-reduce
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.
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.
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
First, create a simple C file, name it ‘hello.c’:
#include

int
main
()
printf
"hello world
\n
);
return
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).
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
and
lli
hello.bc
The second example 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, particularly on Linux and Windows. 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
-DLLVM_TARGETS_TO_BUILD
Set this equal to the target you wish to build. You may wish to set this to
only your host architecture. For example
X86
if you are using an Intel or
AMD machine. 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 compiler during your
build, even if that build is a
Debug
build. This will significantly improve
your build time. You should not enable this if your intention is to debug the
TableGen compiler.
-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. Instead, the debug
information is in a separate DWARF object file (with the extension
.dwo
).
This only applies to host platforms using ELF, such as Linux.
-DBUILD_SHARED_LIBS
Setting this to
ON
will build shared libraries instead of static
libraries. This will ease memory pressure on the linker. However, this should
only be used when developing llvm. See
BUILD_SHARED_LIBS
for more information.
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
index
next
previous
LLVM Home
Documentation
Getting Started/Tutorials
Getting Started with the LLVM System
© Copyright 2003-2026, LLVM Project.
Created using
Sphinx
7.2.6.