This is an update of my previous post “How to build LLVM from source” (2018-04-16), which is now out-of-date. LLVM has moved to a “monorepo” design instead of a collection of smaller tool-specific repositories. So this new post has fewer steps!
The LLVM codebase’s official home is
(However, to submit patches to LLVM projects, you must use
the official Phabricator;
don’t submit GitHub pull requests against
At least not as of November 2019.)
Step 1: Fork!
Go to your GitHub account and fork the following repository:
Step 2: Get the code!
Locally clone the repo to the right place.
cd $ROOT git clone firstname.lastname@example.org:llvm/llvm-project
This is a good time to set up the
.git/config for the repo
you just cloned (for example,
I set it up this way:
[remote "origin"] url = email@example.com:Quuxplusone/llvm-project.git fetch = +refs/heads/*:refs/remotes/origin/* [remote "upstream"] url = firstname.lastname@example.org:llvm/llvm-project.git fetch = +refs/heads/*:refs/remotes/upstream/* [branch "master"] remote = upstream merge = refs/heads/master
This gives me two remotes: one named
upstream whence I can pull,
and one named
origin whither I can push. My local
will both track
upstream/master. Anything I do in my local repo, I will do in a
feature branch; my feature branches will track
Step 3: Build!
mkdir $ROOT/llvm-project/build cd $ROOT/llvm-project/build cmake -G 'Unix Makefiles' \ -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi" \ -DCMAKE_BUILD_TYPE=RelWithDebInfo ../llvm make -j5 clang make -j5 check-clang
If you omit
-DCMAKE_BUILD_TYPE=RelWithDebInfo, this first part will still work, but you’ll produce
a “debug-build” version of
clang that is super slow, and then the “bootstrap” step below will
take days instead of minutes. So watch out for that.
make -j5 clang takes about 96 minutes on my laptop.
make -j5 check-clang takes another 38 minutes
(24 minutes to build a bunch of additional tools, and then another 14 minutes to run the actual tests).
clang will build both
cxx will build
cxxabi will build
check-cxx will build
However, I have found that
make cxx doesn’t work with the system compiler on OS X 10.13.
I get errors like this (where
__ZTIDu is the mangled name of
typeinfo for char8_t):
[ 60%] Linking CXX shared library ../../../lib/libc++abi.dylib Undefined symbols for architecture x86_64: "__ZTIDu", referenced from: -exported_symbol[s_list] command line option
If you get these same errors with
make cxx, then you should just
make clang and then follow the
instructions under “Bootstrap,” below, to build
cxx with your bootstrapped
Clang instead of the system compiler.
If something goes wrong, you can usually recover via
and, absolute worst case, you can blow away
$ROOT/llvm-project/build and start over.
Step 4: Run specific tests.
Running a specific test or directory-of-tests for any product is easy:
cd $ROOT/llvm-project/build ./bin/llvm-lit -sv ../llvm/test/Analysis ./bin/llvm-lit -sv ../clang/test/ARCMT ./bin/llvm-lit -sv ../libcxx/test/std/re
However, it looks like before you can successfully run one of these lines,
you must have run the corresponding one of
at least once, to initialize the right stuff in the
(Thanks to Brian Cain for documenting this recipe.)
But watch out! Both
make check-cxx and
llvm-lit will by default use your system compiler
to run the libc++ tests! This is not what you want! Tell
llvm-lit to use your newly built Clang
by passing the
cxx_under_test parameter, like this:
make -j5 clang make -j5 cxx ./bin/llvm-lit -sv --param cxx_under_test=`pwd`/bin/clang ../libcxx/test/
Step 5: Bootstrap!
Here we will not be installing Clang over top of the system compiler (super dangerous!); but we will instruct CMake to build Clang using the previously built Clang.
There is apparently an official way to bootstrap Clang (probably out-of-date).
However, when I bootstrap Clang, I use this crude approach inspired by the CMake FAQ:
mkdir $ROOT/llvm-project/build2 cd $ROOT/llvm-project/build2 CXX="$ROOT/llvm-project/build/bin/clang++" \ cmake -G 'Unix Makefiles' \ -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi" \ -DCMAKE_BUILD_TYPE=RelWithDebInfo ../llvm make -j5 clang VERBOSE=1
(This takes the same 96 minutes as the original
make -j5 clang did.)
Now you have built two versions of
$ROOT/llvm-project/build/bin/clang++ is the version
built with your system compiler, and
$ROOT/llvm-project/build2/bin/clang++ is the version built with
that version. You can extend this to