Notes on Building Clang

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2024

How I built Clang on Debian Sid, using GCC 14.2.0. See their GettingStarted page (hint: it has become easier).

Clone the repository

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git clone https://github.com/llvm/llvm-project.git

The above clones the whole repo. Alternatively, if you’re not interested in the commit history, you can do a shallow clone. Not only that would suffice for a build, but it would substantially reduce the amount of data to be downloaded.

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git clone --depth 1 https://github.com/llvm/llvm-project.git  # shallow clone

Or, if you’re looking for something more stable, you can checkout a specific release.

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git clone --depth 1 --branch llvmorg-19.1.2 https://github.com/llvm/llvm-project.git

Configure

Now, from the llvm-project directory you’ve just cloned into, run CMake. I’m using Ninja as generator, but others should work as well. The -DLLVM_USE_LINKER=lld flag can really speed up the build process if you already have a version of LLD installed on your system, but it is optional. The -DLLVM_ENABLE_PROJECTS flag is used to specify which projects you want to build. I’m only interested in Clang, LLD and LLDB. You may also add runtime libraries, such as OpenMP.

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cmake -S llvm -B build -G "Ninja" -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=lld -DLLVM_ENABLE_PROJECTS="clang;lld;lldb" -DLLVM_ENABLE_RUNTIMES="openmp"

Build

Then finally, build the project. The -j flag is used to specify the number of threads to use.

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ninja -j30 -C build

You’ll find the binaries in llvm-project/build/bin. Enjoy!

2019

How I built Clang on Ubuntu 18.04.1 LTS, using GCC.

Requirements

Make sure you have the required software. Here’s what I used:

  • GNU Make 4.1
  • GCC 7.3.0
  • CMake 3.10.2
  • Python 3.6.5 (for running tests)
  • zlib 1.2.11 (optional)

Before installing new packages, I usually update & upgrade.

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# Update the list of available packages and their version
sudo apt update

# Actually install newer versions of packages
sudo apt upgrade

Checkout

Checkout LLVM from Git. Take a look at their releases. I chose version 7.0.0. That’s on branch release/7.x, commit 00b161b8971bc6d3cb55f13502288b8fe0dbaa42.

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git clone --single-branch --branch release/7.x https://github.com/llvm/llvm-project.git
git checkout 00b161b8971bc6d3cb55f13502288b8fe0dbaa42

Configure LLVM

After the checkout was complete, I created a folder where the object files would be placed.

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cd llvm-project
mkdir release
cd release

CMake generates the build files. Build variables are used to specify the build configuration. 

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cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_PROJECTS="clang;compiler-rt" -DCLANG_ENABLE_BOOTSTRAP=On ../llvm

Check the CMake output and install any missing software. I got a few errors myself:

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Could NOT find LibXml2
Could NOT find Doxygen
Could NOT find Curses
Could NOT find SWIG

Fixed:

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sudo apt install libxml2-dev
sudo apt install doxygen
sudo apt install libncurses5-dev
sudo apt install swig

Compile

Generate binaries. While still in /release, run make -j5. The -j parameter represents the number of workers used to parallelize the build process. Depending on the configuration, your machine and the number of worker threads you assign, this might take a while.

Bootstrap

Bootstrapping refers to using the newly built compiler in order to build itself again. It is only possible if makefiles were generated with CLANG_ENABLE_BOOTSTRAP=On. This step is optional and it takes time. If you’ve had enough, consider skipping it.

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make stage2

I didn’t add any more workers this time because during linking the entire process would eat up more than 8GB of RAM.

Test

To make sure everything went as planned, run the tests to validate your work. 

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make check-clang

Or, to check everything:

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make check-all

Install

By default, the install location is /usr/local. You probably need root permission to install in there.

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make install

If for some reason you want to undo this step:

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xargs rm < install_manifest.txt

After installing, I removed the /release folder because I don’t plan on doing another build. It occupies quite a lot of disk space, but if you plan on building Clang again, make sure you keep it around.