The Clang UPC compiler (aka cupc) is a compiler for the UPC language. Depending on how it is configured, it can target multiple UPC runtime library implementations. However, this document is focused primarily on building CUPC for use with Berkeley UPC. When properly configured, Berkeley UPC’s upcc compiler driver will invoke Clang UPC transparently and link with its runtime library.

Supported Platforms

While Clang UPC can be compiled on several platforms where LLVM 9 builds, it is currently supported only on the following platforms (each of which receives periodic regression testing):

  • Linux/x86-64
  • Linux/ppc64le
  • macOS/x86-64


The current release and its license file are available from GitHub:

Build Instructions

These instructions assume you begin from a source directory, such as from unpacking a release tar archive (such as from the download link above) or comprised of git sources (described in the next section: “Git Instructions”).


  • CMake version 3.14 or newer
  • Either gcc/g++ (5.1.0 or newer) or clang/clang++ (4.0.0 or newer)
    • On macOS this typically means use of the Xcode Command Line Tools.
  • GNU Make or other standard make utility
    • CUPC has not been tested with other CMake generators, such as Ninja
  • Approximately 1GB to 2GB temporary disk space for the build
  • Approximately 200MB disk space for the installed CUPC

Disk space estimates assume you follow the configuration recommendations given below.


There are four high-level steps involved in installing CUPC, described in more detail in the paragraphs which follow.

  1. Create a build directory
  2. Configure CUPC
  3. Build CUPC
  4. Install CUPC

1. Create a build directory

The LLVM infrastructure does not support builds in the source tree. So, you must create a distinct directory for building CUPC. This build directory is also distinct from the installation directory, and therefore is normally deleted after the installation is complete. Something like /tmp/cupc-build may be appropriate. When configured as recommended, you should expect to need no more than 2GB of disk space for the build. However, without the recommended options to cmake one could need as much as 20GB.

In the following steps [SRCDIR] and [BLDDIR] will be used as placeholders for the CUPC source directory and build directory, respectively.

2. Configure CUPC

There are a mixture of required, recommended and optional arguments to pass to cmake, to configure your build of CUPC. They are described in detail on the CUPC CMake Options page. However, in most cases the following are sufficient with the appropriate substitutions for the [PREFIX] (see below), [BLDDIR] and [SRCDIR] placeholders.

On a Linux system:

$ cd [BLDDIR]
$ cmake \
        -DCMAKE_BUILD_TYPE=Release \
        -DLLVM_TARGETS_TO_BUILD=host \
        -S [SRCDIR]

For macOS, you should add the following to the Linux example above:

        -DCMAKE_CXX_FLAGS="-Wno-deprecated-declarations" \
        -DDEFAULT_SYSROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk \

On some systems, CMake version 3 may be installed as cmake3 rather than cmake.

The [PREFIX] placeholder must be replaced by the full path to an installation directory. The “Install” step will copy the built CUPC software to directories below this prefix, such as [PREFIX]/bin and [PREFIX]/lib. We recommend choosing an initially empty location as the prefix, and strongly discourage choices such as /usr and /usr/local. This ensures that you can reliably remove the entirety of a CUPC installation at a later time.

3. Build CUPC

This step consists of running make or make -j[N], where [N] is the number of concurrent processes to be used in building CUPC. Generally, values of [N] as large as the system’s CPU core count can be used to speed up the build. However, the link steps can be very memory-intensive such that too-large [N] leads to running out of memory.

If your build fails when passing -j[N], please retry without that option. This should be attempted even without anything to suggest an out-of-memory condition. For instance, insufficient disk space in /tmp could also lead to failures exclusive to parallel builds.

It is normal for this build step to generate many harmless warnings.

4. Install CUPC

This step consists simply of running make install.

After this step, you may safely remove [SRCDIR] and [BLDDIR]. The install is self-contained, without dependencies on either of these directories.

Troubleshooting a failed build

As noted earlier, any build failure experienced with make -j[N] should be retried with just make. If that is not sufficient to recover, some known failure modes and possible resolutions are described in the CUPC Troubleshooting page.

Git Instructions

The “Build Instructions” above assume you start from a complete source directory. The simplest means to obtain one (and our strong recommendation) is to use the tar archive in the download section. However, if there is a need, it is also possible to construct such a directory from sources obtained using git.

The development of CUPC is spread over two git repositories, each having distinct branches for the CUPC2C and CUPC products. Therefore, you should use the following steps to ensure a complete and consistent source tree. With appropriate substitutions for [SRCDIR] and [BRANCH] (see below):

$ git clone -b [BRANCH]  [SRCDIR]
$ git clone -b [BRANCH] [SRCDIR]/tools/clang

Where the [BRANCH] placeholder should be replaced with one of the following:

  • For the current CUPC release: main-cupc
  • For a specific CUPC release: clang-upc-[VERSION]
    • Where 3.9.1-2 is an example substitution for [VERSION]
  • For the current CUPC development branch: develop-cupc

Using the installed CUPC

Once CUPC has been installed, you should install Berkeley UPC, following its instructions for use of CUPC as a compiler. In doing so, we recommend using the clang and clang++ built from the CUPC sources as CC and CXX.

Contact Information

Bug Reports

Bugs in Clang UPC can be reported on the Clang UPC issues page.

Bugs in the Berkeley UPC driver or libraries can be reported in the corresponding Bugzilla server under the “BerkeleyUPC” product.

If in doubt as to which to use, please use the Berkeley UPC Bugzilla.
In either case, please search the bug database for a possible solution to your problem before entering a new report.

General Info

To reach the developers, free to drop us a note at upc-devel[non-robots should remove this part]