Information for PoCL developers


Before changes are committed to the upstream PoCL, the code must pass the Main Test Matrix Github workflow.

Under the ‘examples’ directory there are directories for external OpenCL application projects which are used as test suites for pocl (e.g. ViennaCL). These test suites can be enabled for cmake with -DENABLE_TESTSUITES (you can specify a list of test suites if you do not want to enabled all of them, see examples/CMakeLists.txt for the available list). Note that these additional test suites require additional software (tools and libraries). CMake will check some of them, but the checks are not exhaustive. Testsuites are disabled if the dependency checks fail.

You can run the tests or built examples using “ctest” directly; ctest --print-labels prints the available labels (testsuites); Invoke ctest with -jX option to run X tests in parallel.

In order to prepare the external OpenCL examples for the testsuite, you need to run the following build command once:

make prepare_examples

IMPORTANT: using the ICD for in tree ‘make check’ requires an icd loader that allows overriding the icd search path. Other ICD loaders wont be able to work in tree (they require the ICD config file to be installed in the system). There are now two options for such a loader: the open source ocl-icd loader and the Khronos supplied loader with a patch applied.

Debugging a Failed Test

If there are failing tests in the suite, the usual way to start debugging is to look what was printed to the logs for the failing cases. After running the test suite, the logs are stored under Testing/Temporary/*.log Or one could re-run the test with more verbose output. Useful ctest options are “-V” and “–output-on-failure”; to make pocl more chatty, use the POCL_DEBUG env variable.


Ocl-icd is packaged for most popular linux distributions, but can also be downloaded from:

It allows overriding the path from which the icd files are searched which is used to select only the OpenCL library in the build tree of pocl for the make check. Note, however, if you run the tests or examples manually this overriding is not done automatically. To direct the ocl-icd to use only the pocl in the build tree, export the following environment variable in your shell:


Inside the ‘ocl-vendors’ directory there’s a single .icd file which is generated to point to the pocl library in the build tree.

Coding Style

The code base of pocl consists most of pure C sources and C++ sources.

  1. In the C sources, follow the GNU C style, but with spaces for indent.

    The GNU C style guide is here:

    This guide should be followed except please use 2 spaces instead of the confusing “smart” mix of tabs and spaces for indentation.

  2. In the C++ sources (mostly the LLVM passes), follow the LLVM coding guidelines so it is easier to upstream general code to the LLVM project at any point.

It’s acknowledged that the pocl code base does not fully adhere to these principles at the moment, but the aim is to gradually fix the style with every new commit improving the style.

There are clang-format scripts to help in getting the style gradually improved. Running tools/scripts/ in the root of the repository diffs against a master branch and formats the difference, and leaves the diff uncommitted in the working tree. tools/scripts/ formats only the last commit and can be used in an interactive rebase session. Both scripts require “clang-format” binary present in PATH.

An example emacs configuration to help get the pocl code style correct:

(setq default-tab-width 2)
(setq-default indent-tabs-mode nil)
(setq-default show-trailing-whitespace t)

(defun my-c-mode-common-hook ()
  (c-set-style "gnu")
  (setq tab-width 2)
  (setq c-basic-offset 2)
(add-hook 'c-mode-common-hook 'my-c-mode-common-hook)

(defun my-cpp-mode-common-hook ()
  (c-set-style "stroustrup")
  (setq tab-width 4)
  (setq c-basic-offset 4)
(add-hook 'c++-mode-hook 'my-cpp-mode-common-hook)

(add-to-list 'auto-mode-alist '("\\.cl$" . c-mode))
(add-to-list 'auto-mode-alist '("\\.icc$" . c++-mode))
(add-to-list 'auto-mode-alist '("\\.cc$" . c++-mode))

Khronos ICD Loader

The ICD loader supplied by Khronos can be used for pocl development by applying a minor patch that enables overriding the ICD search path as explained above (OCL-ICD).

The steps to build and install the Khronos ICD loader so it can be used to run the pocl test suite:

  1. Download the loader from Unpack it. Copy the OpenCL headers to inc/CL like instructed in inc/README.txt.

  2. Apply a patch from the pocl checkout::

    cd icd

    patch -p1 < ~/pocl/tools/patches/khronos-icd-loader.patch

  3. Build it with ‘make’.

  4. Copy the loader to a library search path: sudo cp bin/libOpenCL* /usr/lib

Now it should use the Khronos loader for ICD dispatching and you (and the pocl build system) should be able to override the icd search path with OCL_ICD_VENDORS environment variable.

Using pocl from the Build Tree

If you want use the pocl from the build tree, you must export POCL_BUILDING=1 so pocl searches for its utility scripts from the build tree first, then the installation location. Running PoCL’s ctest from the build root will do this automatically.

There’s a helper script that, when sourced, in addition to setting POCL_BUILDING setups the OCL_ICD_VENDORS path to point to the pocl in the build tree. This removes the need to install pocl to test the built version. It should be executed in the build root, typically:

. ../tools/scripts/

Target and Host CPU Architectures for CPU Devices

By default, pocl build system compiles the kernel libraries for the host CPU architecture, to be used by CPU devices (‘cpu’ and ‘cpu-minimal’).

LLVM is used to detect the CPU variant to be used as target. This can be overridden by passing -DLLC_HOST_CPU=… to CMake. See the documentation for LLC_HOST_CPU build option.

Cross-compilation where ‘build’ is different from ‘host’ has not been tested. Cross-compilation where ‘host’ is a different architecture from ‘target’ has not been tested for CPU devices.

Writing Documentation

The documentation is written using the Sphinx documentation generator and the reStructuredText markup.

This Sphinx documentation can be built by:

cd doc/sphinx
make html

This builds the html version of the documents under the ‘build/html’ directory.

Maintenance Policy

pocl development is currently managed mostly by researchers and research assistants of the Customized Parallel Computing group of Tampere University. We provide general maintenance for pocl on the side of our research projects (which on the other hand might use and/or extend it) because we consider it an important project that helps the “heterogeneous parallel programming cause”. However, doing maintenance “on the side” unfortunately means that there is limited time to respond to external support requests due to other activities.

To make pocl maintenance feasible within our limited time, we have set the following policy regarding releases: External projects using OpenCL that have a test suite included in “regularly tested suites” (we later call ‘tier-1’ test suites) will be kept regression free, but for the rest we cannot make any promises.

Tier-1 tests will be executed successfully before the lead developer pushes new pull requests (PR) to the master branch, and some of them are additionally executed with multiple continuous integration (buildbot) servers on different platforms. Active developers are also assumed to run them locally before submitting PRs. Thus, regressions on these suites should be detected early. The required testsuites can be enabled at buildtime with -DENABLE_TESTSUITES=tier1 cmake option.

Currently (2023-02-28) the following are included in the tier-1 test suites:

  • The standard test suite of pocl.

  • PyOpenCL test suite

  • piglit test suite

  • conformance_suite_micro_main test suite

  • SHOC test suite

  • CHIP-SPV test suite

Please note that not necessarily all the tests currently pass in the suites, we just ensure the currently passing ones do not regress with new commits (expected failing ones are disabled or skipped). The primary test platform is x86-64.

The latest LLVM release is given priority when testing, and we cannot guarantee older LLVM versions keep working over pocl releases due to the constantly changing library API.

If you would like get your favourite OpenCL-using project’s test suite included in the tier-1 suite, please send a pull request that adds the suite under the ‘examples’ dir and the main CMakeLists.txt along with instructions (a README will do) on how to setup it so it is included in the ‘make check’ run. Please make the test suite short enough to be suitable for frequent “smoke testing” (under 5 minutes per typical run preferred). If your favourite project is already under ‘example’, but not listed as a tier-1 test suite, please update its status so that ‘make check’ passes with the current HEAD of pocl and let us know, and we do our best to add it.

Release management

We aim to make a new release according to the Clang/LLVM release schedule.

For each release, a release manager is assigned. Release manager is responsible for creating and uploading new release candidate tar balls and requesting for testers from different platforms. After a release candidate round with success reports and no failure reports, a release is published.

See the maintenance-policy for the current release criteria.

A checklist and hints for testing and making a release successfully:

  • Check that notes_<VERSION>.rst in doc/sphinx/source has the most interesting updates done during the release cycle. Add missing changes from git log.

  • Create a single commit in master branch: change the version to the release one (without -pre), in all relevant places (doc/**/, CMakeLists.txt, etc); update the .so version (if required); check that supported LLVM versions in cmake/LLVM.cmake are correct. Create the release branch from this commit and push it to github.

  • In the master branch, create a new commit: increase version number (with -pre) in all relevant places; update the .so version; increase the supported LLVM versions in cmake/LLVM.cmake. Commit, push master to github. Now development can go on in master while the release branch is being stabilized.

  • The previous two steps ensure that merge-base of release & master is the start of release branch, which ensures that merging release to the master will not screw up the version numbers in the master. Bugs which need to be fixed in both branches, should be committed to the release branch, then release branch merged to master.

  • Create a new release on Github. Mark it as pre-release. This should create both a tarball and a git tag.

  • Upload the package to via SFTP or to the sourceforge file listing for the pocl project.

  • Request for testers in Twitter and/or mailing list. Point the testers to send their test reports to you privately or by adding them to the wiki. A good way is to create a wiki page for the release schedule and a test log. See for an example.

  • To publish a release, create a new release on Github without the checking the pre-release checkbox. Upload the tar ball to the sourceforge download page and to

  • Update the CHANGES and ANNOUNCEMENT text files in these directories. ANNOUNCEMENT is a copy of the latest release notes. A direct link to it can be easily circulated in IRC, for example.

  • Update the web page with the release information.

  • Advertise everywhere you can. At least in Twitter and the mailing list.

In case of any problems, ask any previous release manager for help. Previous releases were managed by the following pocl developers:

  • 0.14: Pekka Jääskeläinen

  • 0.11: Michal Babej

  • 0.10: Pekka Jääskeläinen

  • 0.9: Kalle Raiskila

  • 0.8: Erik Schnetter

  • 0.6 and 0.7: Pekka Jääskeläinen