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Support, Getting Involved, and FAQ
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==================================
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Please do not hesitate to reach out to us via openmp-dev@lists.llvm.org or join
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one of our :ref:`regular calls <calls>`. Some common questions are answered in
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the :ref:`faq`.
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.. _calls:
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Calls
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-----
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OpenMP in LLVM Technical Call
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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- Development updates on OpenMP (and OpenACC) in the LLVM Project, including Clang, optimization, and runtime work.
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- Join `OpenMP in LLVM Technical Call <https://bluejeans.com/544112769//webrtc>`__.
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- Time: Weekly call on every Wednesday 7:00 AM Pacific time.
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- Meeting minutes are `here <https://docs.google.com/document/d/1Tz8WFN13n7yJ-SCE0Qjqf9LmjGUw0dWO9Ts1ss4YOdg/edit>`__.
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- Status tracking `page <https://openmp.llvm.org/docs>`__.
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OpenMP in Flang Technical Call
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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- Development updates on OpenMP and OpenACC in the Flang Project.
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- Join `OpenMP in Flang Technical Call <https://bit.ly/39eQW3o>`_
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- Time: Weekly call on every Thursdays 8:00 AM Pacific time.
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- Meeting minutes are `here <https://docs.google.com/document/d/1yA-MeJf6RYY-ZXpdol0t7YoDoqtwAyBhFLr5thu5pFI>`__.
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- Status tracking `page <https://docs.google.com/spreadsheets/d/1FvHPuSkGbl4mQZRAwCIndvQx9dQboffiD-xD0oqxgU0/edit#gid=0>`__.
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.. _faq:
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FAQ
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---
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.. note::
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The FAQ is a work in progress and most of the expected content is not
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yet available. While you can expect changes, we always welcome feedback and
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additions. Please contact, e.g., through ``openmp-dev@lists.llvm.org``.
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Q: How to contribute a patch to the webpage or any other part?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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All patches go through the regular `LLVM review process
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<https://llvm.org/docs/Contributing.html#how-to-submit-a-patch>`_.
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.. _build_offload_capable_compiler:
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Q: How to build an OpenMP offload capable compiler?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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To build an *effective* OpenMP offload capable compiler, only one extra CMake
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option, `LLVM_ENABLE_RUNTIMES="openmp"`, is needed when building LLVM (Generic
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information about building LLVM is available `here <https://llvm.org/docs/GettingStarted.html>`__.).
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Make sure all backends that are targeted by OpenMP to be enabled. By default,
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Clang will be built with all backends enabled.
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If your build machine is not the target machine or automatic detection of the
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available GPUs failed, you should also set:
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- `CLANG_OPENMP_NVPTX_DEFAULT_ARCH=sm_XX` where `XX` is the architecture of your GPU, e.g, 80.
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- `LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES=YY` where `YY` is the numeric compute capacity of your GPU, e.g., 75.
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.. note::
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The compiler that generates the offload code should be the same (version) as
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the compiler that builds the OpenMP device runtimes. The OpenMP host runtime
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can be built by a different compiler.
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.. _advanced_builds: https://llvm.org//docs/AdvancedBuilds.html
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Q: Does OpenMP offloading support work in pre-packaged LLVM releases?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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For now, the answer is most likely *no*. Please see :ref:`build_offload_capable_compiler`.
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Q: Does OpenMP offloading support work in packages distributed as part of my OS?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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For now, the answer is most likely *no*. Please see :ref:`build_offload_capable_compiler`.
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.. _math_and_complex_in_target_regions:
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Q: Does Clang support `<math.h>` and `<complex.h>` operations in OpenMP target on GPUs?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Yes, LLVM/Clang allows math functions and complex arithmetic inside of OpenMP target regions
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that are compiled for GPUs.
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Clang provides a set of wrapper headers that are found first when `math.h` and
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`complex.h`, for C, `cmath` and `complex`, for C++, or similar headers are
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included by the application. These wrappers will eventually include the system
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version of the corresponding header file after setting up a target device
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specific environment. The fact that the system header is included is important
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because they differ based on the architecture and operating system and may
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contain preprocessor, variable, and function definitions that need to be
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available in the target region regardless of the targeted device architecture.
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However, various functions may require specialized device versions, e.g.,
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`sin`, and others are only available on certain devices, e.g., `__umul64hi`. To
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provide "native" support for math and complex on the respective architecture,
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Clang will wrap the "native" math functions, e.g., as provided by the device
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vendor, in an OpenMP begin/end declare variant. These functions will then be
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picked up instead of the host versions while host only variables and function
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definitions are still available. Complex arithmetic and functions are support
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through a similar mechanism. It is worth noting that this support requires
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`extensions to the OpenMP begin/end declare variant context selector
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<https://clang.llvm.org/docs/AttributeReference.html#pragma-omp-declare-variant>`__
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that are exposed through LLVM/Clang to the user as well.
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Q: What is a way to debug errors from mapping memory to a target device?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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An experimental way to debug these errors is to use :ref:`remote process
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offloading <remote_offloading_plugin>`.
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By using ``libomptarget.rtl.rpc.so`` and ``openmp-offloading-server``, it is
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possible to explicitly perform memory transfers between processes on the host
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CPU and run sanitizers while doing so in order to catch these errors.
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Q: Why does my application say "Named symbol not found" and abort when I run it?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This is most likely caused by trying to use OpenMP offloading with static
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libraries. Static libraries do not contain any device code, so when the runtime
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attempts to execute the target region it will not be found and you will get an
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an error like this.
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.. code-block:: text
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CUDA error: Loading '__omp_offloading_fd02_3231c15__Z3foov_l2' Failed
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CUDA error: named symbol not found
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Libomptarget error: Unable to generate entries table for device id 0.
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Currently, the only solution is to change how the application is built and avoid
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the use of static libraries.
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