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This introduces a remote offloading plugin for libomptarget. This implementation relies on gRPC and protobuf, so this library will only build if both libraries are available on the system. The corresponding server is compiled to `openmp-offloading-server`. This is a large change, but the only way to split this up is into RTL/server but I fear that could introduce an inconsistency amongst them. Ideally, tests for this should be added to the current ones that but that is problematic for at least one reason. Given that libomptarget registers plugin on a first-come-first-serve basis, if we wanted to offload onto a local x86 through a different process, then we'd have to either re-order the plugin list in `rtl.cpp` (which is what I did locally for testing) or find a better solution for runtime plugin registration in libomptarget. Differential Revision: https://reviews.llvm.org/D95314
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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. |