1Support, Getting Involved, and FAQ 2================================== 3 4Please do not hesitate to reach out to us via openmp-dev@lists.llvm.org or join 5one of our :ref:`regular calls <calls>`. Some common questions are answered in 6the :ref:`faq`. 7 8.. _calls: 9 10Calls 11----- 12 13OpenMP in LLVM Technical Call 14^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 15 16- Development updates on OpenMP (and OpenACC) in the LLVM Project, including Clang, optimization, and runtime work. 17- Join `OpenMP in LLVM Technical Call <https://bluejeans.com/544112769//webrtc>`__. 18- Time: Weekly call on every Wednesday 7:00 AM Pacific time. 19- Meeting minutes are `here <https://docs.google.com/document/d/1Tz8WFN13n7yJ-SCE0Qjqf9LmjGUw0dWO9Ts1ss4YOdg/edit>`__. 20- Status tracking `page <https://openmp.llvm.org/docs>`__. 21 22 23OpenMP in Flang Technical Call 24^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 25- Development updates on OpenMP and OpenACC in the Flang Project. 26- Join `OpenMP in Flang Technical Call <https://bit.ly/39eQW3o>`_ 27- Time: Weekly call on every Thursdays 8:00 AM Pacific time. 28- Meeting minutes are `here <https://docs.google.com/document/d/1yA-MeJf6RYY-ZXpdol0t7YoDoqtwAyBhFLr5thu5pFI>`__. 29- Status tracking `page <https://docs.google.com/spreadsheets/d/1FvHPuSkGbl4mQZRAwCIndvQx9dQboffiD-xD0oqxgU0/edit#gid=0>`__. 30 31 32.. _faq: 33 34FAQ 35--- 36 37.. note:: 38 The FAQ is a work in progress and most of the expected content is not 39 yet available. While you can expect changes, we always welcome feedback and 40 additions. Please contact, e.g., through ``openmp-dev@lists.llvm.org``. 41 42 43Q: How to contribute a patch to the webpage or any other part? 44^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 45 46All patches go through the regular `LLVM review process 47<https://llvm.org/docs/Contributing.html#how-to-submit-a-patch>`_. 48 49 50.. _build_offload_capable_compiler: 51 52Q: How to build an OpenMP GPU offload capable compiler? 53^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 54To build an *effective* OpenMP offload capable compiler, only one extra CMake 55option, `LLVM_ENABLE_RUNTIMES="openmp"`, is needed when building LLVM (Generic 56information about building LLVM is available `here 57<https://llvm.org/docs/GettingStarted.html>`__.). Make sure all backends that 58are targeted by OpenMP to be enabled. By default, Clang will be built with all 59backends enabled. When building with `LLVM_ENABLE_RUNTIMES="openmp"` OpenMP 60should not be enabled in `LLVM_ENABLE_PROJECTS` because it is enabled by 61default. 62 63For Nvidia offload, please see :ref:`build_nvidia_offload_capable_compiler`. 64For AMDGPU offload, please see :ref:`build_amdgpu_offload_capable_compiler`. 65 66.. note:: 67 The compiler that generates the offload code should be the same (version) as 68 the compiler that builds the OpenMP device runtimes. The OpenMP host runtime 69 can be built by a different compiler. 70 71.. _advanced_builds: https://llvm.org//docs/AdvancedBuilds.html 72 73.. _build_nvidia_offload_capable_compiler: 74 75Q: How to build an OpenMP NVidia offload capable compiler? 76^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 77The Cuda SDK is required on the machine that will execute the openmp application. 78 79If your build machine is not the target machine or automatic detection of the 80available GPUs failed, you should also set: 81 82- `CLANG_OPENMP_NVPTX_DEFAULT_ARCH=sm_XX` where `XX` is the architecture of your GPU, e.g, 80. 83- `LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES=YY` where `YY` is the numeric compute capacity of your GPU, e.g., 75. 84 85 86.. _build_amdgpu_offload_capable_compiler: 87 88Q: How to build an OpenMP AMDGPU offload capable compiler? 89^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 90A subset of the `ROCm <https://github.com/radeonopencompute>`_ toolchain is 91required to build the LLVM toolchain and to execute the openmp application. 92Either install ROCm somewhere that cmake's find_package can locate it, or 93build the required subcomponents ROCt and ROCr from source. 94 95The two components used are ROCT-Thunk-Interface, roct, and ROCR-Runtime, rocr. 96Roct is the userspace part of the linux driver. It calls into the driver which 97ships with the linux kernel. It is an implementation detail of Rocr from 98OpenMP's perspective. Rocr is an implementation of `HSA 99<http://www.hsafoundation.com>`_. 100 101.. code-block:: text 102 103 SOURCE_DIR=same-as-llvm-source # e.g. the checkout of llvm-project, next to openmp 104 BUILD_DIR=somewhere 105 INSTALL_PREFIX=same-as-llvm-install 106 107 cd $SOURCE_DIR 108 git clone git@github.com:RadeonOpenCompute/ROCT-Thunk-Interface.git -b roc-4.1.x \ 109 --single-branch 110 git clone git@github.com:RadeonOpenCompute/ROCR-Runtime.git -b rocm-4.1.x \ 111 --single-branch 112 113 cd $BUILD_DIR && mkdir roct && cd roct 114 cmake $SOURCE_DIR/ROCT-Thunk-Interface/ -DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX \ 115 -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF 116 make && make install 117 118 cd $BUILD_DIR && mkdir rocr && cd rocr 119 cmake $SOURCE_DIR/ROCR-Runtime/src -DIMAGE_SUPPORT=OFF \ 120 -DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX -DCMAKE_BUILD_TYPE=Release \ 121 -DBUILD_SHARED_LIBS=ON 122 make && make install 123 124``IMAGE_SUPPORT`` requires building rocr with clang and is not used by openmp. 125 126Provided cmake's find_package can find the ROCR-Runtime package, LLVM will 127build a tool ``bin/amdgpu-arch`` which will print a string like ``gfx906`` when 128run if it recognises a GPU on the local system. LLVM will also build a shared 129library, libomptarget.rtl.amdgpu.so, which is linked against rocr. 130 131With those libraries installed, then LLVM build and installed, try: 132 133.. code-block:: shell 134 135 clang -O2 -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa example.c -o example && ./example 136 137Q: What are the known limitations of OpenMP AMDGPU offload? 138^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 139LD_LIBRARY_PATH is presently required to find the openmp libraries. 140 141There is no libc. That is, malloc and printf do not exist. Also no libm, so 142functions like cos(double) will not work from target regions. 143 144Cards from the gfx10 line, 'navi', that use wave32 are not yet implemented. 145 146Some versions of the driver for the radeon vii (gfx906) will error unless the 147environment variable 'export HSA_IGNORE_SRAMECC_MISREPORT=1' is set. 148 149It is a recent addition to LLVM and the implementation differs from that which 150has been shipping in ROCm and AOMP for some time. Early adopters will encounter 151bugs. 152 153Q: Does OpenMP offloading support work in pre-packaged LLVM releases? 154^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 155For now, the answer is most likely *no*. Please see :ref:`build_offload_capable_compiler`. 156 157Q: Does OpenMP offloading support work in packages distributed as part of my OS? 158^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 159For now, the answer is most likely *no*. Please see :ref:`build_offload_capable_compiler`. 160 161 162.. _math_and_complex_in_target_regions: 163 164Q: Does Clang support `<math.h>` and `<complex.h>` operations in OpenMP target on GPUs? 165^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 166 167Yes, LLVM/Clang allows math functions and complex arithmetic inside of OpenMP 168target regions that are compiled for GPUs. 169 170Clang provides a set of wrapper headers that are found first when `math.h` and 171`complex.h`, for C, `cmath` and `complex`, for C++, or similar headers are 172included by the application. These wrappers will eventually include the system 173version of the corresponding header file after setting up a target device 174specific environment. The fact that the system header is included is important 175because they differ based on the architecture and operating system and may 176contain preprocessor, variable, and function definitions that need to be 177available in the target region regardless of the targeted device architecture. 178However, various functions may require specialized device versions, e.g., 179`sin`, and others are only available on certain devices, e.g., `__umul64hi`. To 180provide "native" support for math and complex on the respective architecture, 181Clang will wrap the "native" math functions, e.g., as provided by the device 182vendor, in an OpenMP begin/end declare variant. These functions will then be 183picked up instead of the host versions while host only variables and function 184definitions are still available. Complex arithmetic and functions are support 185through a similar mechanism. It is worth noting that this support requires 186`extensions to the OpenMP begin/end declare variant context selector 187<https://clang.llvm.org/docs/AttributeReference.html#pragma-omp-declare-variant>`__ 188that are exposed through LLVM/Clang to the user as well. 189 190Q: What is a way to debug errors from mapping memory to a target device? 191^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 192 193An experimental way to debug these errors is to use :ref:`remote process 194offloading <remote_offloading_plugin>`. 195By using ``libomptarget.rtl.rpc.so`` and ``openmp-offloading-server``, it is 196possible to explicitly perform memory transfers between processes on the host 197CPU and run sanitizers while doing so in order to catch these errors. 198 199Q: Why does my application say "Named symbol not found" and abort when I run it? 200^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 201 202This is most likely caused by trying to use OpenMP offloading with static 203libraries. Static libraries do not contain any device code, so when the runtime 204attempts to execute the target region it will not be found and you will get an 205an error like this. 206 207.. code-block:: text 208 209 CUDA error: Loading '__omp_offloading_fd02_3231c15__Z3foov_l2' Failed 210 CUDA error: named symbol not found 211 Libomptarget error: Unable to generate entries table for device id 0. 212 213Currently, the only solution is to change how the application is built and avoid 214the use of static libraries. 215 216Q: Can I use dynamically linked libraries with OpenMP offloading? 217^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 218 219Dynamically linked libraries can be only used if there is no device code split 220between the library and application. Anything declared on the device inside the 221shared library will not be visible to the application when it's linked. 222 223Q: How to build an OpenMP offload capable compiler with an outdated host compiler? 224^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 225 226Enabling the OpenMP runtime will perform a two-stage build for you. 227If your host compiler is different from your system-wide compiler, you may need 228to set the CMake variable `GCC_INSTALL_PREFIX` so clang will be able to find the 229correct GCC toolchain in the second stage of the build. 230 231For example, if your system-wide GCC installation is too old to build LLVM and 232you would like to use a newer GCC, set the CMake variable `GCC_INSTALL_PREFIX` 233to inform clang of the GCC installation you would like to use in the second stage. 234 235Q: How can I include OpenMP offloading support in my CMake project? 236^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 237 238Currently, there is an experimental CMake find module for OpenMP target 239offloading provided by LLVM. It will attempt to find OpenMP target offloading 240support for your compiler. The flags necessary for OpenMP target offloading will 241be loaded into the ``OpenMPTarget::OpenMPTarget_<device>`` target or the 242``OpenMPTarget_<device>_FLAGS`` variable if successful. Currently supported 243devices are ``AMDGPU`` and ``NVPTX``. 244 245To use this module, simply add the path to CMake's current module path and call 246``find_package``. The module will be installed with your OpenMP installation by 247default. Including OpenMP offloading support in an application should now only 248require a few additions. 249 250.. code-block:: cmake 251 252 cmake_minimum_required(VERSION 3.13.4) 253 project(offloadTest VERSION 1.0 LANGUAGES CXX) 254 255 list(APPEND CMAKE_MODULE_PATH "${PATH_TO_OPENMP_INSTALL}/lib/cmake/openmp") 256 257 find_package(OpenMPTarget REQUIRED NVPTX) 258 259 add_executable(offload) 260 target_link_libraries(offload PRIVATE OpenMPTarget::OpenMPTarget_NVPTX) 261 target_sources(offload PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src/Main.cpp) 262 263Using this module requires at least CMake version 3.13.4. Supported languages 264are C and C++ with Fortran support planned in the future. Compiler support is 265best for Clang but this module should work for other compiler vendors such as 266IBM, GNU. 267