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[OpenMP][Documentation] Add FAQ entry for CMake module

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This patch adds documentation for using the CMake find module for OpenMP
target offloading provided by LLVM. It also removes the requirement for
AMD's architecture to be set as this isn't necessary for upstream LLVM.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D105051
This commit is contained in:
Joseph Huber 2021-06-28 15:34:15 -04:00
parent c9f3240c9d
commit 2190c48fde
3 changed files with 77 additions and 37 deletions
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108
openmp/docs/SupportAndFAQ.rst
View File

@ -53,14 +53,15 @@ Q: How to build an OpenMP GPU offload capable compiler?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To build an *effective* OpenMP offload capable compiler, only one extra CMake
option, `LLVM_ENABLE_RUNTIMES="openmp"`, is needed when building LLVM (Generic
information about building LLVM is available `here <https://llvm.org/docs/GettingStarted.html>`__.).
Make sure all backends that are targeted by OpenMP to be enabled. By default,
Clang will be built with all backends enabled.
When building with `LLVM_ENABLE_RUNTIMES="openmp"` OpenMP should not be enabled
in `LLVM_ENABLE_PROJECTS` because it is enabled by default.
information about building LLVM is available `here
<https://llvm.org/docs/GettingStarted.html>`__.). Make sure all backends that
are targeted by OpenMP to be enabled. By default, Clang will be built with all
backends enabled. When building with `LLVM_ENABLE_RUNTIMES="openmp"` OpenMP
should not be enabled in `LLVM_ENABLE_PROJECTS` because it is enabled by
default.
For Nvidia offload, please see :ref:`_build_nvidia_offload_capable_compiler`.
For AMDGPU offload, please see :ref:`_build_amdgpu_offload_capable_compiler`.
For Nvidia offload, please see :ref:`build_nvidia_offload_capable_compiler`.
For AMDGPU offload, please see :ref:`build_amdgpu_offload_capable_compiler`.
.. note::
The compiler that generates the offload code should be the same (version) as
@ -86,41 +87,51 @@ available GPUs failed, you should also set:
Q: How to build an OpenMP AMDGPU offload capable compiler?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A subset of the `ROCm <https://github.com/radeonopencompute>` toolchain is
A subset of the `ROCm <https://github.com/radeonopencompute>`_ toolchain is
required to build the LLVM toolchain and to execute the openmp application.
Either install ROCm somewhere that cmake's find_package can locate it, or
build the required subcomponents ROCt and ROCr from source.
The two components used are ROCT-Thunk-Interface, roct, and ROCR-Runtime,
rocr. Roct is the userspace part of the linux driver. It calls into the
driver which ships with the linux kernel. It is an implementation detail of
Rocr from OpenMP's perspective. Rocr is an implementation of `HSA <http://www.hsafoundation.com>`.
The two components used are ROCT-Thunk-Interface, roct, and ROCR-Runtime, rocr.
Roct is the userspace part of the linux driver. It calls into the driver which
ships with the linux kernel. It is an implementation detail of Rocr from
OpenMP's perspective. Rocr is an implementation of `HSA
<http://www.hsafoundation.com>`_.
SOURCE_DIR=same-as-llvm-source # e.g. the checkout of llvm-project, next to openmp
BUILD_DIR=somewhere
INSTALL_PREFIX=same-as-llvm-install
cd $SOURCE_DIR
git clone git@github.com:RadeonOpenCompute/ROCT-Thunk-Interface.git -b roc-4.1.x --single-branch
git clone git@github.com:RadeonOpenCompute/ROCR-Runtime.git -b rocm-4.1.x --single-branch
cd $BUILD_DIR && mkdir roct && cd roct
cmake $SOURCE_DIR/ROCT-Thunk-Interface/ -DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF
make && make install
.. code-block:: text
cd $BUILD_DIR && mkdir rocr && cd rocr
cmake $SOURCE_DIR/ROCR-Runtime/src -DIMAGE_SUPPORT=OFF -DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON
make && make install
SOURCE_DIR=same-as-llvm-source # e.g. the checkout of llvm-project, next to openmp
BUILD_DIR=somewhere
INSTALL_PREFIX=same-as-llvm-install
cd $SOURCE_DIR
git clone git@github.com:RadeonOpenCompute/ROCT-Thunk-Interface.git -b roc-4.1.x \
--single-branch
git clone git@github.com:RadeonOpenCompute/ROCR-Runtime.git -b rocm-4.1.x \
--single-branch
cd $BUILD_DIR && mkdir roct && cd roct
cmake $SOURCE_DIR/ROCT-Thunk-Interface/ -DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX \
-DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF
make && make install
IMAGE_SUPPORT requires building rocr with clang and is not used by openmp.
cd $BUILD_DIR && mkdir rocr && cd rocr
cmake $SOURCE_DIR/ROCR-Runtime/src -DIMAGE_SUPPORT=OFF \
-DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX -DCMAKE_BUILD_TYPE=Release \
-DBUILD_SHARED_LIBS=ON
make && make install
``IMAGE_SUPPORT`` requires building rocr with clang and is not used by openmp.
Provided cmake's find_package can find the ROCR-Runtime package, LLVM will
build a tool `bin/amdgpu-arch` which will print a string like 'gfx906' when
build a tool ``bin/amdgpu-arch`` which will print a string like ``gfx906`` when
run if it recognises a GPU on the local system. LLVM will also build a shared
library, libomptarget.rtl.amdgpu.so, which is linked against rocr.
With those libraries installed, then LLVM build and installed, try:
.. code-block:: shell
clang -O2 -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa example.c -o example && ./example
Q: What are the known limitations of OpenMP AMDGPU offload?
@ -153,8 +164,8 @@ For now, the answer is most likely *no*. Please see :ref:`build_offload_capable_
Q: Does Clang support `<math.h>` and `<complex.h>` operations in OpenMP target on GPUs?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Yes, LLVM/Clang allows math functions and complex arithmetic inside of OpenMP target regions
that are compiled for GPUs.
Yes, LLVM/Clang allows math functions and complex arithmetic inside of OpenMP
target regions that are compiled for GPUs.
Clang provides a set of wrapper headers that are found first when `math.h` and
`complex.h`, for C, `cmath` and `complex`, for C++, or similar headers are
@ -202,8 +213,8 @@ an error like this.
Currently, the only solution is to change how the application is built and avoid
the use of static libraries.
Q: Can I use dynamically linked libraries with OpenMP offloading
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Q: Can I use dynamically linked libraries with OpenMP offloading?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dynamically linked libraries can be only used if there is no device code split
between the library and application. Anything declared on the device inside the
@ -220,3 +231,36 @@ correct GCC toolchain in the second stage of the build.
For example, if your system-wide GCC installation is too old to build LLVM and
you would like to use a newer GCC, set the CMake variable `GCC_INSTALL_PREFIX`
to inform clang of the GCC installation you would like to use in the second stage.
Q: How can I include OpenMP offloading support in my CMake project?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Currently, there is an experimental CMake find module for OpenMP target
offloading provided by LLVM. It will attempt to find OpenMP target offloading
support for your compiler. The flags necessary for OpenMP target offloading will
be loaded into the ``OpenMPTarget::OpenMPTarget_<device>`` target or the
``OpenMPTarget_<device>_FLAGS`` variable if successful. Currently supported
devices are ``AMDGCN`` and ``NVPTX``.
To use this module, simply add the path to CMake's current module path and call
``find_package``. The module will be installed with your OpenMP installation by
default. Including OpenMP offloading support in an application should now only
require a few additions.
.. code-block:: cmake
cmake_minimum_required(VERSION 3.13.4)
project(offloadTest VERSION 1.0 LANGUAGES CXX)
list(APPEND CMAKE_MODULE_PATH "${PATH_TO_OPENMP_INSTALL}/lib/cmake/openmp")
find_package(OpenMPTarget REQUIRED NVPTX)
add_executable(offload)
target_link_libraries(offload PRIVATE OpenMPTarget::OpenMPTarget_NVPTX)
target_sources(offload PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src/Main.cpp)
Using this module requires at least CMake version 3.13.4. Supported languages
are C and C++ with Fortran support planned in the future. Compiler support is
best for Clang but this module should work for other compiler vendors such as
IBM, GNU.

4
openmp/tools/Modules/FindOpenMPTarget.cmake
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@ -140,10 +140,6 @@ endfunction()
# Get flags for setting the device's architecture for each compiler.
function(_OPENMP_TARGET_DEVICE_ARCH_CANDIDATES LANG DEVICE DEVICE_FLAG)
# AMD requires the architecture, default to gfx908 if not provided.
if((NOT OpenMPTarget_${DEVICE}_ARCH) AND ("${DEVICE}" STREQUAL "AMDGCN"))
set(OpenMPTarget_${DEVICE}_ARCH "gfx908")
endif()
if(OpenMPTarget_${DEVICE}_ARCH)
# Only Clang supports selecting the architecture for now.
set(OMPTarget_ARCH_Clang "-Xopenmp-target=${DEVICE_FLAG} -march=${OpenMPTarget_${DEVICE}_ARCH}")

2
openmp/tools/Modules/README.rst
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@ -15,7 +15,7 @@ This module will attempt to find OpenMP target offloading support for a given
device. The module will attempt to compile a test program using known compiler
flags for each requested architecture. If successful, the flags required for
offloading will be loaded into the ``OpenMPTarget::OpenMPTarget_<device>``
target or the ``OpenMPTarget_NVPTX_FLAGS`` variable. Currently supported target
target or the ``OpenMPTarget_<device>_FLAGS`` variable. Currently supported target
devices are ``NVPTX`` and ``AMDGCN``. This module is still under development so
some features may be missing.