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[OpenMP][Docs] Updated the faq about building an OpenMP offloading capable compiler 

After some issues about building runtimes along with LLVM were fixed,
building an OpenMP offloading capable compiler is pretty simple. This patch updates
the FAQ part in the doc.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93671
This commit is contained in:
Shilei Tian 2020-12-22 13:14:34 -05:00
parent 1432ae57bf
commit 612ddc3117
1 changed files with 5 additions and 16 deletions
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21
openmp/docs/SupportAndFAQ.rst
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@ -52,22 +52,11 @@ All patches go through the regular `LLVM review process
Q: How to build an OpenMP offload capable compiler?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To build an *effective* OpenMP offload capable compiler we recommend a two
stage build. The first stage Clang does not require to be offload capable but
all backends that are targeted by OpenMP need to be enabled. By default, Clang
will be build with all backends enabled. This initial (stage 1) Clang is used
to create a second Clang compiler that is offload capable as well as the
:ref:`device runtime libraries <device_runtime>` that will be linked into the
offloaded code to provide OpenMP runtime support on the device.
Generic information about building LLVM is available `here
<https://llvm.org/docs/GettingStarted.html>`__. The CMake options for the
second stage Clang should include:
- `LIBOMPTARGET_NVPTX_CUDA_COMPILER=$STAGE1/bin/clang` to use the stage one
compiler for the device runtime compilation.
- `LIBOMPTARGET_NVPTX_ENABLE_BCLIB=ON` to enable efficient device runtimes in
bitcode format.
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 build with all backends enabled.
If your build machine is not the target machine or automatic detection of the
available GPUs failed, you should also set: