[OpenMP] Update FAQ with additional instructions for enabling cuda offloading

[ashermancinelli]

Summary

Add an FAQ entry and add a few lines to another one. Document the use of CCC_OVERRIDE_OPTIONS for injecting GCC toolchain path.

[jhuber6]

Thanks for improving the documentation. Some comments, the documentation should be generic over specific cmake invocations / gcc versions and errors.

openmp/docs/SupportAndFAQ.rst

61

I'd just say something like "If building OpenMP with LLVM_ENABLE_RUNTIMES, OpenMP should not be enabled as a project in LLVM_ENABLE_PROJECTS because it is enabled by default."

222

No need to be specific to GCC 9.1.0.

229

Don't give a specific cmake invocation, we can just refer to the existing documentation for setting up CMake as above.

[ashermancinelli]

Remove CMake example. Use more generic language.

[ashermancinelli]

I've hopefully addressed your comments. Thanks for the review @jhuber6!

[ashermancinelli]

Document use of CMake variable GCC_INSTALL_PREFIX over ugly workaround.

[jhuber6]

openmp/docs/SupportAndFAQ.rst

217

No longer relevant.

[ashermancinelli]

Update language to use cmake variables over ugly environment variable workaround.

openmp/docs/SupportAndFAQ.rst

 
 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.
 
 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
   the compiler that builds the OpenMP device runtimes. The OpenMP host runtime
   can be built by a different compiler.
 

 the use of static libraries.
 
 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
 shared library will not be visible to the application when it's linked.
+
+Q: How to build an OpenMP offload capable compiler with an outdated host compiler?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Enabling the OpenMP runtime will perform a two-stage build for you.
+If your host compiler is different from your system-wide compiler, you may need
+to set the CMake variable `GCC_INSTALL_PREFIX` so clang will be able to find the
+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.