[OpenMP] Update FAQ for enabling cuda offloading

Add an FAQ entry and add a few lines to an existing one. Document
the use of `GCC_INSTALL_PREFIX` for pointing clang to correct
GCC installation for two-stage build.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D104474

diff --git a/openmp/docs/SupportAndFAQ.rst b/openmp/docs/SupportAndFAQ.rst
index a469b5e..2eb15d2 100644 (file)
--- a/openmp/docs/SupportAndFAQ.rst
+++ b/openmp/docs/SupportAndFAQ.rst
@@ -56,6 +56,8 @@ 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`.
@@ -206,3 +208,15 @@ 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+Q: How to build an OpenMP offload capable compiler with an outdated host compiler?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.