[HIP] Fix libclang_rt.builtins.a not found (#532)

* The solution to the libclang_rt builtins problem has changed with newer version of ROCm, as used by the Codeplay AMD plugin. The old suggested solution does not work.
* This PR:
  * Suggests a new solution, with accompanying minor CMake work.
  * Updates documentation.

cmake/FindCompiler.cmake

@@ -69,4 +69,15 @@ if(is_dpcpp)
  INTERFACE_LINK_LIBRARIES ${SYCL_LIBRARY})
  endif()
 
-endif()
+ if(ENABLE_ROCBLAS_BACKEND OR ENABLE_ROCRAND_BACKEND OR ENABLE_ROCSOLVER_BACKEND)
+ # Allow find_package(HIP) to find the correct path to libclang_rt.builtins.a
+ # HIP's CMake uses the command `${HIP_CXX_COMPILER} -print-libgcc-file-name --rtlib=compiler-rt` to find this path.
+ # This can print a non-existing file if the compiler used is icpx.
+ if(NOT HIP_CXX_COMPILER)
+ find_path(HIP_CXX_COMPILER clang++
+ HINTS ENV HIPROOT ENV ROCM_PATH
+ )
+ endif()
+ endif()
+
+endif(is_dpcpp)

docs/building_the_project_with_dpcpp.rst

@@ -457,11 +457,14 @@ Build FAQ
 
 clangrt builtins lib not found
  Encountered when trying to build oneMKL with some ROCm libraries. There are
- several possible solutions: * If building Open DPC++ from source, add
- ``compiler-rt`` to the external projects compile option:
- ``--llvm-external-projects compiler-rt``. * The *clangrt* from ROCm can be
- used, depending on ROCm version: ``export
- LIBRARY_PATH=/path/to/rocm-$rocm-version$/llvm/lib/clang/$clang-version$/lib/linux/:$LIBRARY_PATH``
+ several possible solutions:
+
+ * If building Open DPC++ from source, add ``compiler-rt`` to the external
+ projects compile option: ``--llvm-external-projects compiler-rt``.
+ * Manually set the variable ``HIP_CXX_COMPILER`` to HIP's toolkit ``clang++``
+ path, for instance ``-DHIP_CXX_COMPILER=/opt/rocm/6.1.0/llvm/bin/clang++``.
+ oneMKL may fail to link if the clang versions of ``icpx`` and ``rocm`` are
+ not compatible.
 
 Could NOT find CBLAS (missing: CBLAS file)
  Encountered when tests are enabled along with the BLAS domain. The tests