[OpenMP][Offloading] Add a CMake argument LIBOMPTARGET_LIT_ARGS to control behavior of libomptarget lit test

[tianshilei1992]

Summary

By default, lit uses all threads to invoke tests, which can easily cause out
of memory on GPUs because most of OpenMP offloading test usually take about 1GB
GPU memory, but a typical GPU only has 4-8GB memory. This patch introduce a
CMake argument LIBOMPTARGET_LIT_ARGS to allow users to control the behavior of
libomptarget tests, similar to LLVM_LIT_ARGS.

[JonChesterfield]

Could we add this to the openmp docs at the same time?

Aside from whitespace churn, this adds ARGS string to the invocations, which looks plausible.

[tianshilei1992]

rebase and fix comments

[JonChesterfield]

Thanks!

openmp/README.rst

  **LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES** = ``35``
   List of CUDA compute capabilities that should be supported by the NVPTX
   device RTL. E.g. for compute capabilities 6.0 and 7.0, the option "60;70"
   should be used. Compute capability 3.5 is the minimum required.
 
  **LIBOMPTARGET_NVPTX_DEBUG** = ``OFF|ON``
   Enable printing of debug messages from the NVPTX device RTL.
 
+**LIBOMPTARGET_LIT_ARGS** = ``""``
+  Arguments given to lit. ``make check-libomptarget`` and
+  ``make check-libomptarget-*`` are affected. For example, use
+  ``LIBOMPTARGET_LIT_ARGS="-j4"`` to force ``lit`` to start only four parallel
+  jobs instead of by default the number of threads in the system.
+
 Example Usages of CMake
 =======================
 
 Typical Invocations
 -------------------
 
 .. code-block:: console
 

openmp/libomptarget/test/CMakeLists.txt

if(LIBOMPTARGET_ENABLE_DEBUG)
   set(LIBOMPTARGET_DEBUG True)
 else()
   set(LIBOMPTARGET_DEBUG False)
 endif()
 
 string(REGEX MATCHALL "([^\ ]+\ |[^\ ]+$)" SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS}")
 foreach(CURRENT_TARGET IN LISTS SYSTEM_TARGETS)
   string(STRIP "${CURRENT_TARGET}" CURRENT_TARGET)
-  add_openmp_testsuite(check-libomptarget-${CURRENT_TARGET} "Running libomptarget tests" ${CMAKE_CURRENT_BINARY_DIR}/${CURRENT_TARGET} DEPENDS omptarget omp ${LIBOMPTARGET_TESTED_PLUGINS})
+  add_openmp_testsuite(check-libomptarget-${CURRENT_TARGET}
+    "Running libomptarget tests"
+    ${CMAKE_CURRENT_BINARY_DIR}/${CURRENT_TARGET}
+    DEPENDS omptarget omp ${LIBOMPTARGET_TESTED_PLUGINS}
+    ARGS ${LIBOMPTARGET_LIT_ARGS})
   list(APPEND LIBOMPTARGET_LIT_TESTSUITES ${CMAKE_CURRENT_BINARY_DIR}/${CURRENT_TARGET})
 
   # Configure the lit.site.cfg.in file
   set(AUTO_GEN_COMMENT "## Autogenerated by libomptarget configuration.\n# Do not edit!")
   get_filename_component(CUDA_LIBDIR "${CUDA_cudart_static_LIBRARY}" DIRECTORY)
   configure_file(lit.site.cfg.in ${CURRENT_TARGET}/lit.site.cfg @ONLY)
 endforeach()
 
-add_openmp_testsuite(check-libomptarget "Running libomptarget tests" ${LIBOMPTARGET_LIT_TESTSUITES} EXCLUDE_FROM_CHECK_ALL DEPENDS omptarget omp ${LIBOMPTARGET_TESTED_PLUGINS})
+add_openmp_testsuite(check-libomptarget
+  "Running libomptarget tests"
+  ${LIBOMPTARGET_LIT_TESTSUITES}
+  EXCLUDE_FROM_CHECK_ALL
+  DEPENDS omptarget omp ${LIBOMPTARGET_TESTED_PLUGINS}
+  ARGS ${LIBOMPTARGET_LIT_ARGS})