[libomptarget][cuda] Detect missing symbols in plugin at build time

[JonChesterfield]

Summary

[libomptarget][cuda] Detect missing symbols in plugin at build time

Passes -z,defs to the linker. Error on unresolved symbol references.

Otherwise, those unresolved symbols present as target code running on the host
as the plugin fails to load. This is significantly harder to debug than a link
time error. Flag matches that passed by amdgcn and ve plugins.

[tianshilei1992]

This is a nice change, but not all plugins can implement all interfaces listed in exports. For example, if the plugin doesn't support async operations, then those interfaces suffix with _async will not be implemented by the plugin, and libomptarget can acknowledge that because dlopen returns nullptr.

[JonChesterfield]

True but orthogonal. This will raise a link error on libomptarget.rtl.cuda.so if it has symbols that are not resolved by the dynamic libraries it is linked against. E.g. amdgpu links with this and doesn't yet implement the device to device copy function.

Otherwise, the same missing symbol causes dlopen to fail when libomptarget tries to load the plugin.

edit: s/dlopen/dlsym/

[tianshilei1992]

LGTM.

openmp/libomptarget/plugins/cuda/CMakeLists.txt

 add_library(omptarget.rtl.cuda SHARED src/rtl.cpp)
 
 # Install plugin under the lib destination folder.
 install(TARGETS omptarget.rtl.cuda LIBRARY DESTINATION "${OPENMP_INSTALL_LIBDIR}")
 
 target_link_libraries(omptarget.rtl.cuda
   ${LIBOMPTARGET_DEP_CUDA_DRIVER_LIBRARIES}
   ${LIBOMPTARGET_DEP_LIBELF_LIBRARIES}
-  "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/../exports")
+  "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/../exports"
+  "-Wl,-z,defs")
 
 # Report to the parent scope that we are building a plugin for CUDA.
 set(LIBOMPTARGET_SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS} nvptx64-nvidia-cuda" PARENT_SCOPE)