[openmp] [elf_common] Fix linking against LLVM dylib

[mgorny]

Summary

The hand-rolled linking logic in elf_common does not account for
the possibility of using LLVM dylib rather than a dozen static
libraries. Since it does not seem to be easily convertible
to add_llvm_library, just hand-roll support for LLVM_LINK_LLVM_DYLIB.
This is necessary to support stand-alone builds against installed LLVM.

[JonChesterfield]

Thanks! These cmake files (libomptarget) seem to be quite high maintenance. I don't know whether it's possible to reuse cmake functions from the rest of llvm instead of iterating on these

[mgorny]

In D111038#3039250, @JonChesterfield wrote:

Thanks! These cmake files (libomptarget) seem to be quite high maintenance. I don't know whether it's possible to reuse cmake functions from the rest of llvm instead of iterating on these

I don't really know. Back in the day, I think the idea was that openmp isn't supposed to depend on LLVM but this seems to be no longer the case.

[JonChesterfield]

Current state is libomp builds without use of llvm, libomptarget requires llvm. Initially just headers, but partway through implementing yet another elf wrapper library (for windows) that policy changed to depending on llvm directly. Aside from some rough edges in the build scripts this doesn't seem to have broken anyone.

openmp/libomptarget/plugins/common/elf_common/CMakeLists.txt

 ##===----------------------------------------------------------------------===##
 
 add_library(elf_common OBJECT elf_common.cpp)
 
 # Build elf_common with PIC to be able to link it with plugin shared libraries.
 set_property(TARGET elf_common PROPERTY POSITION_INDEPENDENT_CODE ON)
 llvm_update_compile_flags(elf_common)
 set(LINK_LLVM_LIBS LLVMBinaryFormat LLVMObject LLVMSupport)
+if (LLVM_LINK_LLVM_DYLIB)
+  set(LINK_LLVM_LIBS LLVM)
+endif()
 target_link_libraries(elf_common INTERFACE ${LINK_LLVM_LIBS})
 include_directories(${LIBOMPTARGET_LLVM_INCLUDE_DIRS})
 add_dependencies(elf_common ${LINK_LLVM_LIBS})
 
 # The code uses Debug.h, which requires threads support.
 target_link_libraries(elf_common INTERFACE ${OPENMP_PTHREAD_LIB})
 
 # Expose elf_common.h directory to the users of this library.
 target_include_directories(elf_common INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})