[cmake] Add LLVM_UNITTEST_LINK_FLAGS option

[nikic]

Summary

Add an option to specify additional linker flags for unit tests only. For example, this allows doing something like -DLLVM_UNITTEST_LINK_FLAGS="-Wl,-plugin-opt=O0" if you're doing LTO builds, or -DLLVM_UNITTEST_LINK_FLAGS="-fno-lto" if you're using fat LTO objects.

The build system already does this itself if the LLVM_ENABLE_LTO flag is used, but this does not cover all possible LTO configurations. (E.g. we're using ld.bfd with LLVMgold.so, which isn't one of the supported configurations.)

[phosek]

llvm/cmake/modules/AddLLVM.cmake

1613–1614

For variables that are intended to be used by users, I'd prefer to keep them in the top-level CMakeLists.txt file for easier discoverability.

1615–1616

LINK_FLAGS property is deprecated and superseded by LINK_OPTIONS, see https://cmake.org/cmake/help/latest/prop_tgt/LINK_FLAGS.html. Can you use target_link_options instead?

[nikic]

llvm/cmake/modules/AddLLVM.cmake

1613–1614

The reason I put it here is that the option is shared across all subprojects using unit tests, so just putting it in llvm/CMakeLists.txt wouldn't work. Maybe HandleLLVMOptions would be the right place to define such a shared option?

[nikic]

Move option to HandleLLVMOptions, use target_link_options.

[phosek]

LGTM

llvm/cmake/modules/AddLLVM.cmake

elseif(LINKER_IS_LLD_LINK)
       set_property(TARGET ${test_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " /opt:lldlto=0")
     elseif(APPLE AND NOT uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
       set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " -Wl,-mllvm,-O0")
     endif()
   endif()
 
+  target_link_options(${test_name} PRIVATE "${LLVM_UNITTEST_LINK_FLAGS}")
+
   set(outdir ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR})
   set_output_directory(${test_name} BINARY_DIR ${outdir} LIBRARY_DIR ${outdir})
   # libpthreads overrides some standard library symbols, so main
   # executable must be linked with it in order to provide consistent
   # API for all shared libaries loaded by this executable.
   target_link_libraries(${test_name} PRIVATE llvm_gtest_main llvm_gtest ${LLVM_PTHREAD_LIB})
 
   add_dependencies(${test_suite} ${test_name})
   get_target_property(test_suite_folder ${test_suite} FOLDER)
   if (test_suite_folder)

llvm/cmake/modules/HandleLLVMOptions.cmake

if(LLVM_USE_RELATIVE_PATHS_IN_FILES)
   append_if(SUPPORTS_FFILE_PREFIX_MAP "-ffile-prefix-map=${CMAKE_BINARY_DIR}=${relative_root}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   append_if(SUPPORTS_FFILE_PREFIX_MAP "-ffile-prefix-map=${source_root}/=${LLVM_SOURCE_PREFIX}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   add_flag_if_supported("-no-canonical-prefixes" NO_CANONICAL_PREFIXES)
 endif()
 
 set(LLVM_THIRD_PARTY_DIR  ${CMAKE_CURRENT_SOURCE_DIR}/../third-party CACHE STRING
     "Directory containing third party software used by LLVM (e.g. googletest)")
 
+set(LLVM_UNITTEST_LINK_FLAGS "" CACHE STRING
+    "Additional linker flags for unit tests")
+
 if(LLVM_ENABLE_LLVM_LIBC)
   check_library_exists(llvmlibc printf "" HAVE_LLVM_LIBC)
   if(NOT HAVE_LLVM_LIBC)
     message(WARNING "Unable to link against LLVM libc. LLVM will be built without linking against the LLVM libc overlay.")
   endif()
 endif()