Make static linking for LLVM plugins more reliable.

[efriedma]

Summary

Linking a static library into a binary doesn't do anything if none of the symbols are used. For some combinations of plugin/host binary, we apparently get lucky (I think due to inline definitions from headers). But in general, it won't work.

To make sure this works properly, use an object library from CMake.

This is a bit hacky; suggestions welcome for a better way to solve the issue.

[serge-sans-paille]

llvm/cmake/modules/AddLLVM.cmake

928

In that case set_property(TARGET ${llvm_plugin_target} APPEND PROPERTY LINK_LIBRARIES ${llvm_extension}) is no longer needed, is it?

[Meinersbur]

Symbols should be pulled by the responsible tools, e.g. bugpoint: https://reviews.llvm.org/D61446#C1743754NL232 . AFAIK there is no requirements that plugins generate object libraries. For instance, in my typical build configuration, there are no "obj.LLVM*" targets. It depends on the build configuration I guess, whether shared objects and static libraries are both built.

[beanz]

Yea... this is not how plugin linking should work.

Plugin host tools should link against the libraries. We do pass linker flags where necessary to reduce symbol dead stripping, but there is no expectation that all static archive symbols are in the tools. Which is why plugins themselves should be linked against first the plugin loader tool, then the LLVM static libraries that they themselves rely on.

See the LLVMHello Transform library plugin as an example.

[efriedma]

Oh, I didn't realize there was an extra mechanism to pull in static plugins; RegisterStandardPasses doesn't depend on the callback.

I'll look into adding the appropriate calls to LTO.

[Meinersbur]

This is one of the things I insisted on in https://reviews.llvm.org/D61446#1681841. There was a previous discussion to also load plugins in lld for LTO (see https://reviews.llvm.org/D61446?id=198588#inline-548338), but we wanted to finish D61446 first.

[Meinersbur]

In my point of view, having only the new PM cause the plugin symbols to be pulled-in was sufficient, thinking that we will move away from the legacy pass manager anyway.

llvm/cmake/modules/AddLLVM.cmake

else()
       include(LLVMConfigExtensions)
   endif()
 
   # Add static plugins to each plugin target.
   foreach(llvm_extension ${LLVM_STATIC_EXTENSIONS})
     get_property(llvm_plugin_targets GLOBAL PROPERTY LLVM_PLUGIN_TARGETS)
     foreach(llvm_plugin_target ${llvm_plugin_targets})
       set_property(TARGET ${llvm_plugin_target} APPEND PROPERTY LINK_LIBRARIES ${llvm_extension})
+      set_property(TARGET ${llvm_plugin_target} APPEND PROPERTY LINK_LIBRARIES "obj.${llvm_extension}")
       set_property(TARGET ${llvm_plugin_target} APPEND PROPERTY INTERFACE_LINK_LIBRARIES ${llvm_extension})
     endforeach()
   endforeach()
 
   # Eventually generate the extension header, and store config to a cmake file
   # for usage in third-party configuration.
   if(ARG_GEN_CONFIG)
       set(LLVM_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm)