[lld] Support building shared libLLD.so

[mgorny]

Summary

Support building a single shared library in addition to split libraries.
The concept and code is copied from clang's libclang-cpp.so. This will
permit building external tools against LLD without having to resort
to static linking or BUILD_SHARED_LIBS.

Support linking LLD to the dylib to avoid duplication.

[mgorny]

Ping.

[mgorny]

@ruiu, any chance you could review this?

[mgorny]

@thakis, you seem to have been touching lld recently.

[MaskRay]

lld/tools/lld-shlib/CMakeLists.txt

16

Hmm. I think LLVM_BUILD_LLVM_DYLIB & CLANG_BUILD_CLANG_DYLIB are incompatible with BUILD_SHARED_LIBS

24

libclang-cpp.so?

27

Minimum CMake version has been bumped

lld/tools/lld-shlib/lld-shlib.cpp

1

Can you just merge lld-shlib/* to the main CMakeLists.txt to avoid a directory?

[mgorny]

lld/tools/lld-shlib/lld-shlib.cpp

1

I suppose I could but is this really desirable?

[thakis]

From what I understand, linking lld as a shared library is an explicit non-goal of lld. It trusts inputs and crashes on bad inputs, since it's designed to be run as a separate process.

Can you say more about what you want this for? It seems like something we don't want to do.

[mgorny]

In D85278#2434383, @thakis wrote:

From what I understand, linking lld as a shared library is an explicit non-goal of lld. It trusts inputs and crashes on bad inputs, since it's designed to be run as a separate process.

Can you say more about what you want this for? It seems like something we don't want to do.

It's for zig. It apparently tries to link to LLD's static libraries, and we really don't want to install static libraries on Gentoo.

[tstellar]

In D85278#2434383, @thakis wrote:

From what I understand, linking lld as a shared library is an explicit non-goal of lld. It trusts inputs and crashes on bad inputs, since it's designed to be run as a separate process.

Can you say more about what you want this for? It seems like something we don't want to do.

We ship the lld libraries and headers in Fedora too, and this change would be an improvement (1 shared library is better than having many). As @mgorny said there is at least one project already using lld this way. If lld is really not meant to be used as a library, then this needs to be formalized some way with documentation and by removing the install targets for the libraries and headers. I don't personally have an opinion about whether or not lld should be used as a library, but if we do have libraries I prefer libLLD.so over what we have now.

[tstellar]

I think this could be simplified by using object libraries like this patch: https://reviews.llvm.org/D95727 does for libLLVM.so

[mgorny]

I'm not going to put any more effort into this. If you have time and motivation to work on this, feel free to take it over.

lld/CMakeLists.txt

if (VTUNE_FOUND)
     list(APPEND LLVM_COMMON_LIBS ${VTune_LIBRARIES})
     add_definitions(-DLLD_HAS_VTUNE)
   endif()
 endif()
 
 option(LLD_BUILD_TOOLS
   "Build the lld tools. If OFF, just generate build targets." ON)
 
+set(LLD_LINK_LLD_DYLIB ${LLVM_LINK_LLVM_DYLIB} CACHE BOOL
+    "Link tools against libLLD.so")
+
+if (NOT LLVM_LINK_LLVM_DYLIB AND LLD_LINK_LLD_DYLIB)
+  message(FATAL_ERROR "Cannot set LLD_LINK_LLD_DYLIB=ON when "
+                      "LLVM_LINK_LLVM_DYLIB=OFF")
+endif()
+
 if (MSVC)
   add_definitions(-wd4530) # Suppress 'warning C4530: C++ exception handler used, but unwind semantics are not enabled.'
   add_definitions(-wd4062) # Suppress 'warning C4062: enumerator X in switch of enum Y is not handled' from system header.
 endif()
 
 include_directories(BEFORE
   ${CMAKE_CURRENT_BINARY_DIR}/include
   ${CMAKE_CURRENT_SOURCE_DIR}/include
   )
 
 if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   install(DIRECTORY include/
     DESTINATION include
     FILES_MATCHING
     PATTERN "*.h"
     PATTERN ".svn" EXCLUDE
     )
 endif()
 
 add_subdirectory(Common)
 add_subdirectory(lib)
-add_subdirectory(tools/lld)
 
 if (LLVM_INCLUDE_TESTS)
   add_subdirectory(test)
   add_subdirectory(unittests)
 endif()
 
 add_subdirectory(docs)
 add_subdirectory(COFF)
 add_subdirectory(ELF)
 add_subdirectory(MachO)
 add_subdirectory(MinGW)
 add_subdirectory(wasm)
 
+add_subdirectory(tools/lld)
+add_subdirectory(tools/lld-shlib)
+
 add_subdirectory(cmake/modules)

lld/cmake/modules/AddLLD.cmake

 macro(add_lld_library name)
   cmake_parse_arguments(ARG
-    "SHARED"
+    "SHARED;INSTALL_WITH_TOOLCHAIN"
     ""
     ""
     ${ARGN})
   if(ARG_SHARED)
-    set(ARG_ENABLE_SHARED SHARED)
+    set(LIBTYPE SHARED)
+  else()
+    set(LIBTYPE STATIC OBJECT)
+    set_property(GLOBAL APPEND PROPERTY LLD_STATIC_LIBS ${name})
   endif()
-  llvm_add_library(${name} ${ARG_ENABLE_SHARED} ${ARG_UNPARSED_ARGUMENTS})
+  llvm_add_library(${name} ${LIBTYPE} ${ARG_UNPARSED_ARGUMENTS})
   set_target_properties(${name} PROPERTIES FOLDER "lld libraries")
 
-  if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
+  if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY OR ARG_INSTALL_WITH_TOOLCHAIN)
     if(${name} IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR
         NOT LLVM_DISTRIBUTION_COMPONENTS)
       set(export_to_lldtargets EXPORT LLDTargets)
       set_property(GLOBAL PROPERTY LLD_HAS_EXPORTS True)
     endif()
 
     install(TARGETS ${name}
       COMPONENT ${name}

lld/tools/lld-shlib/CMakeLists.txt

+# Building libLLD.so fails if LLVM_ENABLE_PIC=Off
+if (NOT LLVM_ENABLE_PIC)
+  return()
+endif()
+
+get_property(lld_libs GLOBAL PROPERTY LLD_STATIC_LIBS)
+
+foreach (lib ${lld_libs})
+  if(XCODE)
+    # Xcode doesn't support object libraries, so we have to trick it into
+    # linking the static libraries instead.
+    list(APPEND _DEPS "-force_load" ${lib})
+  else()
+    list(APPEND _OBJECTS $<TARGET_OBJECTS:obj.${lib}>)
+  endif()
+  if (BUILD_SHARED_LIBS)
+    # If we are building static libraries, then we don't need to add the static
+    # libraries as a depedency, because we are already linking against the
+    # individual object files.
+    list(APPEND _DEPS $<TARGET_PROPERTY:${lib},INTERFACE_LINK_LIBRARIES>)
+  endif()
+
+  # lld libraries are redundant since we are linking all the individual
+  # object files into libclang-cpp.so, so filter them out from _DEPS.
+  # This avoids problems with LLVM global data when building with
+  # BUILD_SHARED_LIBS=ON
+  # FIXME: We could use list(FILTER) with cmake >= 3.6
+  # FIXME: With cmake >= 3.15 we could use the generator expression
+  # $<FILTER:list,INCLUDE|EXCLUDE,regex>
+  get_target_property(interface ${lib} LINK_LIBRARIES)
+  if (interface)
+    foreach(lib ${interface})
+      if (NOT ${lib} MATCHES "^LLD")
+        list(APPEND _DEPS ${lib})
+      endif()
+    endforeach()
+  endif()
+endforeach ()
+
+if (LLD_LINK_LLD_DYLIB)
+  set(INSTALL_WITH_TOOLCHAIN INSTALL_WITH_TOOLCHAIN)
+endif()
+
+add_lld_library(LLD
+                SHARED
+                ${INSTALL_WITH_TOOLCHAIN}
+                lld-shlib.cpp
+                ${_OBJECTS}
+                LINK_LIBS
+                ${_DEPS})

lld/tools/lld-shlib/lld-shlib.cpp

+// Intentionally empty source file to make CMake happy

lld/tools/lld/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   Support
   )
 
 add_lld_tool(lld
   lld.cpp
 
   SUPPORT_PLUGINS
   )
 export_executable_symbols_for_plugins(lld)
 
+if (LLD_LINK_LLD_DYLIB)
+  target_link_libraries(lld
+    PRIVATE
+    LLD)
+else()
   target_link_libraries(lld
     PRIVATE
     lldCommon
     lldCOFF
     lldDriver
     lldELF
     lldMachO2
     lldMinGW
     lldWasm
     )
+endif()
 
 install(TARGETS lld
   RUNTIME DESTINATION bin)
 
 if(NOT LLD_SYMLINKS_TO_CREATE)
   set(LLD_SYMLINKS_TO_CREATE lld-link ld.lld ld64.lld wasm-ld)
 endif()
 
 foreach(link ${LLD_SYMLINKS_TO_CREATE})
   add_lld_symlink(${link} lld)
 endforeach()