[libc++] Populated library dependencies recursively for linker script

[ismailp]

Summary

When LIBCXX_ENABLE_PARALLEL_ALGORITHMS is set to ON, libc++ will try
to link with pstl::ParallelSTL. However, pstl::ParallelSTL is an
(alias to an) interface library, and OUTPUT_NAME property isn't white
listed on such targets. pstl::ParallelSTL uses TBB::tbb as an
interface library. Therefore, libc++ must link with whatever TBB::tbb
points at.

[ldionne]

Would creating a non-interface library for the PSTL solve the problem? I think so, right? If so, this should be preferred as we're going to need some non header-only content anyway when implementing other backends.

libcxx/cmake/Modules/DefineLinkerScript.cmake

+# This function tries to find library file name without the prefix and the
+# extension.
+#
+# Arguments:
+#   <tgt>: Target name.
+#   <var>: Output variable that the result be written into.
+
+function(get_imported_library_file_base_name tgt var)
+  get_target_property(imported_loc ${tgt} IMPORTED_LOCATION)
+
+  if (NOT imported_loc)
+    get_target_property(imported_loc_debug ${tgt} IMPORTED_LOCATION_DEBUG)
+    get_target_property(imported_loc_release ${tgt} IMPORTED_LOCATION_RELEASE)
+    if (NOT imported_loc_debug AND NOT imported_loc_release)
+      message(FATAL_ERROR "Cannot determine import location for library ${tgt}")
+    endif()
+    set(imported_loc "$<$<CONFIG:DEBUG>:${imported_loc_debug}>$<$<CONFIG:RELEASE>:${imported_loc_release}>")
+  endif()
+
+  get_filename_component(lib_name "${imported_loc}" NAME_WE)
+  unset(imported_loc)
+  string(LENGTH "${CMAKE_SHARED_LIBRARY_PREFIX}" prefix_length)
+
+  if (prefix_length)
+    string(FIND "${lib_name}" "${CMAKE_SHARED_LIBRARY_PREFIX}" prefix_pos)
+    if (NOT prefix_pos EQUAL 0)
+      message(FATAL_ERROR "Library name '${tgt}' doesn't start with expected prefix")
+    endif(NOT prefix_pos EQUAL 0)
+    unset(prefix_pos)
+    string(SUBSTRING "${lib_name}" ${prefix_length} -1 dep_lib_base_name)
+    set(${var} "${dep_lib_base_name}" PARENT_SCOPE)
+  endif(prefix_length)
+endfunction()
+
+# This function builds list of dependencies for given target.
+#
+# Arguments:
+#   <tgt>: Target to build list of dependencies for.
+#   <var>: Output variable that the list be written into.
+
+function(get_dependencies_recursive tgt var)
+  if (TARGET ${tgt})
+    get_target_property(my_deps ${tgt} INTERFACE_LINK_LIBRARIES)
+    if (NOT my_deps)
+      return()
+    endif()
+  endif()
+
+  foreach(dep ${my_deps})
+    if (TARGET ${dep})
+      get_target_property(lib_type ${dep} TYPE)
+    endif()
+
+    if (lib_type STREQUAL "INTERFACE_LIBRARY" OR
+        lib_type STREQUAL "STATIC_LIBRARY" OR
+        lib_type STREQUAL "SHARED_LIBRARY")
+      set(transitive_deps)
+      get_dependencies_recursive(${dep} transitive_deps)
+
+      list(APPEND result ${transitive_deps})
+
+      if (NOT lib_type STREQUAL "INTERFACE_LIBRARY")
+        get_target_property(is_imported ${dep} IMPORTED)
+        # The generator expression $<TARGET_FILE_BASE_NAME:${dep}> yields the
+        # target's name when applied to an imported target. We will try to get
+        # its imported location and do some post-processing to obtain a usable
+        # file name.
+        if (is_imported)
+          get_imported_library_file_base_name(${dep} dep_file_base_name)
+          list(APPEND result "${CMAKE_LINK_LIBRARY_FLAG}${dep_file_base_name}")
+          unset(dep_file_base_name)
+        else(is_imported)
+          list(APPEND result "${CMAKE_LINK_LIBRARY_FLAG}$<TARGET_FILE_BASE_NAME:${dep}>")
+        endif(is_imported)
+        unset(is_imported)
+      endif()
+    endif()
+    unset(lib_type)
+  endforeach()
+  set(${var} ${result} PARENT_SCOPE)
+endfunction()
+
 # This function defines a linker script in place of the symlink traditionally
 # created for shared libraries.
 #
 # More specifically, this function goes through the PUBLIC and INTERFACE
 # library dependencies of <target> and gathers them into a linker script,
 # such that those libraries are linked against when the shared library for
 # <target> is linked against.
 #
function(define_linker_script target)
   get_target_property(interface_libs "${target}" INTERFACE_LINK_LIBRARIES)
 
   set(link_libraries)
   if (interface_libs)
     foreach(lib IN LISTS interface_libs)
       if (TARGET "${lib}" OR
           (${lib} MATCHES "cxxabi(_static|_shared)?" AND HAVE_LIBCXXABI) OR
           (${lib} MATCHES "unwind(_static|_shared)?" AND HAVE_LIBUNWIND))
-        list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}$<TARGET_PROPERTY:${lib},OUTPUT_NAME>")
+        get_dependencies_recursive(${lib} recursive_deps)
+        list(APPEND link_libraries "${recursive_deps}")
       else()
         list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}${lib}")
       endif()
     endforeach()
   endif()
   string(REPLACE ";" " " link_libraries "${link_libraries}")
 
   set(linker_script "INPUT(${soname} ${link_libraries})")
   add_custom_command(TARGET "${target}" POST_BUILD
     COMMAND "${CMAKE_COMMAND}" -E remove "${symlink}"
     COMMAND "${CMAKE_COMMAND}" -E echo "${linker_script}" > "${symlink}"
     COMMENT "Generating linker script: '${linker_script}' as file ${symlink}"
     VERBATIM
   )
 endfunction()

pstl/CMakeLists.txt

 
 if (PSTL_PARALLEL_BACKEND STREQUAL "serial")
     message(STATUS "Parallel STL uses the serial backend")
     set(_PSTL_PAR_BACKEND_SERIAL ON)
 elseif (PSTL_PARALLEL_BACKEND STREQUAL "tbb")
     find_package(TBB 2018 REQUIRED tbb OPTIONAL_COMPONENTS tbbmalloc)
     message(STATUS "Parallel STL uses TBB ${TBB_VERSION} (interface version: ${TBB_INTERFACE_VERSION})")
     target_link_libraries(ParallelSTL INTERFACE TBB::tbb)
+    set_target_properties(TBB::tbb PROPERTIES IMPORTED_GLOBAL TRUE)
     set(_PSTL_PAR_BACKEND_TBB ON)
 else()
     message(FATAL_ERROR "Requested unknown Parallel STL backend '${PSTL_PARALLEL_BACKEND}'.")
 endif()
 
 set(PSTL_GENERATED_HEADERS_DIR "${CMAKE_CURRENT_BINARY_DIR}/generated_headers")
 set(PSTL_CONFIG_SITE_PATH "${PSTL_GENERATED_HEADERS_DIR}/__pstl_config_site")
 configure_file("include/__pstl_config_site.in"