[libc++][CMake] Refactor how we link against system libraries

[ldionne]

Summary

Instead of populating the global LIBCXX_LIBRARIES, we use the link-time
dependency management built into CMake to propagate link flags. This
leads to a cleaner and easier-to-follow build.

[ldionne]

libcxx/lib/CMakeLists.txt

394

This is now handled by CMake automatically, because benchmarks link against cxx_shared or cxx_static, and those have PUBLIC link dependencies.

[smeenai]

This is awesome!

Can we get rid of add_library_flags and add_library_flags_if now?

libcxx/benchmarks/CMakeLists.txt

 set(libcxx_benchmark_targets)
 
 function(add_benchmark_test name source_file)
   set(libcxx_target ${name}_libcxx)
   list(APPEND libcxx_benchmark_targets ${libcxx_target})
   add_executable(${libcxx_target} EXCLUDE_FROM_ALL ${source_file})
   add_dependencies(${libcxx_target} cxx cxx-headers google-benchmark-libcxx)
   add_dependencies(cxx-benchmarks ${libcxx_target})
-  target_link_libraries(${libcxx_target} ${LIBCXX_LIBRARIES})
   if (LIBCXX_ENABLE_SHARED)
     target_link_libraries(${libcxx_target} cxx_shared)
   else()
     target_link_libraries(${libcxx_target} cxx_static)
   endif()
   if (TARGET cxx_experimental)
     target_link_libraries(${libcxx_target} cxx_experimental)
   endif()

libcxx/cmake/Modules/HandleLibcxxFlags.cmake

if(${condition})
     add_library_flags(${ARGN})
   endif()
 endmacro()
 
 # Turn a comma separated CMake list into a space separated string.
 macro(split_list listname)
   string(REPLACE ";" " " ${listname} "${${listname}}")
 endmacro()
+
+# For each specified flag, add that link flag to the provided target.
+# The flags are added with the given visibility, i.e. PUBLIC|PRIVATE|INTERFACE.
+function(target_add_link_flags_if_supported target visibility)
+  foreach(flag ${ARGN})
+    mangle_name("${flag}" flagname)
+    check_cxx_compiler_flag("${flag}" "LIBCXX_SUPPORTS_${flagname}_FLAG")
+    if (LIBCXX_SUPPORTS_${flagname}_FLAG)
+      target_link_libraries(${target} ${visibility} ${flag})
+    endif()
+  endforeach()
+endfunction()
+
+# For each specified flag, add that compile flag to the provided target.
+# The flags are added with the given visibility, i.e. PUBLIC|PRIVATE|INTERFACE.
+function(target_add_compile_flags_if_supported target visibility)
+  foreach(flag ${ARGN})
+    mangle_name("${flag}" flagname)
+    check_cxx_compiler_flag("${flag}" "LIBCXX_SUPPORTS_${flagname}_FLAG")
+    if (LIBCXX_SUPPORTS_${flagname}_FLAG)
+      target_compile_options(${target} ${visibility} ${flag})
+    endif()
+  endforeach()
+endfunction()

libcxx/lib/CMakeLists.txt

if (LIBFILE)
     set(LIBCXX_SANITIZER_LIBRARY "${LIBDIR}/${LIBFILE}")
     set(LIBCXX_SANITIZER_LIBRARY "${LIBCXX_SANITIZER_LIBRARY}" PARENT_SCOPE)
     message(STATUS "Manually linking compiler-rt library: ${LIBCXX_SANITIZER_LIBRARY}")
     add_library_flags("${LIBCXX_SANITIZER_LIBRARY}")
     add_link_flags("-Wl,-rpath,${LIBDIR}")
   endif()
 endif()
 
-# Generate private library list.
-add_library_flags_if(LIBCXX_HAS_PTHREAD_LIB pthread)
-add_library_flags_if(LIBCXX_HAS_C_LIB c)
-add_library_flags_if(LIBCXX_HAS_M_LIB m)
-add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
+function(cxx_link_system_libraries target)
+  target_add_link_flags_if_supported(${target} PRIVATE "-nodefaultlibs")
+  target_add_compile_flags_if_supported(${target} PRIVATE "/Zl")
+  target_add_link_flags_if_supported(${target} PRIVATE "/nodefaultlib")
+
+  if (LIBCXX_HAS_PTHREAD_LIB)
+    target_link_libraries(${target} PUBLIC pthread)
+  endif()
+
+  if (LIBCXX_HAS_C_LIB)
+    target_link_libraries(${target} PUBLIC c)
+  endif()
+
+  if (LIBCXX_HAS_M_LIB)
+    target_link_libraries(${target} PUBLIC m)
+  endif()
+
+  if (LIBCXX_HAS_RT_LIB)
+    target_link_libraries(${target} PUBLIC rt)
+  endif()
+
   if (LIBCXX_USE_COMPILER_RT)
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)
-  add_library_flags_if(LIBCXX_BUILTINS_LIBRARY "${LIBCXX_BUILTINS_LIBRARY}")
-else()
-  add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
+    if (LIBCXX_BUILTINS_LIBRARY)
+      target_link_libraries(${target} PUBLIC "${LIBCXX_BUILTINS_LIBRARY}")
+    endif()
+  elseif (LIBCXX_HAS_GCC_S_LIB)
+    target_link_libraries(${target} PUBLIC gcc_s)
+  endif()
+
+  if (LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
+    target_link_libraries(${target} PUBLIC atomic)
   endif()
-add_library_flags_if(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB atomic)
-add_library_flags_if(MINGW "${MINGW_LIBRARIES}")
 
-# Setup flags.
-add_link_flags_if_supported(-nodefaultlibs)
+  if (MINGW)
+    target_link_libraries(${target} PUBLIC "${MINGW_LIBRARIES}")
+  endif()
 
   if (LIBCXX_TARGETING_MSVC)
     if (LIBCXX_DEBUG_BUILD)
       set(LIB_SUFFIX "d")
     else()
       set(LIB_SUFFIX "")
     endif()
-  add_compile_flags(/Zl)
-  add_link_flags(/nodefaultlib)
 
-  add_library_flags(ucrt${LIB_SUFFIX}) # Universal C runtime
-  add_library_flags(vcruntime${LIB_SUFFIX}) # C++ runtime
-  add_library_flags(msvcrt${LIB_SUFFIX}) # C runtime startup files
-  add_library_flags(msvcprt${LIB_SUFFIX}) # C++ standard library. Required for exception_ptr internals.
+    target_link_libraries(${target} PUBLIC ucrt${LIB_SUFFIX}) # Universal C runtime
+    target_link_libraries(${target} PUBLIC vcruntime${LIB_SUFFIX}) # C++ runtime
+    target_link_libraries(${target} PUBLIC msvcrt${LIB_SUFFIX}) # C runtime startup files
+    target_link_libraries(${target} PUBLIC msvcprt${LIB_SUFFIX}) # C++ standard library. Required for exception_ptr internals.
     # Required for standards-complaint wide character formatting functions
     # (e.g. `printfw`/`scanfw`)
-  add_library_flags(iso_stdio_wide_specifiers)
+    target_link_libraries(${target} PUBLIC iso_stdio_wide_specifiers)
   endif()
+endfunction()
 
 function(cxx_set_common_defines name)
   if(LIBCXX_CXX_ABI_HEADER_TARGET)
     add_dependencies(${name} ${LIBCXX_CXX_ABI_HEADER_TARGET})
   endif()
   if(WIN32 AND NOT MINGW)
     target_compile_definitions(${name}
                                PRIVATE

 split_list(LIBCXX_LINK_FLAGS)
 
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(cxx_shared)
   endif()
+  cxx_link_system_libraries(cxx_shared)
   target_link_libraries(cxx_shared PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
       VERSION       "${LIBCXX_ABI_VERSION}.0"
       SOVERSION     "${LIBCXX_ABI_VERSION}"
       DEFINE_SYMBOL ""
   )
   cxx_set_common_defines(cxx_shared)
 
   # Link against LLVM libunwind
   if (LIBCXXABI_USE_LLVM_UNWINDER)
     if (NOT LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY AND (TARGET unwind_shared OR HAVE_LIBUNWIND))
-      target_link_libraries(cxx_shared PRIVATE unwind_shared)
+      target_link_libraries(cxx_shared PUBLIC unwind_shared)
       list(APPEND LIBCXX_INTERFACE_LIBRARIES unwind_shared) # For the linker script
     elseif (LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY AND (TARGET unwind_static OR HAVE_LIBUNWIND))
       # libunwind is already included in libc++abi
     else()
       target_link_libraries(cxx_shared PRIVATE unwind)
       list(APPEND LIBCXX_INTERFACE_LIBRARIES unwind) # For the linker script
     endif()
   endif()
 
   # Link against libc++abi
   if (LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY)
     if (APPLE)
       target_link_libraries(cxx_shared PRIVATE "-Wl,-force_load" "${LIBCXX_CXX_STATIC_ABI_LIBRARY}")
     else()
       target_link_libraries(cxx_shared PRIVATE "-Wl,--whole-archive,-Bstatic" "${LIBCXX_CXX_STATIC_ABI_LIBRARY}" "-Wl,-Bdynamic,--no-whole-archive")
     endif()
   else()
-    target_link_libraries(cxx_shared PRIVATE "${LIBCXX_CXX_SHARED_ABI_LIBRARY}")
+    target_link_libraries(cxx_shared PUBLIC "${LIBCXX_CXX_SHARED_ABI_LIBRARY}")
     list(APPEND LIBCXX_INTERFACE_LIBRARIES "${LIBCXX_CXX_SHARED_ABI_LIBRARY}") # For the linker script
-    list(APPEND LIBCXX_LIBRARIES "${LIBCXX_CXX_SHARED_ABI_LIBRARY}") # For the benchmarks
   endif()
 
   # Maybe re-export symbols from libc++abi
   if (APPLE AND (LIBCXX_CXX_ABI_LIBNAME STREQUAL "libcxxabi" OR
                  LIBCXX_CXX_ABI_LIBNAME STREQUAL "default")
             AND NOT DEFINED LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS)
     set(LIBCXX_OSX_REEXPORT_LIBCXXABI_SYMBOLS ON)
   endif()
if(WIN32 AND NOT MINGW AND NOT "${CMAKE_HOST_SYSTEM_NAME}" STREQUAL "Windows")
     set_target_properties(cxx_shared PROPERTIES
                           APPEND_STRING PROPERTY LINK_FLAGS " /MANIFEST:NO")
   endif()
 endif()
 
 # Build the static library.
 if (LIBCXX_ENABLE_STATIC)
   add_library(cxx_static STATIC ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
+  cxx_link_system_libraries(cxx_static)
   target_link_libraries(cxx_static PRIVATE ${LIBCXX_LIBRARIES})
   set(CMAKE_STATIC_LIBRARY_PREFIX "lib")
   set_target_properties(cxx_static
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
   )
add_custom_target(install-cxx-stripped
                               ${experimental_lib_install_target}
                               ${header_install_target}
                       COMMAND "${CMAKE_COMMAND}"
                       -DCMAKE_INSTALL_COMPONENT=cxx
                       -DCMAKE_INSTALL_DO_STRIP=1
                       -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")
     add_custom_target(install-libcxx DEPENDS install-cxx)
 endif()
-
-# TODO: This is needed by cxx-benchmarks but this variable isn't
-# available outside of the scope of this file so we need to export
-# it. This is not necessarily the cleanest solution.
-set(LIBCXX_LIBRARIES ${LIBCXX_LIBRARIES} PARENT_SCOPE)