[flang-rt] Use --as-needed for linking flang-rt libraries. (#130856)

This change makes sure that there is no unnecessary library
dependencies like libc++/libstdc++.

Author: vzakhari

flang-rt/CMakeLists.txt

@@ -244,6 +244,12 @@ check_cxx_compiler_flag("-UTESTFLAG" FLANG_RT_SUPPORTS_UNDEFINE_FLAG)
 # Check whether -fno-lto is supported.
 check_cxx_compiler_flag(-fno-lto FLANG_RT_HAS_FNO_LTO_FLAG)
 
+# Check whether -Wl,--as-needed is supported.
+check_linker_flag(C "LINKER:--as-needed" LINKER_SUPPORTS_AS_NEEDED)
+if (LINKER_SUPPORTS_AS_NEEDED)
+  set(LINKER_AS_NEEDED_OPT "LINKER:--as-needed")
+endif()
+
 # Different platform may have different name for the POSIX thread library.
 # For example, libpthread.a on AIX. Search for it as it is needed when
 # building the shared flang_rt.runtime.so.

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -145,6 +145,20 @@ function (add_flangrt_library name)
     if (Threads_FOUND) 
       target_link_libraries(${name_shared} PUBLIC Threads::Threads)
     endif ()
+
+    # Special dependencies handling for shared libraries only:
+    #
+    # flang-rt libraries must not depend on libc++/libstdc++,
+    # so set the linker language to C to avoid the unnecessary
+    # library dependence. Note that libc++/libstdc++ may still
+    # come through CMAKE_CXX_IMPLICIT_LINK_LIBRARIES.
+    set_target_properties(${name_shared} PROPERTIES LINKER_LANGUAGE C)
+    # Use --as-needed to avoid unnecessary dependencies.
+    if (LINKER_AS_NEEDED_OPT)
+      target_link_options(${name_shared} BEFORE PRIVATE
+        "${LINKER_AS_NEEDED_OPT}"
+        )
+    endif()
   endif ()
 
   if (libtargets)