[flang-rt] Use --as-needed for linking flang-rt libraries.

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

Author: vzakhari

flang-rt/CMakeLists.txt

@@ -245,6 +245,12 @@ 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 "-Wl,--as-needed" LINKER_SUPPORTS_AS_NEEDED)
+if (LINKER_SUPPORTS_AS_NEEDED)
+  set(LINKER_AS_NEEDED_OPT "-Wl,--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.
@@ -295,7 +301,6 @@ elseif (FLANG_RT_GCC_RESOURCE_DIR)
 endif ()
 
 
-
 #####################
 # Build Preparation #
 #####################

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -186,6 +186,17 @@ function (add_flangrt_library name)
     else ()
       set_target_properties(${tgtname} PROPERTIES FOLDER "Flang-RT/Libraries")
     endif ()
+
+    # flang-rt libraries must not depend on libc++/libstdc++,
+    # so set the linker language to C to avoid the unnecessary
+    # library dependence. Note that libc++/libstdc++ may still
+    # come through CMAKE_CXX_IMPLICIT_LINK_LIBRARIES.
+    set_target_properties(${tgtname} PROPERTIES LINKER_LANGUAGE C)
+    # Use --as-needed to avoid unnecessary dependencies.
+    if (LINKER_AS_NEEDED_OPT)
+      set_property(TARGET ${tgtname} APPEND_STRING PROPERTY
+        LINK_FLAGS "${LINKER_AS_NEEDED_OPT}")
+    endif()
   endforeach ()
 
   # Define how to compile and link the library.