[SystemZ][ZOS] Prefer -nostdlib++ as opposed to -nodefaultlibs when building c++ libraries

Let's use -nostdlib++ rather than -nodefaultlibs when building libc++/libc++abi/libunwind libraries. The default is -nostdlib++ if supported by a build compiler like it is the case with clang, otherwise -nodefaultlibs is used as before.

This change is needed to avoid additional changes at the link step and not to increase the maintenance costs. If clang with -nodefaultlibs is used all the libraries which are removed but required would have to be manually added in. This set of libraries are unique and will send out.

The propose change will allow to make the link step simple for other platforms as well.

Reviewed By: #libc, #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D95875

libcxx/CMakeLists.txt

@@ -725,9 +725,19 @@ endif()
 
 # Link system libraries =======================================================
 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(${target} PRIVATE "/nodefaultlib")
+
+# In order to remove just libc++ from the link step
+# we need to use -nostdlib++ whenever it is supported.
+# Unfortunately this cannot be used universally because for example g++ supports
+# only -nodefaultlibs in which case all libraries will be removed and
+# all libraries but c++ have to be added in manually.
+  if (LIBCXX_SUPPORTS_NOSTDLIBXX_FLAG)
+    target_add_link_flags_if_supported(${target} PRIVATE "-nostdlib++")
+  else()
+    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")
+  endif()
 
   if (LIBCXX_HAS_SYSTEM_LIB)
     target_link_libraries(${target} PRIVATE System)

libcxx/cmake/config-ix.cmake

@@ -24,16 +24,26 @@ if (NOT LIBCXX_USE_COMPILER_RT)
   endif()
 endif()
 
-# libc++ is built with -nodefaultlibs, so we want all our checks to also
-# use this option, otherwise we may end up with an inconsistency between
+# libc++ is using -nostdlib++ at the link step when available,
+# otherwise -nodefaultlibs is used. We want all our checks to also
+# use one of these options, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
-# performed without -nodefaultlibs) and the flags that are actually
-# required during compilation (which has the -nodefaultlibs). libc is
+# performed without one of those options) and the flags that are actually
+# required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
-check_c_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
-if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+
+check_c_compiler_flag(-nostdlib++ LIBCXX_SUPPORTS_NOSTDLIBXX_FLAG)
+if (LIBCXX_SUPPORTS_NOSTDLIBXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+else()
+  check_c_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+  if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+  endif()
+endif()
+
+if (LIBCXX_SUPPORTS_NOSTDLIBXX_FLAG OR LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBCXX_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()

libcxxabi/cmake/config-ix.cmake

@@ -14,16 +14,26 @@ if (NOT LIBCXXABI_USE_COMPILER_RT)
   endif ()
 endif ()
 
-# libc++abi is built with -nodefaultlibs, so we want all our checks to also
-# use this option, otherwise we may end up with an inconsistency between
+# libc++abi is using -nostdlib++ at the link step when available,
+# otherwise -nodefaultlibs is used. We want all our checks to also
+# use one of these options, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
 # performed without -nodefaultlibs) and the flags that are actually
 # required during compilation (which has the -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
-check_c_compiler_flag(-nodefaultlibs LIBCXXABI_HAS_NODEFAULTLIBS_FLAG)
-if (LIBCXXABI_HAS_NODEFAULTLIBS_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+
+check_c_compiler_flag(-nostdlib++ LIBCXXABI_SUPPORTS_NOSTDLIBXX_FLAG)
+if (LIBCXXABI_SUPPORTS_NOSTDLIBXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+else()
+  check_c_compiler_flag(-nodefaultlibs LIBCXXABI_SUPPORTS_NODEFAULTLIBS_FLAG)
+  if (LIBCXXABI_SUPPORTS_NODEFAULTLIBS_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+  endif()
+endif()
+
+if (LIBCXXABI_SUPPORTS_NOSTDLIBXX_FLAG OR LIBCXXABI_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBCXXABI_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()

libcxxabi/src/CMakeLists.txt

@@ -109,7 +109,11 @@ if (NOT LIBCXXABI_USE_COMPILER_RT)
 endif ()
 
 # Setup flags.
-add_link_flags_if_supported(-nodefaultlibs)
+if (LIBCXXABI_SUPPORTS_NOSTDLIBXX_FLAG)
+  add_link_flags_if_supported(-nostdlib++)
+else()
+  add_link_flags_if_supported(-nodefaultlibs)
+endif()
 
 if ( APPLE )
   if (LLVM_USE_SANITIZER)

libunwind/cmake/config-ix.cmake

@@ -16,16 +16,26 @@ if (NOT LIBUNWIND_USE_COMPILER_RT)
   endif ()
 endif()
 
-# libunwind is built with -nodefaultlibs, so we want all our checks to also
-# use this option, otherwise we may end up with an inconsistency between
+# libunwind is using -nostdlib++ at the link step when available,
+# otherwise -nodefaultlibs is used. We want all our checks to also
+# use one of these options, otherwise we may end up with an inconsistency between
 # the flags we think we require during configuration (if the checks are
-# performed without -nodefaultlibs) and the flags that are actually
-# required during compilation (which has the -nodefaultlibs). libc is
+# performed without one of those options) and the flags that are actually
+# required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
-check_c_compiler_flag(-nodefaultlibs LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
-if (LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+
+check_c_compiler_flag(-nostdlib++ LIBUNWIND_SUPPORTS_NOSTDLIBXX_FLAG)
+if (LIBUNWIND_SUPPORTS_NOSTDLIBXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+else()
+  check_c_compiler_flag(-nodefaultlibs LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG)
+  if (LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+  endif()
+endif()
+
+if (LIBUNWIND_SUPPORTS_NOSTDLIBXX_FLAG OR LIBUNWIND_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBUNWIND_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()

libunwind/src/CMakeLists.txt

@@ -80,7 +80,11 @@ if (LIBUNWIND_ENABLE_THREADS)
 endif()
 
 # Setup flags.
-add_link_flags_if_supported(-nodefaultlibs)
+if (LIBUNWIND_SUPPORTS_NOSTDLIBXX_FLAG)
+  add_link_flags_if_supported(-nostdlib++)
+else()
+  add_link_flags_if_supported(-nodefaultlibs)
+endif()
 
 # MINGW_LIBRARIES is defined in config-ix.cmake
 add_library_flags_if(MINGW "${MINGW_LIBRARIES}")