[libc++][libunwind] Fixes to allow GCC 13 to compile libunwind/libc++abi/libc++

[philnik]

Summary

These are changes to allow GCC 13 to successfully compile the runtimes stack.

[arichardson]

libunwind/src/CMakeLists.txt

149

Why is this needed? Shouldn't it work without linking c++abi/standard library?

[mstorsjo]

libunwind/src/CMakeLists.txt

149

Indeed, libunwind must not rely on the C++ standard library - that's a layering violation. What's the issue that this tries to fix?

libunwind explicitly doesn't even include C++ standard library headers (building with -nostdinc++) in order to stay clear of any dependencies on it. Not sure if any of the CI configurations would show the issue here, but if we'd have the new CI configuration from D150766 merged, that one most definitely would fail on this change.

[philnik]

libunwind/src/CMakeLists.txt

149

GCC 13 doesn't accept -nostdlib++ in C mode. Given that these flags are passed explicitly, I don't think there should be a problem with invoking the driver in C++ mode.

[ldionne]

This LGTM. I think it should be fine to link libunwind as C++ since we pass -nostdlib++, but please make sure @arichardson and @mstorsjo are on board before landing.

libcxx/include/__type_traits/remove_cv.h

22–38

#if __has_builtin(__remove_cv)

// GCC can't mangle builtins so they can't appear in an alias directly
#   if !defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
using __remove_cv_t = __remove_cv(_Tp);
#else
template <class _Tp>
struct __libcpp_remove_cv {
  using type _LIBCPP_NODEBUG = __remove_cv(_Tp);
};

template <class _Tp>
using __remove_cv_t = typename __libcpp_remove_cv<_Tp>::type;
#endif

#else

template <class _Tp>
using __remove_cv_t = __remove_volatile_t<__remove_const_t<_Tp> >;

#endif // __has_builtin(__remove_cv)

This is equivalent but the logic seems a bit easier to follow.

libcxx/include/__type_traits/remove_cvref.h

23

Same comments here.

libunwind/src/CMakeLists.txt

1

@power-llvm-team Can you please help us understand the failure on AIX at https://buildkite.com/llvm-project/libcxx-ci/builds/24846#01885016-92a1-4955-a7cb-e87fc56b59e5 ?

All the tests on AIX are failing with something like:

Could not load program /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/test/std/utilities/expected/expected.void/observers/Output/error.pass.cpp.dir/t.tmp.exe:
Symbol resolution failed for /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/lib/libunwind.a(libunwind.so.1) because:
	Symbol unw_getcontext (number 14) is not exported from dependent
	  module /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/lib/libunwind.a(libunwind.so.1).
	Symbol unw_init_local (number 15) is not exported from dependent
	  module /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/lib/libunwind.a(libunwind.so.1).
	Symbol unw_step (number 16) is not exported from dependent
	  module /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/lib/libunwind.a(libunwind.so.1).
	Symbol unw_get_reg (number 17) is not exported from dependent
	  module /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/lib/libunwind.a(libunwind.so.1).
	Symbol unw_get_fpreg (number 18) is not exported from dependent
	  module /scratch/powerllvm/cpap8008/llvm-project/libcxx-ci/build/aix/lib/libunwind.a(libunwind.so.1).
        [...]
Examine .loader section symbols with the 'dump -Tv' command.

error: command failed with exit status: 255

This is likely related to changing the linker language from C to CXX and it seems like suddenly the symbol names exported by libunwind are not correct anymore, but that's weird cause everything should be in extern "C" context anyway.

[mstorsjo]

libunwind/src/CMakeLists.txt

149

Oh, right, we use that too - yes, with -nostdlib++ it's probably fine. But it would be nice to have D150766 in place so we'd have better CI coverage of that aspect.

I can run this patch through a test build from scratch in my config to be sure.

[mstorsjo]

libunwind/src/CMakeLists.txt

149

This did build fine for me, so consider this objection withdrawn.

[MaskRay]

libunwind/src/CMakeLists.txt

149

D95875 added -nostdlib++ here. Can we remove it or make it z/OS specific @zibi @SeanP

[MaskRay]

libunwind LGTM, but I suspect the LINKER_LANGUAGE CXX change in libunwind is adding a workaround on top of another one D95875

Newer GCC supports g++ -nostdlib++ but not gcc -nostdlib++. If we don't add -nostdlib++ to link flags, we can keep LINKER_LANGUAGE C.

[zibi]

libunwind/src/CMakeLists.txt

149

The -nostdlib++ can be removed. It was initially added for consistency with libcxx and libcxxabi configurations but those are built with CXX.

[philnik]

Remove -nostdinc++

[philnik]

@MaskRay I've removed both -nostdinc++ and -nodefaultlibs, since the latter doesn't add anything that we don't re-add anyways. I'll try to make some cleanup patches later, but we should really get this fixed, since LLVM17 should support GCC13.

libcxx/include/__type_traits/remove_cv.h

 #include <__type_traits/remove_volatile.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__remove_cv)
+#if __has_builtin(__remove_cv) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
 struct remove_cv {
   using type _LIBCPP_NODEBUG = __remove_cv(_Tp);
 };
 
 template <class _Tp>
 using __remove_cv_t = __remove_cv(_Tp);
 #else
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS remove_cv {
   typedef __remove_volatile_t<__remove_const_t<_Tp> > type;
 };
 
 template <class _Tp>
 using __remove_cv_t = __remove_volatile_t<__remove_const_t<_Tp> >;
 #endif // __has_builtin(__remove_cv)
 
 #if _LIBCPP_STD_VER >= 14
 template <class _Tp>
 using remove_cv_t = __remove_cv_t<_Tp>;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_REMOVE_CV_H

libcxx/include/__type_traits/remove_cvref.h

 #include <__type_traits/remove_reference.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__remove_cvref)
+#if __has_builtin(__remove_cvref) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
 using __remove_cvref_t _LIBCPP_NODEBUG = __remove_cvref(_Tp);
 #else
 template <class _Tp>
 using __remove_cvref_t _LIBCPP_NODEBUG = __remove_cv_t<__libcpp_remove_reference_t<_Tp> >;
 #endif // __has_builtin(__remove_cvref)
 
 template <class _Tp, class _Up>

libunwind/src/CMakeLists.txt

 # Get sources
 
 set(LIBUNWIND_CXX_SOURCES
     libunwind.cpp
     Unwind-EHABI.cpp
     Unwind-seh.cpp
     )
 
 if(${CMAKE_SYSTEM_NAME} MATCHES "AIX")

 add_library_flags_if(LIBUNWIND_HAS_DL_LIB dl)
 if (LIBUNWIND_ENABLE_THREADS)
   add_library_flags_if(LIBUNWIND_HAS_PTHREAD_LIB pthread)
   add_compile_flags_if(LIBUNWIND_WEAK_PTHREAD_LIB -DLIBUNWIND_USE_WEAK_PTHREAD=1)
 endif()
 
 # Setup flags.
 add_link_flags_if(CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG --unwindlib=none)
-if (CXX_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}")
 
 if (LIBUNWIND_ENABLE_SHARED AND
     NOT (CXX_SUPPORTS_FNO_EXCEPTIONS_FLAG AND
          CXX_SUPPORTS_FUNWIND_TABLES_FLAG))
   message(FATAL_ERROR

 endif()
 
 if (LIBUNWIND_ENABLE_SHARED)
   add_library(unwind_shared SHARED)
   target_link_libraries(unwind_shared PUBLIC unwind_shared_objects)
   set_target_properties(unwind_shared
     PROPERTIES
       LINK_FLAGS "${LIBUNWIND_LINK_FLAGS}"
       LINKER_LANGUAGE C
       OUTPUT_NAME "${LIBUNWIND_SHARED_OUTPUT_NAME}"
       VERSION "1.0"
       SOVERSION "1"
   )
 
   list(APPEND LIBUNWIND_BUILD_TARGETS "unwind_shared")
   if (LIBUNWIND_INSTALL_SHARED_LIBRARY)
     list(APPEND LIBUNWIND_INSTALL_TARGETS "unwind_shared")