[libcxx][CMake] Add an option to include -lgcc_s in the linker script

[tstellar]

Summary

This adds the LIBCXXABI_USE_LIBGCC_UNWINDER cmake option, which is the
libgcc version of LIBCXXABI_USE_LLVM_UNWINDER. When enabled, it will
add -lgcc_s to the libc++.so linker script, so that users will not
need to explicitly link to an unwind library when linking with libc++.so.

[phosek]

libcxx/test/lit.site.cfg.in

30

No need to wrap the value in " to make it a string since it's already Pythonized as bool.

libcxx/utils/libcxx/test/target_info.py

263

s/eanbled/enabled/

[EricWF]

I don't think this is correct.

The compiler should be linking gcc unwind for us, and there should be no need for users to link it themselves.

We only do it in the test suite because we can't drop the C++ standard library without dropping the rest of the link line (At least before -nostdlibc++ existed).

Also I don't think a libc++ build is tied to any one particular unwind library?

libcxx/utils/libcxx/test/target_info.py

253

We really should be replacing this whole thing with -nostdlib++ (And I would if GCC supported it).

[tstellar]

In D60794#1469761, @EricWF wrote:

I don't think this is correct.

The compiler should be linking gcc unwind for us, and there should be no need for users to link it themselves.

Ok, I wasn't sure the correct place to fix this. clang does not currently add the gcc unwind library. I thought clang was assuming that the unwind library would always be specified in the linker script since this is what happens with the LLVM unwinder.

Here is an example to reproduce the problem in clang:

cat << EOF | ./bin/clang++ -x c++ -fuse-ld=lld -rtlib=compiler-rt -stdlib=libc++ -

#include <iostream>
int main(int argc, char **argv) {
  std::cout << "Hello World\n";
  return 0;
}
EOF

ld.lld: error: undefined symbol: _Unwind_Resume
>>> referenced by -
>>>               /tmp/--697774.o:(std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::__put_character_sequence<char, std::__1::char_traits<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, char const*, unsigned long))

ld.lld: error: undefined symbol: _Unwind_Resume
>>> referenced by -
>>>               /tmp/--697774.o:(std::__1::ostreambuf_iterator<char, std::__1::char_traits<char> > std::__1::__pad_and_output<char, std::__1::char_traits<char> >(std::__1::ostreambuf_iterator<char, std::__1::char_traits<char> >, char const*, char const*, char const*, std::__1::ios_base&, char))
clang-9: error: linker command failed with exit code 1 (use -v to see invocation)

We only do it in the test suite because we can't drop the C++ standard library without dropping the rest of the link line (At least before -nostdlibc++ existed).

Also I don't think a libc++ build is tied to any one particular unwind library?

It can be when the option LIBCXXABI_USE_LLVM_UNWINDER=ON is used, at least on Linux.

[ldionne]

I agree with Eric this seems to be the wrong place to do this. Is this still a problem we want to solve? If not, let's abandon this to clear up the review queue.

libcxx/CMakeLists.txt

option(LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS
     be used to disable the definitions when libc++abi is expected to provide
     them" ${ENABLE_NEW_DELETE_DEFAULT})
 
 # Build libc++abi with libunwind. We need this option to determine whether to
 # link with libunwind or libgcc_s while running the test cases.
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 
+option(LIBCXXABI_USE_LIBGCC_UNWINDER "Use libgcc unwind library" OFF)
+if (LIBCXXABI_USE_LLVM_UNWINDER AND LIBCXXABI_USE_LIBGCC_UNWINDER)
+  message(FATAL_ERROR "Cannot specify both LIBCXXABI_USE_LLVM_UNWINDER and
+                       LIBCXXABI_USE_LIBGCC_UNWINDER.")
+endif()
+
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++." ${LLVM_BUILD_32_BITS})
 set(LIBCXX_TARGET_TRIPLE "" CACHE STRING "Use alternate target triple.")
 set(LIBCXX_SYSROOT "" CACHE STRING "Use alternate sysroot.")
 set(LIBCXX_GCC_TOOLCHAIN "" CACHE STRING "Use alternate GCC toolchain.")
 
 # Feature options -------------------------------------------------------------
 option(LIBCXX_ENABLE_EXCEPTIONS "Use exceptions." ON)

libcxx/lib/CMakeLists.txt

 if (LIBCXXABI_USE_LLVM_UNWINDER)
   if (NOT LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_shared OR HAVE_LIBUNWIND))
     add_interface_library(unwind_shared)
   elseif (LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
     add_interface_library(unwind_static)
   else()
     add_interface_library(unwind)
   endif()
+elseif (LIBCXXABI_USE_LIBGCC_UNWINDER)
+  add_interface_library(gcc_s)
 endif()
 
 # Setup flags.
 add_link_flags_if_supported(-nodefaultlibs)
 
 if (LIBCXX_TARGETING_MSVC)
   if (LIBCXX_DEBUG_BUILD)
     set(LIB_SUFFIX "d")

libcxx/test/CMakeLists.txt

 pythonize_bool(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
 pythonize_bool(LIBCXX_ENABLE_RTTI)
 pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBCXX_ENABLE_FILESYSTEM)
 pythonize_bool(LIBCXX_BUILD_32_BITS)
 pythonize_bool(LIBCXX_GENERATE_COVERAGE)
 pythonize_bool(LIBCXXABI_ENABLE_SHARED)
 pythonize_bool(LIBCXXABI_USE_LLVM_UNWINDER)
+pythonize_bool(LIBCXXABI_USE_LIBGCC_UNWINDER)
 pythonize_bool(LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
 pythonize_bool(LIBCXX_USE_COMPILER_RT)
 pythonize_bool(LIBCXX_HAS_ATOMIC_LIB)
 pythonize_bool(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
 pythonize_bool(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
 pythonize_bool(LIBCXX_DEBUG_BUILD)
 
 # By default, for non-standalone builds, libcxx and libcxxabi share a library

libcxx/test/lit.site.cfg.in

 config.gcc_toolchain            = "@LIBCXX_GCC_TOOLCHAIN@"
 config.generate_coverage        = @LIBCXX_GENERATE_COVERAGE@
 config.target_info              = "@LIBCXX_TARGET_INFO@"
 config.test_linker_flags        = "@LIBCXX_TEST_LINKER_FLAGS@"
 config.test_compiler_flags      = "@LIBCXX_TEST_COMPILER_FLAGS@"
 
 config.executor                 = "@LIBCXX_EXECUTOR@"
 config.llvm_unwinder            = @LIBCXXABI_USE_LLVM_UNWINDER@
+config.libgcc_unwinder          = "@LIBCXXABI_USE_LIBGCC_UNWINDER@"
 config.libcxx_is_linker_script  = @LIBCXX_ENABLE_ABI_LINKER_SCRIPT@
 config.builtins_library         = "@LIBCXX_BUILTINS_LIBRARY@"
 config.has_libatomic            = @LIBCXX_HAS_ATOMIC_LIB@
 config.use_libatomic            = @LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB@
 config.debug_build              = @LIBCXX_DEBUG_BUILD@
 config.libcxxabi_shared         = @LIBCXXABI_ENABLE_SHARED@
 config.cxx_ext_threads          = @LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY@
 
 # Let the main config do the real work.
 config.loaded_site_config = True
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")

libcxx/utils/libcxx/test/target_info.py

def add_cxx_compile_flags(self, flags):
         flags += ['-D__STDC_FORMAT_MACROS',
                   '-D__STDC_LIMIT_MACROS',
                   '-D__STDC_CONSTANT_MACROS']
 
     def add_cxx_link_flags(self, flags):
         enable_threads = ('libcpp-has-no-threads' not in
                           self.full_config.config.available_features)
         llvm_unwinder = self.full_config.get_lit_bool('llvm_unwinder', False)
+        libgcc_unwinder = self.full_config.get_lit_bool('libgcc_unwinder', False)
         libcxx_is_linker_script  = self.full_config.get_lit_bool('libcxx_is_linker_script', False)
         shared_libcxx = self.full_config.get_lit_bool('enable_shared', True)
         flags += ['-lm']
         if enable_threads:
             flags += ['-lpthread']
             if not shared_libcxx:
                 flags += ['-lrt']
         flags += ['-lc']
+
+        # Unwinder library link flag matrix.
+        #
+        # Linker Script | Yes | No
+        #-------------------------
+        # unwind        |  X  | unwind 
+        # libgcc        |  X  | gcc 
+        # None          | gcc | gcc
+        #
         if llvm_unwinder and not libcxx_is_linker_script:
             # FIXME: Why does llvm unwinder need -ldl?
             flags += ['-lunwind', '-ldl']
-        elif not llvm_unwinder:
+        elif not libcxx_is_linker_script or (not llvm_unwinder and not libgcc_unwinder):
             # If we aren't using llvm unwinder, then fallback to using libgcc's
             # unwind implementation.
             # FIXME: Why is -lgcc needed for the unwinder?
             flags += ['-lgcc_s', '-lgcc']
-        # If we are using a linker script and llvm_unwinder is eanbled, then we
-        # don't need to explicitly link against -lunwind, since the linker
-        # script will take care of that for us.
+        # If we are using a linker script and llvm_unwinder or libgcc_unwinder
+        # is eanbled, then we don't need to explicitly link against an unwind
+        # implementation, since the linker script will take care of that for us.
         builtins_lib = self.full_config.get_lit_conf('builtins_library')
         if builtins_lib:
             flags += [builtins_lib]
         else:
             flags += ['-lgcc']
         use_libatomic = self.full_config.get_lit_bool('use_libatomic', False)
         if use_libatomic:
             flags += ['-latomic']