[libunwind] Remove checks for -nostdlib++

[philnik777]

libunwind uses a C linker, so it's never even trying to link against any C++ libraries. This removes the code which tries to drop C++ libraries, which makes the CMake configuration simpler and allows for upgrading GCC.

[llvmbot]

@llvm/pr-subscribers-libunwind

Author: Nikolas Klauser (philnik777)

Changes

libunwind uses a C linker, so it's never even trying to link against any C++ libraries. This removes the code which tries to drop C++ libraries, which makes the CMake configuration simpler and allows for upgrading GCC.

---

Full diff: https://github.com/llvm/llvm-project/pull/143162.diff

2 Files Affected:

• (modified) libunwind/cmake/config-ix.cmake (-56)
• (modified) libunwind/src/CMakeLists.txt (-12)

diff --git a/libunwind/cmake/config-ix.cmake b/libunwind/cmake/config-ix.cmake
index 126c872f0d489..d42ceffb1f631 100644
--- a/libunwind/cmake/config-ix.cmake
+++ b/libunwind/cmake/config-ix.cmake
@@ -26,62 +26,6 @@ if (NOT LIBUNWIND_USE_COMPILER_RT)
   endif ()
 endif()
 
-# 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 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.
-
-llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
-else()
-  llvm_check_compiler_linker_flag(C "-nodefaultlibs" C_SUPPORTS_NODEFAULTLIBS_FLAG)
-  if (C_SUPPORTS_NODEFAULTLIBS_FLAG)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
-  endif()
-endif()
-
-# Only link against compiler-rt manually if we use -nodefaultlibs, since
-# otherwise the compiler will do the right thing on its own.
-if (NOT CXX_SUPPORTS_NOSTDLIBXX_FLAG AND C_SUPPORTS_NODEFAULTLIBS_FLAG)
-  if (LIBUNWIND_HAS_C_LIB)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES c)
-  endif ()
-  if (LIBUNWIND_HAS_ROOT_LIB)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES root)
-  endif ()
-  if (LIBUNWIND_USE_COMPILER_RT)
-    include(HandleCompilerRT)
-    find_compiler_rt_library(builtins LIBUNWIND_BUILTINS_LIBRARY
-                             FLAGS ${LIBUNWIND_COMPILE_FLAGS})
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBUNWIND_BUILTINS_LIBRARY}")
-  else ()
-    if (LIBUNWIND_HAS_GCC_S_LIB)
-      list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
-    endif ()
-    if (LIBUNWIND_HAS_GCC_LIB)
-      list(APPEND CMAKE_REQUIRED_LIBRARIES gcc)
-    endif ()
-  endif ()
-  if (MINGW)
-    # Mingw64 requires quite a few "C" runtime libraries in order for basic
-    # programs to link successfully with -nodefaultlibs.
-    if (LIBUNWIND_USE_COMPILER_RT)
-      set(MINGW_RUNTIME ${LIBUNWIND_BUILTINS_LIBRARY})
-    else ()
-      set(MINGW_RUNTIME gcc_s gcc)
-    endif()
-    set(MINGW_LIBRARIES mingw32 ${MINGW_RUNTIME} moldname mingwex msvcrt advapi32
-                        shell32 user32 kernel32 mingw32 ${MINGW_RUNTIME}
-                        moldname mingwex msvcrt)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES ${MINGW_LIBRARIES})
-  endif()
-endif()
-
 if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
diff --git a/libunwind/src/CMakeLists.txt b/libunwind/src/CMakeLists.txt
index 70bd3a017cda7..03818b1bb2512 100644
--- a/libunwind/src/CMakeLists.txt
+++ b/libunwind/src/CMakeLists.txt
@@ -71,18 +71,6 @@ set(LIBUNWIND_SOURCES
     ${LIBUNWIND_ASM_SOURCES})
 
 # Generate library list.
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-  add_link_flags_if_supported(-nostdlib++)
-else()
-  if (LIBUNWIND_USE_COMPILER_RT)
-    add_library_flags("${LIBUNWIND_BUILTINS_LIBRARY}")
-  else()
-    add_library_flags_if(LIBUNWIND_HAS_GCC_S_LIB gcc_s)
-    add_library_flags_if(LIBUNWIND_HAS_GCC_LIB gcc)
-  endif()
-  add_library_flags_if(LIBUNWIND_HAS_C_LIB c)
-endif()
-
 if (NOT APPLE)
   add_library_flags_if(LIBUNWIND_HAS_DL_LIB dl)
 endif()

[ldionne]

LGTM assuming CI passes (modulo the usual infra issues), this indeed seems to have been copy-pasted from libc++ and libc++abi.

[arichardson]

Shouldn't this also be adjusted since CXX_SUPPORTS_NOSTDLIBXX_FLAG is no longer being set?