[libc++] Use -nostdlib++ on GCC unconditionally

[ldionne]

We support GCC 13, which supports the flag. This allows simplifying the CMake logic around the use of -nostdlib++. Note that there are other places where we don't assume -nostdlib++ yet in the build, but this patch is intentionally trying to be small because this part of our CMake is pretty tricky.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

We support GCC 13, which supports the flag. This allows simplifying the CMake logic around the use of -nostdlib++. Note that there are other places where we don't assume -nostdlib++ yet in the build, but this patch is intentionally trying to be small because this part of our CMake is pretty tricky.

---

Full diff: https://github.com/llvm/llvm-project/pull/68832.diff

2 Files Affected:

• (modified) libcxx/CMakeLists.txt (+4-29)
• (modified) libcxx/cmake/config-ix.cmake (-15)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 16540caf68eaf04..f36e2f13b37f350 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -642,18 +642,11 @@ get_sanitizer_flags(SANITIZER_FLAGS "${LLVM_USE_SANITIZER}")
 
 # Link system libraries =======================================================
 function(cxx_link_system_libraries target)
-
-# 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 (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-    target_add_link_flags_if_supported(${target} PRIVATE "-nostdlib++")
+  if (MSVC)
+    target_add_compile_flags(${target} PRIVATE "/Zl")
+    target_add_link_flags(${target} PRIVATE "/nodefaultlib")
   else()
-    target_add_link_flags_if_supported(${target} PRIVATE "-nodefaultlibs")
-    target_add_compile_flags_if_supported(${target} PRIVATE "/Zl")
-    target_add_link_flags_if_supported(${target} PRIVATE "/nodefaultlib")
+    target_add_link_flags(${target} PRIVATE "-nostdlib++")
   endif()
 
   if (CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG AND LIBCXXABI_USE_LLVM_UNWINDER)
@@ -663,24 +656,6 @@ function(cxx_link_system_libraries target)
     target_add_link_flags_if_supported(${target} PRIVATE "--unwindlib=none")
   endif()
 
-  if (NOT APPLE) # On Apple platforms, we always use -nostdlib++ so we don't need to re-add other libraries
-    if (LIBCXX_HAS_PTHREAD_LIB)
-      target_link_libraries(${target} PRIVATE pthread)
-    endif()
-
-    if (LIBCXX_HAS_C_LIB)
-      target_link_libraries(${target} PRIVATE c)
-    endif()
-
-    if (LIBCXX_HAS_M_LIB)
-      target_link_libraries(${target} PRIVATE m)
-    endif()
-
-    if (LIBCXX_HAS_RT_LIB)
-      target_link_libraries(${target} PRIVATE rt)
-    endif()
-  endif()
-
   if (LIBCXX_USE_COMPILER_RT)
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)
     if (LIBCXX_BUILTINS_LIBRARY)
diff --git a/libcxx/cmake/config-ix.cmake b/libcxx/cmake/config-ix.cmake
index 9962d848d85e846..9fed861a4e193c5 100644
--- a/libcxx/cmake/config-ix.cmake
+++ b/libcxx/cmake/config-ix.cmake
@@ -14,14 +14,6 @@ include(CheckCSourceCompiles)
 # link with --uwnindlib=none. Check if that option works.
 llvm_check_compiler_linker_flag(C "--unwindlib=none" CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG)
 
-if(WIN32 AND NOT MINGW)
-  # NOTE(compnerd) this is technically a lie, there is msvcrt, but for now, lets
-  # let the default linking take care of that.
-  set(LIBCXX_HAS_C_LIB NO)
-else()
-  check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
-endif()
-
 if (NOT LIBCXX_USE_COMPILER_RT)
   if(WIN32 AND NOT MINGW)
     set(LIBCXX_HAS_GCC_S_LIB NO)
@@ -54,9 +46,6 @@ else()
 endif()
 
 if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
-  if (LIBCXX_HAS_C_LIB)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES c)
-  endif ()
   if (LIBCXX_USE_COMPILER_RT)
     include(HandleCompilerRT)
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY
@@ -108,22 +97,18 @@ if(WIN32 AND NOT MINGW)
   # TODO(compnerd) do we want to support an emulation layer that allows for the
   # use of pthread-win32 or similar libraries to emulate pthreads on Windows?
   set(LIBCXX_HAS_PTHREAD_LIB NO)
-  set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
   set(LIBCXX_HAS_ATOMIC_LIB NO)
 elseif(APPLE)
   set(LIBCXX_HAS_PTHREAD_LIB NO)
-  set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
   set(LIBCXX_HAS_ATOMIC_LIB NO)
 elseif(FUCHSIA)
-  set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_PTHREAD_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
   check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
 else()
   check_library_exists(pthread pthread_create "" LIBCXX_HAS_PTHREAD_LIB)
-  check_library_exists(m ccos "" LIBCXX_HAS_M_LIB)
   check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)
   check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
 endif()

[EricWF]

LGTM, but could you double check on libm before committing?

Then again, we have great CI coverage now, so if it builds ship it?

[EricWF]

It's nice having CI for all the platforms, otherwise this would be harder to review and be confident about.

LGTM assuming everything passes.