[libcxx] [cmake] Use -print-libgcc-file-name option to find compiler runtime

[mgorny]

Summary

Use the -print-libgcc-file-name compiler option to obtain the path to
libgcc or an equivalent compiler runtime library, rather than hardcoding
-lgcc_s. This aims to make it possible to avoid linking with -lgcc_s
when -rtlib=compiler-rt is used along with libunwind.

If -print-libgcc-file-name call fails, -lgcc_s is used as a fallback.
Furthermore, -lgcc_s is still linked in for unwinding if
LIBCXXABI_USE_LLVM_UNWINDER is not set.

[mgorny]

Fixed indentation.

[EricWF]

This doesn't seem right seeing as print-libgcc_file-name prints the libgcc.a path, and we were previously linking libgcc_s.

[mgorny]

In D25568#570222, @EricWF wrote:

This doesn't seem right seeing as print-libgcc_file-name prints the libgcc.a path, and we were previously linking libgcc_s.

Do you have any other solution in mind? There is no switch to print the shared library path. Should I modify it to detect whether the printed path matches libgcc.a and use libgcc_s in that case?

Just to be clear, since libgcc_s is the shared version of libgcc (+ libgcc_eh), this is mostly a matter of replacing dynamic linking with static. However, it doesn't matter much since in most of the cases no library function is actually used. From my experience, throughout 3.8.1 .. HEAD, no builtin functions were needed on amd64 and only one (__udiv3, if I recall correctly) was needed on 32-bit x86. In this case, I dare say using the static library may actually be less problematic as it makes libc++ less dependent on compiler runtime.

Of course, on the other hand we have the unwinder library which still can be gcc_s and there is no easy way to solve this dependency.

cmake/Modules/HandleLibcxxFlags.cmake

 # Add a list of libraries or link flags to 'LIBCXX_LIBRARIES'.
 macro(add_interface_library)
   foreach(lib ${ARGN})
     list(APPEND LIBCXX_LIBRARIES ${lib})
     list(APPEND LIBCXX_INTERFACE_LIBRARIES ${lib})
   endforeach()
 endmacro()
 
+# Attempt to detect the correct compiler runtime library and add it
+# to 'LIBCXX_LIBRARIES'. Fall back to gcc_s if available.
+macro(add_libgcc_library)
+  # split space-separated vars into a list
+  set(libgcc_cmd ${CMAKE_CXX_COMPILER}
+                 ${CMAKE_SHARED_LIBRARY_CXX_FLAGS}
+                 ${CMAKE_CXX_FLAGS}
+                 ${CMAKE_SHARED_LINKER_FLAGS}
+                 -print-libgcc-file-name)
+
+  execute_process(
+    COMMAND "${libgcc_cmd}"
+    RESULT_VARIABLE libgcc_check_ret
+    OUTPUT_VARIABLE libgcc_path
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+  if (${libgcc_check_ret} EQUAL 0)
+    add_library_flags("${libgcc_path}")
+  else()
+    add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
+  endif()
+endmacro()
+
 # Turn a comma separated CMake list into a space separated string.
 macro(split_list listname)
   string(REPLACE ";" " " ${listname} "${${listname}}")
 endmacro()

lib/CMakeLists.txt

if (APPLE AND LLVM_USE_SANITIZER)
   endif()
 endif()
 
 # Generate private library list.
 add_library_flags_if(LIBCXX_HAS_PTHREAD_LIB pthread)
 add_library_flags_if(LIBCXX_HAS_C_LIB c)
 add_library_flags_if(LIBCXX_HAS_M_LIB m)
 add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
-add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
+add_libgcc_library()
 add_library_flags_if(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB atomic)
 
 # Add the unwinder library.
 if (LIBCXXABI_USE_LLVM_UNWINDER)
   if (TARGET unwind_shared)
     add_interface_library(unwind_shared)
   elseif (TARGET unwind_static)
     add_interface_library(unwind_static)
   else()
     add_interface_library(unwind)
   endif()
+else()
+  add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
 endif()
 
 # Setup flags.
 if (NOT WIN32)
   add_flags_if_supported(-fPIC)
 endif()
 add_link_flags_if_supported(-nodefaultlibs)