[CMake][libunwind] Use -nodefaultlibs for CMake checks

[phosek]

Summary

Since libunwind is built with -nodefaultlibs, we should be using this option even for CMake checks to avoid any inconsistency and also to avoid dependency on a working C++ standard library just for the setting up the build itself, because check_cxx_compile_flags adds the flag to COMPILE_DEFINITIONS when calling try_compile and this variable is only considered for compilation but not for linking where -nodefaultlibs actually makes a difference. The implementation is largely similar to the one used by libc++.

[compnerd]

Sure, using the same flags sounds reasonable.

libunwind/trunk/cmake/config-ix.cmake

 
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckLibraryExists)
 
+check_library_exists(c fopen "" LIBUNWIND_HAS_C_LIB)
+
+# libunwind is built with -nodefaultlibs, so we want all our checks to also
+# use this option, otherwise we may end up with an inconsistency between
+# the flags we think we require during configuration (if the checks are
+# performed without -nodefaultlibs) and the flags that are actually
+# required during compilation (which has the -nodefaultlibs). libc is
+# required for the link to go through. We remove sanitizers from the
+# configuration checks to avoid spurious link errors.
+check_c_compiler_flag(-nodefaultlibs LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
+if (LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+  if (LIBUNWIND_HAS_C_LIB)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES c)
+  endif ()
+  if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
+  endif ()
+  if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")
+  endif ()
+endif ()
+
 # Check compiler flags
 check_c_compiler_flag(-funwind-tables         LIBUNWIND_HAS_FUNWIND_TABLES)
 check_cxx_compiler_flag(-fPIC                 LIBUNWIND_HAS_FPIC_FLAG)
 check_cxx_compiler_flag(-fno-exceptions       LIBUNWIND_HAS_NO_EXCEPTIONS_FLAG)
 check_cxx_compiler_flag(-fno-rtti             LIBUNWIND_HAS_NO_RTTI_FLAG)
 check_cxx_compiler_flag(-fstrict-aliasing     LIBUNWIND_HAS_FSTRICT_ALIASING_FLAG)
-check_cxx_compiler_flag(-nodefaultlibs        LIBUNWIND_HAS_NODEFAULTLIBS_FLAG)
 check_cxx_compiler_flag(-nostdinc++           LIBUNWIND_HAS_NOSTDINCXX_FLAG)
 check_cxx_compiler_flag(-Wall                 LIBUNWIND_HAS_WALL_FLAG)
 check_cxx_compiler_flag(-W                    LIBUNWIND_HAS_W_FLAG)
 check_cxx_compiler_flag(-Wno-unused-function  LIBUNWIND_HAS_WNO_UNUSED_FUNCTION_FLAG)
 check_cxx_compiler_flag(-Wunused-variable     LIBUNWIND_HAS_WUNUSED_VARIABLE_FLAG)
 check_cxx_compiler_flag(-Wunused-parameter    LIBUNWIND_HAS_WUNUSED_PARAMETER_FLAG)
 check_cxx_compiler_flag(-Wstrict-aliasing     LIBUNWIND_HAS_WSTRICT_ALIASING_FLAG)
 check_cxx_compiler_flag(-Wstrict-overflow     LIBUNWIND_HAS_WSTRICT_OVERFLOW_FLAG)

 check_cxx_compiler_flag(/EHa-                 LIBUNWIND_HAS_NO_EHA_FLAG)
 check_cxx_compiler_flag(/GR-                  LIBUNWIND_HAS_NO_GR_FLAG)
 check_cxx_compiler_flag(-std=c++11            LIBUNWIND_HAS_STD_CXX11)
 
 if(LIBUNWIND_HAS_STD_CXX11)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 endif()
 
-check_library_exists(c fopen "" LIBUNWIND_HAS_C_LIB)
 check_library_exists(dl dladdr "" LIBUNWIND_HAS_DL_LIB)
 check_library_exists(pthread pthread_once "" LIBUNWIND_HAS_PTHREAD_LIB)