[libc++] Perform configuration checks with -nodefaultlibs

[smeenai]

Summary

We're compiling libc++ with -nodefaultlibs, so we should also pass this
option during the configuration checks to ensure those checks are
consistent with the actual build.

The primary motivation here is to ease cross-compilation against a
non-standard set of C++ libraries. Previously, the configuration checks
would attempt to link against the standard C++ libraries, which would
cause link failures when cross-compiling, even though the actual library
link would go through correctly (because of the use of -nodefaultlibs
and explicitly specifying any needed libraries). This is more correct
even ignoring the motivation, however.

This redoes D23791 but fixes the configuration errors in sanitizer
builds by disabling the sanitizers for the configuration checks.

Test Plan:

• check-libcxx passes on OS X 10.11
• check-libcxx passes on Ubuntu 16.04
• libcxx configures correctly on Ubuntu 16.04 with LLVM_USE_SANITIZER Address, Memory, Thread, and Undefined

Did some additional testing on CentOS 7 against trunk clang:

• check-libcxx passes with -DLLVM_USE_SANITIZER=Address
• check-libcxx passes with -DLLVM_USE_SANITIZER=Memory
• check-libcxx passes with -DLLVM_USE_SANITIZER=MemoryWithOrigins
• check-libcxx passes with -DLLVM_USE_SANITIZER=Thread
• check-libcxx with -DLLVM_USE_SANITIZER=Undefined has 24 unexpected failures both with and without this diff. Seems to be related to LLVM bug 19302, except I don't know why it's passing on the buildbots and failing locally. I can reproduce the failures on Ubuntu 16.04

[compnerd]

cmake/config-ix.cmake

18

Can we not use CMAKE_SHARED_LINKER_FLAGS instead of CMAKE_REQUIRED_LIBRARIES? It is slightly misleading.

[smeenai]

cmake/config-ix.cmake

18

That would pollute everything building a shared library, whereas this limits it to only configuration checks. I know it's misleading, but I view it similar to how target_link_libraries is also used for specifying linker flags.

Speaking of pollution though, my REQUIRED_* changes in this file will end up affecting other projects for non-standalone builds, correct? What's the best way to limit this to be libc++-only?

[smeenai]

cmake/config-ix.cmake

18

Never mind. cmake automatically limits variables to the scope of the current directory, so this shouldn't pollute anything outside of libc++.

This should be good to review.

[EricWF]

cmake/config-ix.cmake

18

CMAKE_REQUIRED_LIBRARIES should still only really be used for libraries. I think CMAKE_REQUIRED_FLAGS would be a better place to put this, if not CMAKE_SHARED_LINKER_FLAGS.

[smeenai]

cmake/config-ix.cmake

18

I'm fine moving it to CMAKE_REQUIRED_FLAGS. Lemme make that change and test it out and then re-upload it.

[smeenai]

Putting -nodefaultlibs in CMAKE_REQUIRED_FLAGS

[compnerd]

SVN r280015

cmake/Modules/CheckLibcxxAtomic.cmake

 INCLUDE(CheckCXXSourceCompiles)
 
 # Sometimes linking against libatomic is required for atomic ops, if
 # the platform doesn't support lock-free atomics.
 #
 # We could modify LLVM's CheckAtomic module and have it check for 64-bit
 # atomics instead. However, we would like to avoid careless uses of 64-bit
 # atomics inside LLVM over time on 32-bit platforms.
 
 function(check_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
+  set(CMAKE_REQUIRED_FLAGS "-nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
   if (${LIBCXX_GCC_TOOLCHAIN})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
   endif()
+  if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
+  endif()
   check_cxx_source_compiles("
 #include <cstdint>
 #include <atomic>
 std::atomic<uintptr_t> x;
 std::atomic<uintmax_t> y;
 int main() {
   return x + y;
 }

cmake/config-ix.cmake

 include(CheckLibraryExists)
 include(CheckCXXCompilerFlag)
+
+check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
+if (NOT LIBCXX_USE_COMPILER_RT)
+  check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXX_HAS_GCC_S_LIB)
+endif()
+
+# libc++ 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_cxx_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
+  if (LIBCXX_HAS_C_LIB)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES c)
+  endif ()
+  if (LIBCXX_USE_COMPILER_RT)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES -rtlib=compiler-rt)
+  elseif (LIBCXX_HAS_GCC_S_LIB)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
+  endif ()
+  if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
+  endif ()
+endif ()
+
 include(CheckLibcxxAtomic)
 
 # Check compiler flags
 
 check_cxx_compiler_flag(/WX                     LIBCXX_HAS_WX_FLAG)
 check_cxx_compiler_flag(/WX-                    LIBCXX_HAS_NO_WX_FLAG)
 check_cxx_compiler_flag(/EHsc                   LIBCXX_HAS_EHSC_FLAG)
 check_cxx_compiler_flag(/EHs-                   LIBCXX_HAS_NO_EHS_FLAG)
 check_cxx_compiler_flag(/EHa-                   LIBCXX_HAS_NO_EHA_FLAG)
 check_cxx_compiler_flag(/GR-                    LIBCXX_HAS_NO_GR_FLAG)
 
 
 # Check libraries
 check_library_exists(pthread pthread_create "" LIBCXX_HAS_PTHREAD_LIB)
-check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
 check_library_exists(m ccos "" LIBCXX_HAS_M_LIB)
 check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)
-if (NOT LIBCXX_USE_COMPILER_RT)
-  check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXX_HAS_GCC_S_LIB)
-endif()