[libcxx][cmake] Remove libatomic temporarily from CMAKE_REQUIRED_LIBRARIES when configuring

[sdardis]

Summary

When libcxx is built in tree for a host which requires libatomic, LLVM's
configuration steps will determine it is required and add it to
CMAKE_REQUIRED_LIBRARIES. When libcxx is later configured, it tests if it
has C++ atomics without libatomic. The test erroneously passes as libatomic
is already part of the set of required libraries.

In turn, a number of the atomic tests will fail as they require libatomic
but the test suite is configured not to use libatomic.

Address this by always dropping libatomic from the set of required libraries
before determining if LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB is true,
then restoring the set of required libraries.

[sdardis]

Ping.

[sdardis]

Ping.

[sdardis]

Ping.

[sdardis]

Ping.

[EricWF]

LGTM.

I would modify the comment to mention that LLVM's configuration may already be adding libatomic

[sdardis]

Thanks for the review.

cmake/Modules/CheckLibcxxAtomic.cmake

 std::atomic<uintmax_t> y;
 int main() {
   return x + y;
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_cxx_atomics)
 
+# Perform the check for 64bit atomics without libatomic. It may have been added
+# to the required libraries earlier in the configuration, which would cause
+# this check to incorrectly pass.
+set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
+list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES "atomic")
 check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
+set(CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES})
+
 check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
 # If not, check if the library exists, and atomics work with it.
 if(NOT LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB)
   if(LIBCXX_HAS_ATOMIC_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
     check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
     if (NOT LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
       message(WARNING "Host compiler must support std::atomic!")
     endif()
   else()
     message(WARNING "Host compiler appears to require libatomic, but cannot find it.")
   endif()
 endif()