Re-commit "Introduce a cmake module to figure out whether we need to link with libatomic."

This re-applies commit r260235. However, this time we add -gcc-toolchain
to the compiler's flags when the user has specified the LIBCXX_GCC_TOOLCHAIN
variable.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260515 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 46 insertions, 0 deletions

diff --git a/cmake/Modules/CheckLibcxxAtomic.cmake b/cmake/Modules/CheckLibcxxAtomic.cmake
new file mode 100644
index 000000000..d3b9fa466
--- /dev/null
+++ b/cmake/Modules/CheckLibcxxAtomic.cmake
@@ -0,0 +1,41 @@
+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 ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
+  if (${LIBCXX_GCC_TOOLCHAIN})
+    set(CMAKE_REQUIRED_FLAGS "-std=c++11 --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
+  endif()
+  check_cxx_source_compiles("
+#include <cstdint>
+#include <atomic>
+std::atomic<uintptr_t> x;
+std::atomic<uintmax_t> y;
+int main() {
+  return x + y;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_cxx_atomics)
+
+check_cxx_atomics(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
+# If not, check if the library exists, and atomics work with it.
+if(NOT LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
+  check_library_exists(atomic __atomic_fetch_add_8 "" HAVE_LIBATOMIC)
+  if(HAVE_LIBATOMIC)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+    check_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
+    if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
+      message(FATAL_ERROR "Host compiler must support std::atomic!")
+    endif()
+  else()
+    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+  endif()
+endif()
diff --git a/cmake/config-ix.cmake b/cmake/config-ix.cmake
index c05597ea7..1bfa23fad 100644
--- a/cmake/config-ix.cmake
+++ b/cmake/config-ix.cmake
@@ -1,5 +1,6 @@
 include(CheckLibraryExists)
 include(CheckCXXCompilerFlag)
+include(CheckLibcxxAtomic)
 
 # Check compiler flags
 
@@ -17,3 +18,7 @@ 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)
 check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXX_HAS_GCC_S_LIB)
+
+if (NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+  set(LIBCXX_HAS_ATOMIC_LIB True)
+endif()