Introduce a cmake module to figure out whether we need to link with libatomic.

Summary:
This fixes the tests under std/atomics for 32-bit MIPS CPUs where the
8-byte atomic operations call into the libatomic library.

Reviewers: dsanders, mclow.lists, EricWF, jroelofs, joerg

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16613

llvm-svn: 260235

2 files changed, 43 insertions, 0 deletions

diff --git a/libcxx/cmake/Modules/CheckLibcxxAtomic.cmake b/libcxx/cmake/Modules/CheckLibcxxAtomic.cmake
new file mode 100644
index 00000000000..ed3e6cc879e
--- /dev/null
+++ b/libcxx/cmake/Modules/CheckLibcxxAtomic.cmake
@@ -0,0 +1,38 @@
+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")
+  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(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+# If not, check if the library exists, and atomics work with it.
+if(NOT HAVE_CXX_ATOMICS_WITHOUT_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/libcxx/cmake/config-ix.cmake b/libcxx/cmake/config-ix.cmake
index c05597ea73d..1bfa23fadd8 100644
--- a/libcxx/cmake/config-ix.cmake
+++ b/libcxx/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()