[LLVM][MIPS] Introduce 64 bit atomic check in CheckAtomic.cmake.

[nitesh.jain]

Summary

On some target like MIPS32 we need to explicitly link atomic library for 64 bit atomic operations. This module then can be used in LLDB (http://reviews.llvm.org/D20464) or Libcxx (http://reviews.llvm.org/D16613) for explicitly link to atomic library.

[nitesh.jain]

Hi Chandler Carruth,

Please could you find some time to review this ?

Thanks

[zturner]

My own comments:

This looks fine since it removes code duplication across multiple
subprojects. Have you tested that both libcxx and lldb work correctly when
using this module as opposed to the existing ones?

[chandlerc]

This seems generally fine but:

1. I'd love Chris B to look at this to make sure he likes it too.

2. Pretty sure LLVM itself needs to use this. Can you update LLVM's CMake to switch to this routine?

[beanz]

This looks reasonable to me. Is there a reason for having this as a new CMake module instead of just adding it to CheckAtomic.cmake?

It seems to me it could be reasonable to modify CheckAtomic.cmake to test for both 32-bit and 64-bit atomics.

[nitesh.jain]

In D20896#454788, @zturner wrote:

My own comments:

This looks fine since it removes code duplication across multiple
subprojects. Have you tested that both libcxx and lldb work correctly when
using this module as opposed to the existing ones?

This module work fine with LLDB but need to check with libcxx. I will check and let you know.

[nitesh.jain]

In D20896#454949, @beanz wrote:

This looks reasonable to me. Is there a reason for having this as a new CMake module instead of just adding it to CheckAtomic.cmake?

It seems to me it could be reasonable to modify CheckAtomic.cmake to test for both 32-bit and 64-bit atomics.

I will modify CheckAtomic.cmake to test 64-bit atomic too.

Regards,
Nitesh Jain

[nitesh.jain]

Added 64 bit atomic check in CheckAtomic.cmake.

[nitesh.jain]

In D20896#455830, @nitesh.jain wrote:

In D20896#454788, @zturner wrote:

My own comments:

This looks fine since it removes code duplication across multiple
subprojects. Have you tested that both libcxx and lldb work correctly when
using this module as opposed to the existing ones?

This module work fine with LLDB but need to check with libcxx. I will check and let you know.

I am able to build lldb and libcxx with this module.

-nj

[nitesh.jain]

Hi Chandler/beanz,

Please could you find some time to review this ?

Thanks

[beanz]

LGTM.

Thanks!
-Chris

cmake/modules/CheckAtomic64.cmake

+INCLUDE(CheckCXXSourceCompiles)
+
+# To ensure libatomic get link for 64 bit atomic operations.
+
+function(check_cxx_atomics varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11 ${CMAKE_REQUIRED_FLAGS}")
+  check_cxx_source_compiles("
+#include <atomic>
+#include <cstdint>
+std::atomic<uint64_t> x (0);
+int main() {
+  uint64_t i = x.load(std::memory_order_relaxed);
+  return 0;
+}
+" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_cxx_atomics)
+
+if(MSVC)
+    set(HAVE_CXX_ATOMIC64_WITHOUT_LIB True)
+else()
+    check_cxx_atomics(HAVE_CXX_ATOMIC64_WITHOUT_LIB)
+endif()
+
+# If not, check if the library exists, and atomics work with it.
+if(NOT HAVE_CXX_ATOMIC64_WITHOUT_LIB)
+  check_library_exists(atomic __atomic_load_8 "" HAVE_CXX_LIBATOMIC64)
+  if(HAVE_CXX_LIBATOMIC64)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+    check_cxx_atomics(HAVE_CXX_ATOMIC64_WITH_LIB)
+    if (NOT HAVE_CXX_ATOMIC64_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()