CUDA host device code with two code paths

Summary:
Allow CUDA host device functions with two code paths using __CUDA_ARCH__
to differentiate between code path being compiled.

For example:
  __host__ __device__ void host_device_function(void) {
  #ifdef __CUDA_ARCH__
    device_only_function();
  #else
    host_only_function();
  #endif
  }

Patch by Jacques Pienaar.

Reviewed By: rnk

Differential Revision: http://reviews.llvm.org/D6457

llvm-svn: 223271

clang/lib/Frontend/InitPreprocessor.cpp

@@ -870,6 +870,13 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
     Builder.defineMacro("_OPENMP", "201307");
   }
 
+  // CUDA device path compilaton
+  if (LangOpts.CUDAIsDevice) {
+    // The CUDA_ARCH value is set for the GPU target specified in the NVPTX
+    // backend's target defines.
+    Builder.defineMacro("__CUDA_ARCH__");
+  }
+
   // Get other target #defines.
   TI.getTargetDefines(LangOpts, Builder);
 }