Define HOST_DEVICE_CONSTANT to implement constexpr aggregate host/device constants

FWCore/Utilities/interface/HostDeviceConstant.h

@@ -0,0 +1,25 @@
+#ifndef FWCore_Utilities_HostDeviceConstant_h
+#define FWCore_Utilities_HostDeviceConstant_h
+
+// The use of host-side consexpr constants in device code is limited to:
+//   - scalars (other than `long double`)
+//   - scalar elements of aggregates used inside `constexpr` functions
+//
+// See https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#constexpr-variables .
+//
+// In particular, it's not possible to use constexpr scalars by pointer or reference (e.g. std::min() takes arguments
+// by reference), or pass constexpr arrays as pointers, or access elements of constexpr arrays outside of constexpr
+// functions.
+//
+// The workaround is to define a macro that evaluates to "constexpr" on the host, and "__device__ constexpr" on the
+// device. Such macro can be used to declare aggregate objects that are available both on the host and on the device.
+// Note these objects may be at different memory addresses on the host and device, so their pointers will be different
+// -- but the actual values should be the same.
+
+#ifdef __CUDA_ARCH__
+#define HOST_DEVICE_CONSTANT __device__ constexpr
+#else
+#define HOST_DEVICE_CONSTANT constexpr
+#endif
+
+#endif  // FWCore_Utilities_HostDeviceConstant_h