[CUDA] Add test checking our ability to take a function pointer to a __global__ function on the host side.

[jlebar]

Summary

This functionality is used by Thrust.

[tra]

LGTM.

clang/test/SemaCUDA/reference-to-kernel-fn.cu

+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++11 -fcuda-is-device -fsyntax-only -verify -DDEVICE %s
+
+// Check that we can reference (get a function pointer to) a __global__
+// function from the host side, but not the device side.  (We don't yet support
+// device-side kernel launches.)
+
+#include "Inputs/cuda.h"
+
+struct Dummy {};
+
+__global__ void kernel() {}
+// expected-note@-1 {{declared here}}
+#ifdef DEVICE
+// expected-note@-3 {{declared here}}
+#endif
+
+typedef void (*fn_ptr_t)();
+
+__host__ __device__ fn_ptr_t get_ptr_hd() {
+  return kernel;
+#ifdef DEVICE
+  // expected-error@-2 {{reference to __global__ function}}
+#endif
+}
+__host__ fn_ptr_t get_ptr_h() {
+  return kernel;
+}
+__device__ fn_ptr_t get_ptr_d() {
+  return kernel;  // expected-error {{reference to __global__ function}}
+}