Add CUDF_UNREACHABLE macro.

cpp/include/cudf/ast/detail/operators.hpp

@@ -201,13 +201,7 @@ CUDA_HOST_DEVICE_CALLABLE constexpr void ast_operator_dispatcher(ast_operator op
     case ast_operator::CAST_TO_FLOAT64:
       f.template operator()<ast_operator::CAST_TO_FLOAT64>(std::forward<Ts>(args)...);
       break;
-    default:
-#ifndef __CUDA_ARCH__
-      CUDF_FAIL("Invalid operator.");
-#else
-      cudf_assert(false && "Invalid operator.");
-#endif
-      break;
+    default: CUDF_UNREACHABLE("Unsupported operator.");
   }
 }
 

cpp/include/cudf/detail/utilities/assert.cuh

@@ -35,3 +35,26 @@
 #else
 #define cudf_assert(e) (static_cast<void>(0))
 #endif
+
+/**
+ * @brief Macro indicating that a location in the code is unreachable.
+ *
+ * The CUDF_UNREACHABLE macro should only be used where CUDF_FAIL cannot be used
+ * due to performance or due to being used in device code. In the majority of
+ * host code situations, an exception should be thrown in "unreachable" code
+ * paths as those usually aren't tight inner loops like they are in device code.
+ *
+ * One example where this macro may be used is in conjunction with dispatchers
+ * to indicate that a function does not need to return a default value because
+ * it has already exhausted all possible cases in a `switch` statement.
+ *
+ * Example usage:
+ * ```
+ * CUDF_UNREACHABLE("Invalid type_id.");
+ * ```
+ */
+#define CUDF_UNREACHABLE(msg)             \
+  do {                                    \
+    assert(false && "Unreachable: " msg); \
+    __builtin_unreachable();              \
+  } while (0)

cpp/include/cudf/utilities/type_dispatcher.hpp

@@ -509,18 +509,9 @@ CUDF_HDFI constexpr decltype(auto) type_dispatcher(cudf::data_type dtype, Functo
         std::forward<Ts>(args)...);
     default: {
 #ifndef __CUDA_ARCH__
-      CUDF_FAIL("Unsupported type_id.");
+      CUDF_FAIL("Invalid type_id.");
 #else
-      cudf_assert(false && "Unsupported type_id.");
-
-      // The following code will never be reached, but the compiler generates a
-      // warning if there isn't a return value.
-
-      // Need to find out what the return type is in order to have a default
-      // return value and solve the compiler warning for lack of a default
-      // return
-      using return_type = decltype(f.template operator()<int8_t>(std::forward<Ts>(args)...));
-      return return_type();
+      CUDF_UNREACHABLE("Invalid type_id.");
 #endif
     }
   }