Improve error message when creating an op that isn't registered in the context

[mehdi_amini]

Summary

This prints a more helpful error for folks who aren't intrinsically
familiar with the system.

[ftynse]

mlir/include/mlir/IR/Builders.h

425–426

Nit: can we factor this out into a checkHasAbstractOperation() function to avoid duplicating the long message? Also, this copy doesn't have LLVM_UNLIKELY.

[GMNGeoffrey]

Thanks! I second Alex's suggestion :-)

[mehdi_amini]

Refactor in a helper function

mlir/include/mlir/IR/Builders.h

 //===- Builders.h - Helpers for constructing MLIR Classes -------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef MLIR_IR_BUILDERS_H
 #define MLIR_IR_BUILDERS_H
 
 #include "mlir/IR/OpDefinition.h"
+#include "llvm/Support/Compiler.h"
 
 namespace mlir {
 
 class AffineExpr;
 class BlockAndValueMapping;
 class UnknownLoc;
 class FileLineColLoc;
 class Type;
public:
   //===--------------------------------------------------------------------===//
 
   /// Insert the given operation at the current insertion point and return it.
   Operation *insert(Operation *op);
 
   /// Creates an operation given the fields represented as an OperationState.
   Operation *createOperation(const OperationState &state);
 
+private:
+  /// Helper for sanity checking preconditions for create* methods below.
+  void checkHasAbstractOperation(const OperationName &name) {
+    if (LLVM_UNLIKELY(!name.getAbstractOperation()))
+      llvm::report_fatal_error(
+          "Building op `" + name.getStringRef().str() +
+          "` but it isn't registered in this MLIRContext: the dialect may not "
+          "be loaded or this operation isn't registered by the dialect. See "
+          "also https://mlir.llvm.org/getting_started/Faq/"
+          "#registered-loaded-dependent-whats-up-with-dialects-management");
+  }
+
+public:
   /// Create an operation of specific op type at the current insertion point.
   template <typename OpTy, typename... Args>
   OpTy create(Location location, Args &&...args) {
     OperationState state(location, OpTy::getOperationName());
-    if (!state.name.getAbstractOperation())
-      llvm::report_fatal_error("Building op `" +
-                               state.name.getStringRef().str() +
-                               "` but it isn't registered in this MLIRContext");
+    checkHasAbstractOperation(state.name);
     OpTy::build(*this, state, std::forward<Args>(args)...);
     auto *op = createOperation(state);
     auto result = dyn_cast<OpTy>(op);
     assert(result && "builder didn't return the right type");
     return result;
   }
 
   /// Create an operation of specific op type at the current insertion point,
   /// and immediately try to fold it. This functions populates 'results' with
   /// the results after folding the operation.
   template <typename OpTy, typename... Args>
   void createOrFold(SmallVectorImpl<Value> &results, Location location,
                     Args &&...args) {
     // Create the operation without using 'createOperation' as we don't want to
     // insert it yet.
     OperationState state(location, OpTy::getOperationName());
-    if (!state.name.getAbstractOperation())
-      llvm::report_fatal_error("Building op `" +
-                               state.name.getStringRef().str() +
-                               "` but it isn't registered in this MLIRContext");
+    checkHasAbstractOperation(state.name);
     OpTy::build(*this, state, std::forward<Args>(args)...);
     Operation *op = Operation::create(state);
 
     // Fold the operation. If successful destroy it, otherwise insert it.
     if (succeeded(tryFold(op, results)))
       op->destroy();
     else
       insert(op);