Fix MLIR bytecode reader for unregistered dialects

[mehdi_amini]

Summary

At the moment we accept (in tests) unregistered dialects and in particular:

"new_processor_id_and_range"()

where there is no . separator. We probably will remove support for this
from the parser, but for now we're adding compatibility support in the
reader.

mlir/lib/Bytecode/Reader/BytecodeReader.cpp

 //===- BytecodeReader.cpp - MLIR Bytecode Reader --------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // TODO: Support for big-endian architectures.
 
 #include "mlir/Bytecode/BytecodeReader.h"
 #include "mlir/AsmParser/AsmParser.h"
 #include "mlir/Bytecode/BytecodeImplementation.h"
 #include "mlir/Bytecode/Encoding.h"
 #include "mlir/IR/BuiltinDialect.h"
 #include "mlir/IR/BuiltinOps.h"
+#include "mlir/IR/Diagnostics.h"
 #include "mlir/IR/OpImplementation.h"
 #include "mlir/IR/Verifier.h"
 #include "mlir/IR/Visitors.h"
 #include "mlir/Support/LLVM.h"
 #include "mlir/Support/LogicalResult.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallString.h"
BytecodeReader::Impl::parseOpName(EncodingReader &reader) {
   // Check to see if this operation name has already been resolved. If we
   // haven't, load the dialect and build the operation name.
   if (!opName->opName) {
     // Load the dialect and its version.
     DialectReader dialectReader(attrTypeReader, stringReader, resourceReader,
                                 reader);
     if (failed(opName->dialect->load(dialectReader, getContext())))
       return failure();
+    // If the opName is empty, this is because we use to accept names such as
+    // `foo` without any `.` separator. We shouldn't tolerate this in textual
+    // format anymore but for now we'll be backward compatible. This can only
+    // happen with unregistered dialects.
+    if (opName->name.empty()) {
+      if (opName->dialect->getLoadedDialect())
+        return emitError(fileLoc) << "has an empty opname for dialect '"
+                                  << opName->dialect->name << "'\n";
+
+      opName->opName.emplace(opName->dialect->name, getContext());
+    } else {
       opName->opName.emplace((opName->dialect->name + "." + opName->name).str(),
                              getContext());
     }
+  }
   return *opName->opName;
 }
 
 //===----------------------------------------------------------------------===//
 // Resource Section
 
 LogicalResult BytecodeReader::Impl::parseResourceSection(
     EncodingReader &reader, std::optional<ArrayRef<uint8_t>> resourceData,

mlir/test/Bytecode/unregistered_dialect.mlir

+// RUN: mlir-opt -emit-bytecode -allow-unregistered-dialect %s | mlir-opt -allow-unregistered-dialect | FileCheck %s
+
+// verify that we round-trip an op without a dialect name (as long as we support this)
+func.func @map1d(%lb: index, %ub: index, %step: index) {
+// CHECK: "new_processor_id_and_range"
+  %0:2 = "new_processor_id_and_range"() : () -> (index, index)
+  return
+}