[MLIR] add show-dialects option for mlir-opt

[stephenneuendorffer]

Summary

Display the list of dialects known to mlir-opt. This is useful
for ensuring that linkage has happened correctly, for instance.

[rriddle]

mlir/tools/mlir-opt/mlir-opt.cpp

67

Can you please sink the context into the if you avoid unnecessarily initializing it?

70

nit: Remove the trivial brace here.

mlir/tools/mlir-opt/mlir-opt.cpp

 //===- mlir-opt.cpp - MLIR Optimizer Driver -------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 //
 // Main entry function for mlir-opt for when built as standalone binary.
 //
 //===----------------------------------------------------------------------===//
 
 #include "mlir/Analysis/Passes.h"
+#include "mlir/IR/Dialect.h"
+#include "mlir/IR/MLIRContext.h"
 #include "mlir/Pass/Pass.h"
 #include "mlir/Pass/PassManager.h"
 #include "mlir/Support/FileUtilities.h"
 #include "mlir/Support/MlirOptMain.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/ToolOutputFile.h"
verifyDiagnostics("verify-diagnostics",
                                "expected-* lines on the corresponding line"),
                       cl::init(false));
 
 static cl::opt<bool>
     verifyPasses("verify-each",
                  cl::desc("Run the verifier after each transformation pass"),
                  cl::init(true));
 
+static cl::opt<bool>
+    showDialects("show-dialects",
+                 cl::desc("Print the list of registered dialects"),
+                 cl::init(false));
+
 int main(int argc, char **argv) {
   InitLLVM y(argc, argv);
 
   // Register any pass manager command line options.
   registerPassManagerCLOptions();
   PassPipelineCLParser passPipeline("", "Compiler passes to run");
 
   // Parse pass names in main to ensure static initialization completed.
   cl::ParseCommandLineOptions(argc, argv, "MLIR modular optimizer driver\n");
 
+  MLIRContext context;
+  if(showDialects) {
+    llvm::outs() << "Registered Dialects:\n";
+    for(Dialect *dialect : context.getRegisteredDialects()) {
+      llvm::outs() << dialect->getNamespace() << "\n";
+    }
+    return 0;
+  }
+
   // Set up the input file.
   std::string errorMessage;
   auto file = openInputFile(inputFilename, &errorMessage);
   if (!file) {
     llvm::errs() << errorMessage << "\n";
     return 1;
   }