[MLIR] Fix mlir-opt --show-dialects to not require any input (as do…

…cumented)

mlir/lib/Tools/mlir-opt/MlirOptMain.cpp

@@ -31,6 +31,7 @@
 #include "mlir/Pass/Pass.h"
 #include "mlir/Pass/PassManager.h"
 #include "mlir/Support/FileUtilities.h"
+#include "mlir/Support/LogicalResult.h"
 #include "mlir/Support/Timing.h"
 #include "mlir/Support/ToolUtilities.h"
 #include "mlir/Tools/ParseUtilities.h"
@@ -513,15 +514,19 @@ mlir::registerAndParseCLIOptions(int argc, char **argv,
   return std::make_pair(inputFilename.getValue(), outputFilename.getValue());
 }
 
+static LogicalResult printRegisteredDialects(DialectRegistry &registry) {
+  llvm::outs() << "Available Dialects: ";
+  interleave(registry.getDialectNames(), llvm::outs(), ",");
+  llvm::outs() << "\n";
+  return success();
+}
+
 LogicalResult mlir::MlirOptMain(llvm::raw_ostream &outputStream,
                                 std::unique_ptr<llvm::MemoryBuffer> buffer,
                                 DialectRegistry &registry,
                                 const MlirOptMainConfig &config) {
-  if (config.shouldShowDialects()) {
-    llvm::outs() << "Available Dialects: ";
-    interleave(registry.getDialectNames(), llvm::outs(), ",");
-    llvm::outs() << "\n";
-  }
+  if (config.shouldShowDialects())
+    return printRegisteredDialects(registry);
 
   // The split-input-file mode is a very specific mode that slices the file
   // up into small pieces and checks each independently.
@@ -556,6 +561,9 @@ LogicalResult mlir::MlirOptMain(int argc, char **argv,
 
   MlirOptMainConfig config = MlirOptMainConfig::createFromCLOptions();
 
+  if (config.shouldShowDialects())
+    return printRegisteredDialects(registry);
+
   // When reading from stdin and the input is a tty, it is often a user mistake
   // and the process "appears to be stuck". Print a message to let the user know
   // about it!