Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -4355,6 +4355,8 @@
   HelpText<"Measure the parse tree">;
 def fdebug_pre_fir_tree : Flag<["-"], "fdebug-pre-fir-tree">, Group<Action_Group>,
   HelpText<"Dump the pre-FIR tree">;
+def fdebug_module_writer : Flag<["-"],"fdebug-module-writer">, 
+  HelpText<"Enable debug messages while writing module files">;
 
 }
 
Index: clang/lib/Driver/ToolChains/Flang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Flang.cpp
+++ clang/lib/Driver/ToolChains/Flang.cpp
@@ -41,7 +41,7 @@
 }
 
 void Flang::AddOtherOptions(const ArgList &Args, ArgStringList &CmdArgs) const {
-  Args.AddAllArgs(CmdArgs, options::OPT_module_dir);
+  Args.AddAllArgs(CmdArgs, {options::OPT_module_dir, options::OPT_fdebug_module_writer});
 }
 
 void Flang::ConstructJob(Compilation &C, const JobAction &JA,
Index: flang/include/flang/Frontend/CompilerInvocation.h
===================================================================
--- flang/include/flang/Frontend/CompilerInvocation.h
+++ flang/include/flang/Frontend/CompilerInvocation.h
@@ -69,6 +69,8 @@
   // of options.
   std::string moduleDir_ = ".";
 
+  bool debugModuleDir_ = false;
+
   // Fortran Dialect options
   Fortran::common::IntrinsicTypeDefaultKinds defaultKinds_;
 
@@ -91,6 +93,9 @@
   std::string &moduleDir() { return moduleDir_; }
   const std::string &moduleDir() const { return moduleDir_; }
 
+  bool &debugModuleDir() { return debugModuleDir_; }
+  const bool &debugModuleDir() const { return debugModuleDir_; }
+
   Fortran::common::IntrinsicTypeDefaultKinds &defaultKinds() {
     return defaultKinds_;
   }
@@ -106,6 +111,15 @@
       llvm::ArrayRef<const char *> commandLineArgs,
       clang::DiagnosticsEngine &diags);
 
+  /// Useful setters
+  void SetModuleDir(std::string &moduleDir) {
+      moduleDir_ = moduleDir;
+  }
+
+  void SetDebugModuleDir(bool flag) {
+      debugModuleDir_ = flag;
+  }
+
   /// Set the Fortran options to predifined defaults. These defaults are
   /// consistend with f18/f18.cpp.
   // TODO: We should map frontendOpts_ to parserOpts_ instead. For that, we
Index: flang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- flang/lib/Frontend/CompilerInvocation.cpp
+++ flang/lib/Frontend/CompilerInvocation.cpp
@@ -291,9 +291,10 @@
 
 /// Parses all semantic related arguments and populates the variables
 /// options accordingly.
-static void parseSemaArgs(std::string &moduleDir, llvm::opt::ArgList &args,
+static void parseSemaArgs(CompilerInvocation &res, llvm::opt::ArgList &args,
     clang::DiagnosticsEngine &diags) {
 
+  // -J/module-dir option
   auto moduleDirList =
       args.getAllArgValues(clang::driver::options::OPT_module_dir);
   // User can only specify -J/-module-dir once
@@ -305,7 +306,12 @@
     diags.Report(diagID);
   }
   if (moduleDirList.size() == 1)
-    moduleDir = moduleDirList[0];
+    res.SetModuleDir(moduleDirList[0]);
+
+  // -fdebug-module-writer option
+  if (args.hasArg(clang::driver::options::OPT_fdebug_module_writer)) {
+    res.SetDebugModuleDir(true);
+  }
 }
 
 /// Parses all Dialect related arguments and populates the variables
@@ -380,7 +386,7 @@
   // Parse the preprocessor args
   parsePreprocessorArgs(res.preprocessorOpts(), args);
   // Parse semantic args
-  parseSemaArgs(res.moduleDir(), args, diags);
+  parseSemaArgs(res, args, diags);
   // Parse dialect arguments
   parseDialectArgs(res, args, diags);
 
@@ -493,7 +499,6 @@
   semanticsContext_ = std::make_unique<semantics::SemanticsContext>(
       defaultKinds(), fortranOptions.features, allCookedSources);
 
-  auto &moduleDirJ = moduleDir();
-  semanticsContext_->set_moduleDirectory(moduleDirJ)
+  semanticsContext_->set_moduleDirectory(moduleDir())
       .set_searchDirectories(fortranOptions.searchDirectories);
 }
Index: flang/lib/Frontend/FrontendActions.cpp
===================================================================
--- flang/lib/Frontend/FrontendActions.cpp
+++ flang/lib/Frontend/FrontendActions.cpp
@@ -126,7 +126,8 @@
   // Prepare semantics
   setSemantics(std::make_unique<Fortran::semantics::Semantics>(
       ci.invocation().semanticsContext(), parseTree,
-      ci.parsing().cooked().AsCharBlock()));
+      ci.parsing().cooked().AsCharBlock(),
+      ci.invocation().debugModuleDir()));
   auto &semantics = this->semantics();
 
   // Run semantic checks
Index: flang/test/Flang-Driver/driver-help.f90
===================================================================
--- flang/test/Flang-Driver/driver-help.f90
+++ flang/test/Flang-Driver/driver-help.f90
@@ -66,6 +66,7 @@
 ! HELP-FC1-NEXT: -fdebug-dump-symbols    Dump symbols after the semantic analysis
 ! HELP-FC1-NEXT: -fdebug-measure-parse-tree
 ! HELP-FC1-NEXT:                         Measure the parse tree
+! HELP-FC1-NEXT: -fdebug-module-writer   Enable debug messages while writing module files
 ! HELP-FC1-NEXT: -fdebug-pre-fir-tree    Dump the pre-FIR tree
 ! HELP-FC1-NEXT: -fdebug-unparse-with-symbols
 ! HELP-FC1-NEXT:                        Unparse and stop.
Index: flang/test/Semantics/mod-file-rewriter.f90
===================================================================
--- flang/test/Semantics/mod-file-rewriter.f90
+++ flang/test/Semantics/mod-file-rewriter.f90
@@ -1,8 +1,8 @@
 ! RUN: rm -fr %t && mkdir %t && cd %t
-! RUN: %f18 -fsyntax-only -fdebug-module-writer %s 2>&1 | FileCheck %s --check-prefix CHECK_CHANGED
-! RUN: %f18 -fsyntax-only -fdebug-module-writer %s 2>&1 | FileCheck %s --check-prefix CHECK_UNCHANGED
-! RUN: %f18 -fsyntax-only -fdebug-module-writer %p/Inputs/mod-file-unchanged.f90 2>&1 | FileCheck %s --check-prefix CHECK_UNCHANGED
-! RUN: %f18 -fsyntax-only -fdebug-module-writer %p/Inputs/mod-file-changed.f90 2>&1 | FileCheck %s --check-prefix CHECK_CHANGED
+! RUN: %flang_fc1 -fsyntax-only -fdebug-module-writer %s 2>&1 | FileCheck %s --check-prefix CHECK_CHANGED
+! RUN: %flang_fc1 -fsyntax-only -fdebug-module-writer %s 2>&1 | FileCheck %s --check-prefix CHECK_UNCHANGED
+! RUN: %flang_fc1 -fsyntax-only -fdebug-module-writer %p/Inputs/mod-file-unchanged.f90 2>&1 | FileCheck %s --check-prefix CHECK_UNCHANGED
+! RUN: %flang_fc1 -fsyntax-only -fdebug-module-writer %p/Inputs/mod-file-changed.f90 2>&1 | FileCheck %s --check-prefix CHECK_CHANGED
 
 module m
   real :: x(10)