[clang] 4c5906c - [Flang][Driver] Add infrastructure for basic frontend actions and file I/O

Andrzej Warzynski via cfe-commits cfe-commits at lists.llvm.org
Sat Oct 24 06:58:47 PDT 2020


Author: Caroline Concatto
Date: 2020-10-24T14:58:32+01:00
New Revision: 4c5906cffd04202387d2f6b50a47d39c0e4f2c0e

URL: https://github.com/llvm/llvm-project/commit/4c5906cffd04202387d2f6b50a47d39c0e4f2c0e
DIFF: https://github.com/llvm/llvm-project/commit/4c5906cffd04202387d2f6b50a47d39c0e4f2c0e.diff

LOG: [Flang][Driver] Add infrastructure for basic frontend actions and file I/O

This patch introduces the dependencies required to read and manage input files
provided by the command line option. It also adds the infrastructure to create
and write to output files. The output is sent to either stdout or a file
(specified with the `-o` flag).

Separately, in order to be able to test the code for file I/O, it adds
infrastructure to create frontend actions. As a basic testable example, it adds
the `InputOutputTest` FrontendAction. The sole purpose of this action is to
read a file from the command line and print it either to stdout or the output
file.  This action is run by using the `-test-io` flag also introduced in this
patch (available for `flang-new` and `flang-new -fc1`). With this patch:
```
flang-new -test-io input-file.f90
```
will read input-file.f90 and print it in the output file.

The `InputOutputTest` frontend action has been introduced primarily to
facilitate testing. It is hidden from users (i.e. it's only displayed with
`--help-hidden`). Currently Clang doesn’t have an equivalent action.

`-test-io` is used to trigger the InputOutputTest action in the Flang frontend
driver. This patch makes sure that “flang-new” forwards it to “flang-new -fc1"
by creating a preprocessor job. However, in Flang.cpp, `-test-io` is passed to
“flang-new -fc1” without `-E`. This way we make sure that the preprocessor is
_not_ run in the frontend driver. This is the desired behaviour: `-test-io`
should only read the input file and print it to the output stream.

co-authored-by: Andrzej Warzynski <andrzej.warzynski at arm.com>

Differential Revision: https://reviews.llvm.org/D87989

Added: 
    flang/include/flang/Frontend/FrontendAction.h
    flang/include/flang/Frontend/FrontendActions.h
    flang/lib/Frontend/FrontendAction.cpp
    flang/lib/Frontend/FrontendActions.cpp
    flang/test/Flang-Driver/driver-help-hidden.f90
    flang/test/Frontend/Inputs/hello-world.f90
    flang/test/Frontend/input-output-file.f90
    flang/test/Frontend/multiple-input-files.f90
    flang/unittests/Frontend/InputOutputTest.cpp

Modified: 
    clang/include/clang/Driver/Options.h
    clang/include/clang/Driver/Options.td
    clang/lib/Driver/Driver.cpp
    clang/lib/Driver/ToolChains/Flang.cpp
    clang/lib/Driver/Types.cpp
    clang/test/Driver/immediate-options.c
    flang/include/flang/Frontend/CompilerInstance.h
    flang/include/flang/Frontend/CompilerInvocation.h
    flang/include/flang/Frontend/FrontendOptions.h
    flang/include/flang/FrontendTool/Utils.h
    flang/lib/Frontend/CMakeLists.txt
    flang/lib/Frontend/CompilerInstance.cpp
    flang/lib/Frontend/CompilerInvocation.cpp
    flang/lib/Frontend/FrontendOptions.cpp
    flang/lib/FrontendTool/CMakeLists.txt
    flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
    flang/test/Flang-Driver/driver-help.f90
    flang/test/Flang-Driver/emit-obj.f90
    flang/test/lit.cfg.py
    flang/tools/flang-driver/fc1_main.cpp
    flang/unittests/Frontend/CMakeLists.txt
    flang/unittests/Frontend/CompilerInstanceTest.cpp

Removed: 
    


################################################################################
diff  --git a/clang/include/clang/Driver/Options.h b/clang/include/clang/Driver/Options.h
index 06dd3652be94..ae188cec93c2 100644
--- a/clang/include/clang/Driver/Options.h
+++ b/clang/include/clang/Driver/Options.h
@@ -36,7 +36,8 @@ enum ClangFlags {
   LinkOption = (1 << 13),
   FlangOption = (1 << 14),
   FC1Option = (1 << 15),
-  Ignored = (1 << 16),
+  FlangOnlyOption = (1 << 16),
+  Ignored = (1 << 17),
 };
 
 enum ID {

diff  --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 0cab3e8ecc16..0b9817a0cfd4 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -60,6 +60,10 @@ def LinkOption : OptionFlag;
 // flang mode.
 def FlangOption : OptionFlag;
 
+// FlangOnlyOption - This option should only be used by Flang (i.e. it is not
+// available for Clang)
+def FlangOnlyOption : OptionFlag;
+
 // FC1Option - This option should be accepted by flang -fc1.
 def FC1Option : OptionFlag;
 
@@ -2130,8 +2134,8 @@ def gno_embed_source : Flag<["-"], "gno-embed-source">, Group<g_flags_Group>,
     Flags<[DriverOption]>,
     HelpText<"Restore the default behavior of not embedding source text in DWARF debug sections">;
 def headerpad__max__install__names : Joined<["-"], "headerpad_max_install_names">;
-def help : Flag<["-", "--"], "help">, Flags<[CC1Option,CC1AsOption, FC1Option, FlangOption]>,
-  HelpText<"Display available options">;
+def help : Flag<["-", "--"], "help">, Flags<[CC1Option,CC1AsOption, FC1Option,
+    FlangOption]>, HelpText<"Display available options">;
 def ibuiltininc : Flag<["-"], "ibuiltininc">,
   HelpText<"Enable builtin #include directories even when -nostdinc is used "
            "before or after -ibuiltininc. "
@@ -2811,7 +2815,8 @@ def nostdincxx : Flag<["-"], "nostdinc++">, Flags<[CC1Option]>,
 def nostdlib : Flag<["-"], "nostdlib">, Group<Link_Group>;
 def nostdlibxx : Flag<["-"], "nostdlib++">;
 def object : Flag<["-"], "object">;
-def o : JoinedOrSeparate<["-"], "o">, Flags<[DriverOption, RenderAsInput, CC1Option, CC1AsOption]>,
+def o : JoinedOrSeparate<["-"], "o">, Flags<[DriverOption, RenderAsInput,
+  CC1Option, CC1AsOption, FC1Option, FlangOption]>,
   HelpText<"Write output to <file>">, MetaVarName<"<file>">;
 def pagezero__size : JoinedOrSeparate<["-"], "pagezero_size">;
 def pass_exit_codes : Flag<["-", "--"], "pass-exit-codes">, Flags<[Unsupported]>;
@@ -3546,6 +3551,12 @@ def fno_sycl : Flag<["-"], "fno-sycl">, Group<sycl_Group>, Flags<[CoreOption]>,
 def sycl_std_EQ : Joined<["-"], "sycl-std=">, Group<sycl_Group>, Flags<[CC1Option, NoArgumentUnused, CoreOption]>,
   HelpText<"SYCL language standard to compile for.">, Values<"2017, 121, 1.2.1, sycl-1.2.1">;
 
+//===----------------------------------------------------------------------===//
+// FlangOption and FC1 Options
+//===----------------------------------------------------------------------===//
+def test_io : Flag<["-"], "test-io">, Flags<[HelpHidden, FlangOption, FC1Option, FlangOnlyOption]>, Group<Action_Group>,
+  HelpText<"Run the InputOuputTest action. Use for development and testing only.">;
+
 //===----------------------------------------------------------------------===//
 // CC1 Options
 //===----------------------------------------------------------------------===//

diff  --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 6f2a030290ed..aaa66661af75 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -211,6 +211,11 @@ InputArgList Driver::ParseArgStrings(ArrayRef<const char *> ArgStrings,
   std::tie(IncludedFlagsBitmask, ExcludedFlagsBitmask) =
       getIncludeExcludeOptionFlagMasks(IsClCompatMode);
 
+  // Make sure that Flang-only options don't pollute the Clang output
+  // TODO: Make sure that Clang-only options don't pollute Flang output
+  if (!IsFlangMode())
+    ExcludedFlagsBitmask |= options::FlangOnlyOption;
+
   unsigned MissingArgIndex, MissingArgCount;
   InputArgList Args =
       getOpts().ParseArgs(ArgStrings, MissingArgIndex, MissingArgCount,
@@ -1573,6 +1578,8 @@ void Driver::PrintHelp(bool ShowHidden) const {
 
   if (IsFlangMode())
     IncludedFlagsBitmask |= options::FlangOption;
+  else
+    ExcludedFlagsBitmask |= options::FlangOnlyOption;
 
   std::string Usage = llvm::formatv("{0} [options] file...", Name).str();
   getOpts().PrintHelp(llvm::outs(), Usage.c_str(), DriverTitle.c_str(),
@@ -1628,6 +1635,11 @@ void Driver::HandleAutocompletions(StringRef PassedFlags) const {
   unsigned int DisableFlags =
       options::NoDriverOption | options::Unsupported | options::Ignored;
 
+  // Make sure that Flang-only options don't pollute the Clang output
+  // TODO: Make sure that Clang-only options don't pollute Flang output
+  if (!IsFlangMode())
+    DisableFlags |= options::FlangOnlyOption;
+
   // Distinguish "--autocomplete=-someflag" and "--autocomplete=-someflag,"
   // because the latter indicates that the user put space before pushing tab
   // which should end up in a file completion.

diff  --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index f8633b988faa..6d79fbe6aa8f 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -30,12 +30,18 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
 
   CmdArgs.push_back("-fc1");
 
-  CmdArgs.push_back("-triple");
-  CmdArgs.push_back(Args.MakeArgString(TripleStr));
-
+  // TODO: Eventually all actions will require a triple (e.g. `-triple
+  // aarch64-unknown-linux-gnu`). However, `-triple` is currently not supported
+  // by `flang-new -fc1`, so we only add it selectively to actions that we
+  // don't support/execute just yet.
   if (isa<PreprocessJobAction>(JA)) {
-    CmdArgs.push_back("-E");
+    if (C.getArgs().hasArg(options::OPT_test_io))
+      CmdArgs.push_back("-test-io");
+    else
+      CmdArgs.push_back("-E");
   } else if (isa<CompileJobAction>(JA) || isa<BackendJobAction>(JA)) {
+    CmdArgs.push_back("-triple");
+    CmdArgs.push_back(Args.MakeArgString(TripleStr));
     if (JA.getType() == types::TY_Nothing) {
       CmdArgs.push_back("-fsyntax-only");
     } else if (JA.getType() == types::TY_AST) {
@@ -52,6 +58,8 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
       assert(false && "Unexpected output type!");
     }
   } else if (isa<AssembleJobAction>(JA)) {
+    CmdArgs.push_back("-triple");
+    CmdArgs.push_back(Args.MakeArgString(TripleStr));
     CmdArgs.push_back("-emit-obj");
   } else {
     assert(false && "Unexpected action class for Flang tool.");

diff  --git a/clang/lib/Driver/Types.cpp b/clang/lib/Driver/Types.cpp
index 2050dffa6fa0..e898334c3227 100644
--- a/clang/lib/Driver/Types.cpp
+++ b/clang/lib/Driver/Types.cpp
@@ -325,10 +325,12 @@ types::getCompilationPhases(const clang::driver::Driver &Driver,
   // Filter to compiler mode. When the compiler is run as a preprocessor then
   // compilation is not an option.
   // -S runs the compiler in Assembly listing mode.
+  // -test-io is used by Flang to run InputOutputTest action
   if (Driver.CCCIsCPP() || DAL.getLastArg(options::OPT_E) ||
       DAL.getLastArg(options::OPT__SLASH_EP) ||
       DAL.getLastArg(options::OPT_M, options::OPT_MM) ||
-      DAL.getLastArg(options::OPT__SLASH_P))
+      DAL.getLastArg(options::OPT__SLASH_P) ||
+      DAL.getLastArg(options::OPT_test_io))
     LastPhase = phases::Preprocess;
 
   // --precompile only runs up to precompilation.

diff  --git a/clang/test/Driver/immediate-options.c b/clang/test/Driver/immediate-options.c
index d7cd6be40801..723a6fa302f8 100644
--- a/clang/test/Driver/immediate-options.c
+++ b/clang/test/Driver/immediate-options.c
@@ -3,8 +3,12 @@
 // HELP-NOT: ast-dump
 // HELP-NOT: driver-mode
 
+// Make sure that Flang-only options are not available in Clang
+// HELP-NOT: test-io
+
 // RUN: %clang --help-hidden | FileCheck %s -check-prefix=HELP-HIDDEN
 // HELP-HIDDEN: driver-mode
+// HELP-HIDDEN-NOT: test-io
 
 // RUN: %clang -dumpversion | FileCheck %s -check-prefix=DUMPVERSION
 // DUMPVERSION: {{[0-9]+\.[0-9.]+}}

diff  --git a/flang/include/flang/Frontend/CompilerInstance.h b/flang/include/flang/Frontend/CompilerInstance.h
index 298be676ea4a..c259be159366 100644
--- a/flang/include/flang/Frontend/CompilerInstance.h
+++ b/flang/include/flang/Frontend/CompilerInstance.h
@@ -9,6 +9,9 @@
 #define LLVM_FLANG_FRONTEND_COMPILERINSTANCE_H
 
 #include "flang/Frontend/CompilerInvocation.h"
+#include "flang/Frontend/FrontendAction.h"
+#include "flang/Parser/provenance.h"
+#include "llvm/Support/raw_ostream.h"
 
 #include <cassert>
 #include <memory>
@@ -20,18 +23,68 @@ class CompilerInstance {
   /// The options used in this compiler instance.
   std::shared_ptr<CompilerInvocation> invocation_;
 
+  /// Flang file  manager.
+  std::shared_ptr<Fortran::parser::AllSources> allSources_;
+
   /// The diagnostics engine instance.
   llvm::IntrusiveRefCntPtr<clang::DiagnosticsEngine> diagnostics_;
 
+  /// Holds information about the output file.
+  struct OutputFile {
+    std::string filename_;
+    OutputFile(std::string inputFilename)
+        : filename_(std::move(inputFilename)) {}
+  };
+
+  /// Output stream that doesn't support seeking (e.g. terminal, pipe).
+  /// This stream is normally wrapped in buffer_ostream before being passed
+  /// to users (e.g. via CreateOutputFile).
+  std::unique_ptr<llvm::raw_fd_ostream> nonSeekStream_;
+
+  /// The list of active output files.
+  std::list<OutputFile> outputFiles_;
+
+  /// Holds the output stream provided by the user. Normally, users of
+  /// CompilerInstance will call CreateOutputFile to obtain/create an output
+  /// stream. If they want to provide their own output stream, this field will
+  /// facilitate this. It is optional and will normally be just a nullptr.
+  std::unique_ptr<llvm::raw_pwrite_stream> outputStream_;
+
 public:
   explicit CompilerInstance();
 
   ~CompilerInstance();
+
+  /// @name Compiler Invocation
+  /// {
+
   CompilerInvocation &GetInvocation() {
     assert(invocation_ && "Compiler instance has no invocation!");
     return *invocation_;
   };
 
+  /// Replace the current invocation.
+  void SetInvocation(std::shared_ptr<CompilerInvocation> value);
+
+  /// }
+  /// @name File manager
+  /// {
+
+  /// Return the current allSources.
+  Fortran::parser::AllSources &GetAllSources() const { return *allSources_; }
+
+  bool HasAllSources() const { return allSources_ != nullptr; }
+
+  /// }
+  /// @name High-Level Operations
+  /// {
+
+  /// Execute the provided action against the compiler's
+  /// CompilerInvocation object.
+  /// \param act - The action to execute.
+  /// \return - True on success.
+  bool ExecuteAction(FrontendAction &act);
+
   /// }
   /// @name Forwarding Methods
   /// {
@@ -60,7 +113,7 @@ class CompilerInstance {
     return *diagnostics_;
   }
 
-  /// SetDiagnostics - Replace the current diagnostics engine.
+  /// Replace the current diagnostics engine.
   void SetDiagnostics(clang::DiagnosticsEngine *value);
 
   clang::DiagnosticConsumer &GetDiagnosticClient() const {
@@ -75,23 +128,59 @@ class CompilerInstance {
     return *diagnostics_;
   }
 
+  /// {
+  /// @name Output Files
+  /// {
+
+  /// Add an output file onto the list of tracked output files.
+  ///
+  /// \param outFile - The output file info.
+  void AddOutputFile(OutputFile &&outFile);
+
+  /// Clear the output file list.
+  void ClearOutputFiles(bool eraseFiles);
+
+  /// Create the default output file (based on the invocation's options) and
+  /// add it to the list of tracked output files. If the name of the output
+  /// file is not provided, it is derived from the input file.
+  ///
+  /// \param binary     The mode to open the file in.
+  /// \param baseInput  If the invocation contains no output file name (i.e.
+  ///                   outputFile_ in FrontendOptions is empty), the input path
+  ///                   name to use for deriving the output path.
+  /// \param extension  The extension to use for output names derived from
+  ///                   \p baseInput.
+  /// \return           ostream for the output file or nullptr on error.
+  std::unique_ptr<llvm::raw_pwrite_stream> CreateDefaultOutputFile(
+      bool binary = true, llvm::StringRef baseInput = "",
+      llvm::StringRef extension = "");
+
+  /// Create a new output file
+  ///
+  /// \param outputPath   The path to the output file.
+  /// \param error [out]  On failure, the error.
+  /// \param binary       The mode to open the file in.
+  /// \return             ostream for the output file or nullptr on error.
+  std::unique_ptr<llvm::raw_pwrite_stream> CreateOutputFile(
+      llvm::StringRef outputPath, std::error_code &error, bool binary);
+
   /// }
   /// @name Construction Utility Methods
   /// {
 
-  /// Create a DiagnosticsEngine object with a the TextDiagnosticPrinter.
+  /// Create a DiagnosticsEngine object
   ///
-  /// If no diagnostic client is provided, this creates a
-  /// DiagnosticConsumer that is owned by the returned diagnostic
-  /// object, if using directly the caller is responsible for
-  /// releasing the returned DiagnosticsEngine's client eventually.
+  /// If no diagnostic client is provided, this method creates a
+  /// DiagnosticConsumer that is owned by the returned diagnostic object. If
+  /// using directly the caller is responsible for releasing the returned
+  /// DiagnosticsEngine's client eventually.
   ///
   /// \param opts - The diagnostic options; note that the created text
   /// diagnostic object contains a reference to these options.
   ///
-  /// \param client If non-NULL, a diagnostic client that will be
-  /// attached to (and, then, owned by) the returned DiagnosticsEngine
-  /// object.
+  /// \param client - If non-NULL, a diagnostic client that will be attached to
+  /// (and optionally, depending on /p shouldOwnClient, owned by) the returned
+  /// DiagnosticsEngine object.
   ///
   /// \return The new object on success, or null on failure.
   static clang::IntrusiveRefCntPtr<clang::DiagnosticsEngine> CreateDiagnostics(
@@ -99,6 +188,20 @@ class CompilerInstance {
       clang::DiagnosticConsumer *client = nullptr, bool shouldOwnClient = true);
   void CreateDiagnostics(
       clang::DiagnosticConsumer *client = nullptr, bool shouldOwnClient = true);
+
+  /// }
+  /// @name Output Stream Methods
+  /// {
+  void SetOutputStream(std::unique_ptr<llvm::raw_pwrite_stream> outStream) {
+    outputStream_ = std::move(outStream);
+  }
+
+  bool IsOutputStreamNull() { return (outputStream_ == nullptr); }
+
+  // Allow the frontend compiler to write in the output stream.
+  void WriteOutputStream(const std::string &message) {
+    *outputStream_ << message;
+  }
 };
 
 } // end namespace Fortran::frontend

diff  --git a/flang/include/flang/Frontend/CompilerInvocation.h b/flang/include/flang/Frontend/CompilerInvocation.h
index 05f93293d0a5..e7764d49244a 100644
--- a/flang/include/flang/Frontend/CompilerInvocation.h
+++ b/flang/include/flang/Frontend/CompilerInvocation.h
@@ -24,7 +24,7 @@ bool ParseDiagnosticArgs(clang::DiagnosticOptions &opts,
 
 class CompilerInvocationBase {
 public:
-  /// Options controlling the diagnostic engine.$
+  /// Options controlling the diagnostic engine.
   llvm::IntrusiveRefCntPtr<clang::DiagnosticOptions> diagnosticOpts_;
 
   CompilerInvocationBase();

diff  --git a/flang/include/flang/Frontend/FrontendAction.h b/flang/include/flang/Frontend/FrontendAction.h
new file mode 100644
index 000000000000..06121686b0d8
--- /dev/null
+++ b/flang/include/flang/Frontend/FrontendAction.h
@@ -0,0 +1,101 @@
+//===- FrontendAction.h -----------------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Defines the flang::FrontendAction interface.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FLANG_FRONTEND_FRONTENDACTION_H
+#define LLVM_FLANG_FRONTEND_FRONTENDACTION_H
+
+#include "flang/Frontend/FrontendOptions.h"
+#include "llvm/Support/Error.h"
+
+namespace Fortran::frontend {
+class CompilerInstance;
+
+/// Abstract base class for actions which can be performed by the frontend.
+class FrontendAction {
+  FrontendInputFile currentInput_;
+  CompilerInstance *instance_;
+
+protected:
+  /// @name Implementation Action Interface
+  /// @{
+
+  /// Callback to run the program action, using the initialized
+  /// compiler instance.
+  virtual void ExecuteAction() = 0;
+
+  /// Callback at the end of processing a single input, to determine
+  /// if the output files should be erased or not.
+  ///
+  /// By default it returns true if a compiler error occurred.
+  virtual bool ShouldEraseOutputFiles();
+
+  /// @}
+
+public:
+  FrontendAction() : instance_(nullptr) {}
+  virtual ~FrontendAction() = default;
+
+  /// @name Compiler Instance Access
+  /// @{
+
+  CompilerInstance &GetCompilerInstance() const {
+    assert(instance_ && "Compiler instance not registered!");
+    return *instance_;
+  }
+
+  void SetCompilerInstance(CompilerInstance *value) { instance_ = value; }
+
+  /// @}
+  /// @name Current File Information
+  /// @{
+
+  const FrontendInputFile &GetCurrentInput() const { return currentInput_; }
+
+  llvm::StringRef GetCurrentFile() const {
+    assert(!currentInput_.IsEmpty() && "No current file!");
+    return currentInput_.GetFile();
+  }
+
+  llvm::StringRef GetCurrentFileOrBufferName() const {
+    assert(!currentInput_.IsEmpty() && "No current file!");
+    return currentInput_.IsFile()
+        ? currentInput_.GetFile()
+        : currentInput_.GetBuffer()->getBufferIdentifier();
+  }
+  void SetCurrentInput(const FrontendInputFile &currentInput);
+
+  /// @}
+  /// @name Public Action Interface
+  /// @}
+
+  /// Prepare the action for processing the input file \p input.
+  ///
+  /// This is run after the options and frontend have been initialized,
+  /// but prior to executing any per-file processing.
+  /// \param ci - The compiler instance this action is being run from. The
+  /// action may store and use this object.
+  /// \param input - The input filename and kind.
+  /// \return True on success; on failure the compilation of this file should
+  bool BeginSourceFile(CompilerInstance &ci, const FrontendInputFile &input);
+
+  /// Run the action.
+  llvm::Error Execute();
+
+  /// Perform any per-file post processing, deallocate per-file
+  /// objects, and run statistics and output file cleanup code.
+  void EndSourceFile();
+};
+
+} // namespace Fortran::frontend
+
+#endif // LLVM_FLANG_FRONTEND_FRONTENDACTION_H

diff  --git a/flang/include/flang/Frontend/FrontendActions.h b/flang/include/flang/Frontend/FrontendActions.h
new file mode 100644
index 000000000000..c6d9d070b016
--- /dev/null
+++ b/flang/include/flang/Frontend/FrontendActions.h
@@ -0,0 +1,26 @@
+//===- FrontendActions.h -----------------------------------------*- 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 LLVM_FLANG_FRONTEND_FRONTENDACTIONS_H
+#define LLVM_FLANG_FRONTEND_FRONTENDACTIONS_H
+
+#include "flang/Frontend/FrontendAction.h"
+
+namespace Fortran::frontend {
+
+//===----------------------------------------------------------------------===//
+// Custom Consumer Actions
+//===----------------------------------------------------------------------===//
+
+class InputOutputTestAction : public FrontendAction {
+  void ExecuteAction() override;
+};
+
+} // namespace Fortran::frontend
+
+#endif // LLVM_FLANG_FRONTEND_FRONTENDACTIONS_H

diff  --git a/flang/include/flang/Frontend/FrontendOptions.h b/flang/include/flang/Frontend/FrontendOptions.h
index 474086f44e3b..f1857043c8ab 100644
--- a/flang/include/flang/Frontend/FrontendOptions.h
+++ b/flang/include/flang/Frontend/FrontendOptions.h
@@ -8,10 +8,41 @@
 #ifndef LLVM_FLANG_FRONTEND_FRONTENDOPTIONS_H
 #define LLVM_FLANG_FRONTEND_FRONTENDOPTIONS_H
 
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/MemoryBuffer.h"
+
 #include <cstdint>
 #include <string>
+
 namespace Fortran::frontend {
 
+enum ActionKind {
+  InvalidAction = 0,
+
+  /// -test-io mode
+  InputOutputTest,
+
+  // TODO: ADD flags as the Actions are implemented, e.g.
+  // RunPreprocessor, ParserSyntaxOnly, EmitLLVM, EmitLLVMOnly,
+  // EmitCodeGenOnly, EmitAssembly, (...)
+};
+
+inline const char *GetActionKindName(const ActionKind ak) {
+  switch (ak) {
+  case InputOutputTest:
+    return "InputOutputTest";
+  default:
+    return "<unknown ActionKind>";
+    // TODO:
+    // case RunPreprocessor:
+    // case ParserSyntaxOnly:
+    // case EmitLLVM:
+    // case EmitLLVMOnly:
+    // case EmitCodeGenOnly:
+    // (...)
+  }
+}
+
 enum class Language : uint8_t {
   Unknown,
 
@@ -19,9 +50,8 @@ enum class Language : uint8_t {
   /// and compile it to assembly or object code.
   LLVM_IR,
 
-  ///@{ Languages that the frontend can parse and compile.
+  /// @{ Languages that the frontend can parse and compile.
   Fortran,
-  ///@}
 };
 
 /// The kind of a file that we've been handed as an input.
@@ -41,6 +71,43 @@ class InputKind {
   bool IsUnknown() const { return lang_ == Language::Unknown; }
 };
 
+/// An input file for the front end.
+class FrontendInputFile {
+  /// The file name, or "-" to read from standard input.
+  std::string file_;
+
+  /// The input, if it comes from a buffer rather than a file. This object
+  /// does not own the buffer, and the caller is responsible for ensuring
+  /// that it outlives any users.
+  const llvm::MemoryBuffer *buffer_ = nullptr;
+
+  /// The kind of input, atm it contains language
+  InputKind kind_;
+
+public:
+  FrontendInputFile() = default;
+  FrontendInputFile(llvm::StringRef file, InputKind kind)
+      : file_(file.str()), kind_(kind) {}
+  FrontendInputFile(const llvm::MemoryBuffer *buffer, InputKind kind)
+      : buffer_(buffer), kind_(kind) {}
+
+  InputKind GetKind() const { return kind_; }
+
+  bool IsEmpty() const { return file_.empty() && buffer_ == nullptr; }
+  bool IsFile() const { return !IsBuffer(); }
+  bool IsBuffer() const { return buffer_ != nullptr; }
+
+  llvm::StringRef GetFile() const {
+    assert(IsFile());
+    return file_;
+  }
+
+  const llvm::MemoryBuffer *GetBuffer() const {
+    assert(IsBuffer() && "Requested buffer_, but it is empty!");
+    return buffer_;
+  }
+};
+
 /// FrontendOptions - Options for controlling the behavior of the frontend.
 class FrontendOptions {
 public:
@@ -50,8 +117,24 @@ class FrontendOptions {
   /// Show the -version text.
   unsigned showVersion_ : 1;
 
+  /// The input files and their types.
+  std::vector<FrontendInputFile> inputs_;
+
+  /// The output file, if any.
+  std::string outputFile_;
+
+  /// The frontend action to perform.
+  frontend::ActionKind programAction_;
+
 public:
   FrontendOptions() : showHelp_(false), showVersion_(false) {}
+
+  // Return the appropriate input kind for a file extension. For example,
+  /// "*.f" would return Language::Fortran.
+  ///
+  /// \return The input kind for the extension, or Language::Unknown if the
+  /// extension is not recognized.
+  static InputKind GetInputKindForExtension(llvm::StringRef extension);
 };
 } // namespace Fortran::frontend
 

diff  --git a/flang/include/flang/FrontendTool/Utils.h b/flang/include/flang/FrontendTool/Utils.h
index f49c4e6dae62..d62c03d8dc0b 100644
--- a/flang/include/flang/FrontendTool/Utils.h
+++ b/flang/include/flang/FrontendTool/Utils.h
@@ -17,6 +17,13 @@
 namespace Fortran::frontend {
 
 class CompilerInstance;
+class FrontendAction;
+
+/// Construct the FrontendAction of a compiler invocation based on the
+/// options specified for the compiler invocation.
+///
+/// \return - The created FrontendAction object
+std::unique_ptr<FrontendAction> CreateFrontendAction(CompilerInstance &ci);
 
 /// ExecuteCompilerInvocation - Execute the given actions described by the
 /// compiler invocation object in the given compiler instance.

diff  --git a/flang/lib/Frontend/CMakeLists.txt b/flang/lib/Frontend/CMakeLists.txt
index 3cebc959c5ec..22e38595ea4c 100644
--- a/flang/lib/Frontend/CMakeLists.txt
+++ b/flang/lib/Frontend/CMakeLists.txt
@@ -1,6 +1,8 @@
 add_flang_library(flangFrontend
   CompilerInstance.cpp
   CompilerInvocation.cpp
+  FrontendAction.cpp
+  FrontendActions.cpp
   FrontendOptions.cpp
   TextDiagnosticPrinter.cpp
   TextDiagnosticBuffer.cpp
@@ -10,6 +12,7 @@ add_flang_library(flangFrontend
   clangBasic
 
   LINK_LIBS
+  FortranParser
   clangBasic
   clangDriver
 

diff  --git a/flang/lib/Frontend/CompilerInstance.cpp b/flang/lib/Frontend/CompilerInstance.cpp
index dd92639c3dc2..7f86920f3cf8 100644
--- a/flang/lib/Frontend/CompilerInstance.cpp
+++ b/flang/lib/Frontend/CompilerInstance.cpp
@@ -9,13 +9,127 @@
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/CompilerInvocation.h"
 #include "flang/Frontend/TextDiagnosticPrinter.h"
+#include "flang/Parser/provenance.h"
+#include "llvm/Support/Errc.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace Fortran::frontend;
 
-CompilerInstance::CompilerInstance() : invocation_(new CompilerInvocation()) {}
+CompilerInstance::CompilerInstance()
+    : invocation_(new CompilerInvocation()),
+      allSources_(new Fortran::parser::AllSources()) {}
 
-CompilerInstance::~CompilerInstance() = default;
+CompilerInstance::~CompilerInstance() {
+  assert(outputFiles_.empty() && "Still output files in flight?");
+}
+
+void CompilerInstance::SetInvocation(
+    std::shared_ptr<CompilerInvocation> value) {
+  invocation_ = std::move(value);
+}
+
+void CompilerInstance::AddOutputFile(OutputFile &&outFile) {
+  outputFiles_.push_back(std::move(outFile));
+}
+
+// Helper method to generate the path of the output file. The following logic
+// applies:
+// 1. If the user specifies the output file via `-o`, then use that (i.e.
+//    the outputFilename parameter).
+// 2. If the user does not specify the name of the output file, derive it from
+//    the input file (i.e. inputFilename + extension)
+// 3. If the output file is not specified and the input file is `-`, then set
+//    the output file to `-` as well.
+static std::string GetOutputFilePath(llvm::StringRef outputFilename,
+    llvm::StringRef inputFilename, llvm::StringRef extension) {
+
+  // Output filename _is_ specified. Just use that.
+  if (!outputFilename.empty())
+    return std::string(outputFilename);
+
+  // Output filename _is not_ specified. Derive it from the input file name.
+  std::string outFile = "-";
+  if (!extension.empty() && (inputFilename != "-")) {
+    llvm::SmallString<128> path(inputFilename);
+    llvm::sys::path::replace_extension(path, extension);
+    outFile = std::string(path.str());
+  }
+
+  return outFile;
+}
+
+std::unique_ptr<llvm::raw_pwrite_stream>
+CompilerInstance::CreateDefaultOutputFile(
+    bool binary, llvm::StringRef baseName, llvm::StringRef extension) {
+  std::string outputPathName;
+  std::error_code ec;
+
+  // Get the path of the output file
+  std::string outputFilePath =
+      GetOutputFilePath(GetFrontendOpts().outputFile_, baseName, extension);
+
+  // Create the output file
+  std::unique_ptr<llvm::raw_pwrite_stream> os =
+      CreateOutputFile(outputFilePath, ec, binary);
+
+  // Add the file to the list of tracked output files (provided it was created
+  // successfully)
+  if (os)
+    AddOutputFile(OutputFile(outputPathName));
+
+  return os;
+}
+
+std::unique_ptr<llvm::raw_pwrite_stream> CompilerInstance::CreateOutputFile(
+    llvm::StringRef outputFilePath, std::error_code &error, bool binary) {
+
+  // Creates the file descriptor for the output file
+  std::unique_ptr<llvm::raw_fd_ostream> os;
+  std::string osFile;
+  if (!os) {
+    osFile = outputFilePath;
+    os.reset(new llvm::raw_fd_ostream(osFile, error,
+        (binary ? llvm::sys::fs::OF_None : llvm::sys::fs::OF_Text)));
+    if (error)
+      return nullptr;
+  }
+
+  // Return the stream corresponding to the output file.
+  // For non-seekable streams, wrap it in llvm::buffer_ostream first.
+  if (!binary || os->supportsSeeking())
+    return std::move(os);
+
+  assert(!nonSeekStream_ && "The non-seek stream has already been set!");
+  auto b = std::make_unique<llvm::buffer_ostream>(*os);
+  nonSeekStream_ = std::move(os);
+  return std::move(b);
+}
+
+void CompilerInstance::ClearOutputFiles(bool eraseFiles) {
+  for (OutputFile &of : outputFiles_)
+    if (!of.filename_.empty() && eraseFiles)
+      llvm::sys::fs::remove(of.filename_);
+
+  outputFiles_.clear();
+  nonSeekStream_.reset();
+}
+
+bool CompilerInstance::ExecuteAction(FrontendAction &act) {
+
+  // Connect Input to a CompileInstance
+  for (const FrontendInputFile &fif : GetFrontendOpts().inputs_) {
+    if (act.BeginSourceFile(*this, fif)) {
+      if (llvm::Error err = act.Execute()) {
+        consumeError(std::move(err));
+      }
+      act.EndSourceFile();
+    }
+  }
+  return true;
+}
 
 void CompilerInstance::CreateDiagnostics(
     clang::DiagnosticConsumer *client, bool shouldOwnClient) {

diff  --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index aef5e3c95d05..69f1d57adc89 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -87,6 +87,9 @@ static InputKind ParseFrontendArgs(FrontendOptions &opts,
     default: {
       llvm_unreachable("Invalid option in group!");
     }
+    case clang::driver::options::OPT_test_io:
+      opts.programAction_ = InputOutputTest;
+      break;
       // TODO:
       // case clang::driver::options::OPT_E:
       // case clang::driver::options::OPT_emit_obj:
@@ -98,6 +101,7 @@ static InputKind ParseFrontendArgs(FrontendOptions &opts,
     }
   }
 
+  opts.outputFile_ = args.getLastArgValue(clang::driver::options::OPT_o);
   opts.showHelp_ = args.hasArg(clang::driver::options::OPT_help);
   opts.showVersion_ = args.hasArg(clang::driver::options::OPT_version);
 
@@ -122,6 +126,26 @@ static InputKind ParseFrontendArgs(FrontendOptions &opts,
           << a->getAsString(args) << a->getValue();
   }
 
+  // Collect the input files and save them in our instance of FrontendOptions.
+  std::vector<std::string> inputs =
+      args.getAllArgValues(clang::driver::options::OPT_INPUT);
+  opts.inputs_.clear();
+  if (inputs.empty())
+    // '-' is the default input if none is given.
+    inputs.push_back("-");
+  for (unsigned i = 0, e = inputs.size(); i != e; ++i) {
+    InputKind ik = dashX;
+    if (ik.IsUnknown()) {
+      ik = FrontendOptions::GetInputKindForExtension(
+          llvm::StringRef(inputs[i]).rsplit('.').second);
+      if (ik.IsUnknown())
+        ik = Language::Unknown;
+      if (i == 0)
+        dashX = ik;
+    }
+
+    opts.inputs_.emplace_back(std::move(inputs[i]), ik);
+  }
   return dashX;
 }
 

diff  --git a/flang/lib/Frontend/FrontendAction.cpp b/flang/lib/Frontend/FrontendAction.cpp
new file mode 100644
index 000000000000..024cc826aed2
--- /dev/null
+++ b/flang/lib/Frontend/FrontendAction.cpp
@@ -0,0 +1,61 @@
+//===--- FrontendAction.cpp -----------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Frontend/FrontendAction.h"
+#include "flang/Frontend/CompilerInstance.h"
+#include "flang/Frontend/FrontendActions.h"
+#include "llvm/Support/Errc.h"
+
+using namespace Fortran::frontend;
+
+void FrontendAction::SetCurrentInput(const FrontendInputFile &currentInput) {
+  this->currentInput_ = currentInput;
+}
+
+// Call this method if BeginSourceFile fails.
+// Deallocate compiler instance, input and output descriptors
+static void BeginSourceFileCleanUp(FrontendAction &fa, CompilerInstance &ci) {
+  ci.ClearOutputFiles(/*EraseFiles=*/true);
+  fa.SetCurrentInput(FrontendInputFile());
+  fa.SetCompilerInstance(nullptr);
+}
+
+bool FrontendAction::BeginSourceFile(
+    CompilerInstance &ci, const FrontendInputFile &realInput) {
+
+  FrontendInputFile input(realInput);
+  assert(!instance_ && "Already processing a source file!");
+  assert(!realInput.IsEmpty() && "Unexpected empty filename!");
+  SetCurrentInput(realInput);
+  SetCompilerInstance(&ci);
+  if (!ci.HasAllSources()) {
+    BeginSourceFileCleanUp(*this, ci);
+    return false;
+  }
+  return true;
+}
+
+bool FrontendAction::ShouldEraseOutputFiles() {
+  return GetCompilerInstance().getDiagnostics().hasErrorOccurred();
+}
+
+llvm::Error FrontendAction::Execute() {
+  ExecuteAction();
+  return llvm::Error::success();
+}
+
+void FrontendAction::EndSourceFile() {
+  CompilerInstance &ci = GetCompilerInstance();
+
+  // Cleanup the output streams, and erase the output files if instructed by the
+  // FrontendAction.
+  ci.ClearOutputFiles(/*EraseFiles=*/ShouldEraseOutputFiles());
+
+  SetCompilerInstance(nullptr);
+  SetCurrentInput(FrontendInputFile());
+}

diff  --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
new file mode 100644
index 000000000000..fd4580330f18
--- /dev/null
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -0,0 +1,45 @@
+//===--- FrontendActions.cpp ----------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+#include "flang/Frontend/FrontendActions.h"
+#include "flang/Common/Fortran-features.h"
+#include "flang/Common/default-kinds.h"
+#include "flang/Frontend/CompilerInstance.h"
+#include "flang/Parser/source.h"
+#include "clang/Serialization/PCHContainerOperations.h"
+
+using namespace Fortran::frontend;
+
+void InputOutputTestAction::ExecuteAction() {
+
+  // Get the name of the file from FrontendInputFile current.
+  std::string path{GetCurrentFileOrBufferName()};
+  std::string buf;
+  llvm::raw_string_ostream error_stream{buf};
+  bool binaryMode = true;
+
+  // Set/store input file info into CompilerInstance.
+  CompilerInstance &ci = GetCompilerInstance();
+  Fortran::parser::AllSources &allSources{ci.GetAllSources()};
+  const Fortran::parser::SourceFile *sf;
+  sf = allSources.Open(path, error_stream);
+  llvm::ArrayRef<char> fileContent = sf->content();
+
+  // Output file descriptor to receive the content of input file.
+  std::unique_ptr<llvm::raw_ostream> os;
+
+  // Do not write on the output file if using outputStream_.
+  if (ci.IsOutputStreamNull()) {
+    os = ci.CreateDefaultOutputFile(
+        binaryMode, GetCurrentFileOrBufferName(), "txt");
+    if (!os)
+      return;
+    (*os) << fileContent.data();
+  } else {
+    ci.WriteOutputStream(fileContent.data());
+  }
+}

diff  --git a/flang/lib/Frontend/FrontendOptions.cpp b/flang/lib/Frontend/FrontendOptions.cpp
index ea5d54aa7ff0..1757db3bb0d6 100644
--- a/flang/lib/Frontend/FrontendOptions.cpp
+++ b/flang/lib/Frontend/FrontendOptions.cpp
@@ -7,3 +7,15 @@
 //===----------------------------------------------------------------------===//
 
 #include "flang/Frontend/FrontendOptions.h"
+#include "llvm/ADT/StringSwitch.h"
+
+using namespace Fortran::frontend;
+
+InputKind FrontendOptions::GetInputKindForExtension(llvm::StringRef extension) {
+  return llvm::StringSwitch<InputKind>(extension)
+      // TODO: Should match the list in flang/test/lit.cfg.py
+      // FIXME: Currently this API allows at most 9 items per case.
+      .Cases("f", "F", "f77", "f90", "F90", "f95", "F95", "ff95", "f18", "F18",
+          Language::Fortran)
+      .Default(Language::Unknown);
+}

diff  --git a/flang/lib/FrontendTool/CMakeLists.txt b/flang/lib/FrontendTool/CMakeLists.txt
index db0349517af2..65e1dd5188ac 100644
--- a/flang/lib/FrontendTool/CMakeLists.txt
+++ b/flang/lib/FrontendTool/CMakeLists.txt
@@ -5,6 +5,7 @@ add_flang_library(flangFrontendTool
   clangBasic
 
   LINK_LIBS
+  flangFrontend
   clangBasic
   clangDriver
 

diff  --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index 969979273722..171d70bcfac2 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -12,18 +12,52 @@
 //===----------------------------------------------------------------------===//
 
 #include "flang/Frontend/CompilerInstance.h"
+#include "flang/Frontend/FrontendActions.h"
 #include "clang/Driver/Options.h"
 #include "llvm/Option/OptTable.h"
+#include "llvm/Option/Option.h"
+#include "llvm/Support/BuryPointer.h"
 #include "llvm/Support/CommandLine.h"
 
 namespace Fortran::frontend {
+
+static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
+    CompilerInstance &ci) {
+
+  ActionKind ak = ci.GetFrontendOpts().programAction_;
+  switch (ak) {
+  case InputOutputTest:
+    return std::make_unique<InputOutputTestAction>();
+    break;
+  default:
+    break;
+    // TODO:
+    // case RunPreprocessor:
+    // case ParserSyntaxOnly:
+    // case EmitLLVM:
+    // case EmitLLVMOnly:
+    // case EmitCodeGenOnly:
+    // (...)
+  }
+  return 0;
+}
+
+std::unique_ptr<FrontendAction> CreateFrontendAction(CompilerInstance &ci) {
+  // Create the underlying action.
+  std::unique_ptr<FrontendAction> act = CreateFrontendBaseAction(ci);
+  if (!act)
+    return nullptr;
+
+  return act;
+}
 bool ExecuteCompilerInvocation(CompilerInstance *flang) {
   // Honor -help.
   if (flang->GetFrontendOpts().showHelp_) {
     clang::driver::getDriverOptTable().PrintHelp(llvm::outs(),
         "flang-new -fc1 [options] file...", "LLVM 'Flang' Compiler",
         /*Include=*/clang::driver::options::FC1Option,
-        /*Exclude=*/0, /*ShowAllAliases=*/false);
+        /*Exclude=*/llvm::opt::DriverFlag::HelpHidden,
+        /*ShowAllAliases=*/false);
     return true;
   }
 
@@ -33,7 +67,13 @@ bool ExecuteCompilerInvocation(CompilerInstance *flang) {
     return true;
   }
 
-  return true;
+  // Create and execute the frontend action.
+  std::unique_ptr<FrontendAction> act(CreateFrontendAction(*flang));
+  if (!act)
+    return false;
+
+  bool success = flang->ExecuteAction(*act);
+  return success;
 }
 
 } // namespace Fortran::frontend

diff  --git a/flang/test/Flang-Driver/driver-help-hidden.f90 b/flang/test/Flang-Driver/driver-help-hidden.f90
new file mode 100644
index 000000000000..1a8c7394afb7
--- /dev/null
+++ b/flang/test/Flang-Driver/driver-help-hidden.f90
@@ -0,0 +1,38 @@
+! REQUIRES: new-flang-driver
+
+!--------------------------
+! FLANG DRIVER (flang-new)
+!--------------------------
+! RUN: %flang-new --help-hidden 2>&1 | FileCheck %s
+! RUN: not %flang-new  -help-hidden 2>&1 | FileCheck %s --check-prefix=ERROR-FLANG
+
+!----------------------------------------
+! FLANG FRONTEND DRIVER (flang-new -fc1)
+!----------------------------------------
+! RUN: not %flang-new -fc1 --help-hidden 2>&1 | FileCheck %s --check-prefix=ERROR-FLANG-FC1
+! RUN: not %flang-new -fc1  -help-hidden 2>&1 | FileCheck %s --check-prefix=ERROR-FLANG-FC1
+
+!----------------------------------------------------
+! EXPECTED OUTPUT FOR FLANG DRIVER (flang-new)
+!----------------------------------------------------
+! CHECK:USAGE: flang-new
+! CHECK-EMPTY:
+! CHECK-NEXT:OPTIONS:
+! CHECK-NEXT: -fcolor-diagnostics    Enable colors in diagnostics
+! CHECK-NEXT: -fno-color-diagnostics Disable colors in diagnostics
+! CHECK-NEXT: -help     Display available options
+! CHECK-NEXT: -o <file> Write output to <file>
+! CHECK-NEXT: -test-io  Run the InputOuputTest action. Use for development and testing only.
+! CHECK-NEXT: --version Print version information
+
+!-------------------------------------------------------------
+! EXPECTED OUTPUT FOR FLANG DRIVER (flang-new)
+!-------------------------------------------------------------
+! ERROR-FLANG: error: unknown argument '-help-hidden'; did you mean '--help-hidden'?
+
+!-------------------------------------------------------------
+! EXPECTED OUTPUT FOR FLANG FRONTEND DRIVER (flang-new -fc1)
+!-------------------------------------------------------------
+! Frontend driver -help-hidden is not supported
+! ERROR-FLANG-FC1: error: unknown argument: '{{.*}}'
+

diff  --git a/flang/test/Flang-Driver/driver-help.f90 b/flang/test/Flang-Driver/driver-help.f90
index aafc5630b270..aecac33b127e 100644
--- a/flang/test/Flang-Driver/driver-help.f90
+++ b/flang/test/Flang-Driver/driver-help.f90
@@ -1,21 +1,40 @@
+! REQUIRES: new-flang-driver
+
+!--------------------------
+! FLANG DRIVER (flang-new)
+!--------------------------
 ! RUN: %flang-new -help 2>&1 | FileCheck %s --check-prefix=HELP
-! RUN: %flang-new -fc1 -help 2>&1 | FileCheck %s --check-prefix=HELP-FC1
 ! RUN: not %flang-new -helps 2>&1 | FileCheck %s --check-prefix=ERROR
 
-! REQUIRES: new-flang-driver
+!----------------------------------------
+! FLANG FRONTEND DRIVER (flang-new -fc1)
+!----------------------------------------
+! RUN: %flang-new -fc1 -help 2>&1 | FileCheck %s --check-prefix=HELP-FC1
+! RUN: not %flang-new -fc1 -helps 2>&1 | FileCheck %s --check-prefix=ERROR
 
+!-----------------------------
+! EXPECTED OUTPUT (flang-new)
+!-----------------------------
 ! HELP:USAGE: flang-new
 ! HELP-EMPTY:
 ! HELP-NEXT:OPTIONS:
 ! HELP-NEXT: -fcolor-diagnostics    Enable colors in diagnostics
 ! HELP-NEXT: -fno-color-diagnostics Disable colors in diagnostics
-! HELP-NEXT: -help     Display available options
-! HELP-NEXT: --version Print version information
+! HELP-NEXT: -help                  Display available options
+! HELP-NEXT: -o <file>              Write output to <file>
+! HELP-NEXT: --version              Print version information
 
+!----------------------------------
+! EXPECTED OUTPUT (flang-new -fc1)
+!----------------------------------
 ! HELP-FC1:USAGE: flang-new
 ! HELP-FC1-EMPTY:
 ! HELP-FC1-NEXT:OPTIONS:
 ! HELP-FC1-NEXT: -help     Display available options
+! HELP-FC1-NEXT: -o <file> Write output to <file>
 ! HELP-FC1-NEXT: --version Print version information
 
-! ERROR: flang-new: error: unknown argument '-helps'; did you mean '-help'
+!---------------
+! EXPECTED ERROR
+!---------------
+! ERROR: error: unknown argument '-helps'; did you mean '-help'

diff  --git a/flang/test/Flang-Driver/emit-obj.f90 b/flang/test/Flang-Driver/emit-obj.f90
index 4ddd48382862..09c1f84fb87c 100644
--- a/flang/test/Flang-Driver/emit-obj.f90
+++ b/flang/test/Flang-Driver/emit-obj.f90
@@ -1,5 +1,5 @@
-! RUN: not %flang-new  %s 2>&1 | FileCheck %s --check-prefix=ERROR-IMPLICIT
-! RUN: not %flang-new  -emit-obj %s 2>&1 | FileCheck %s --check-prefix=ERROR-EXPLICIT
+! RUN: not %flang-new  %s 2>&1 | FileCheck %s --check-prefix=ERROR
+! RUN: not %flang-new  -emit-obj %s 2>&1 | FileCheck %s --check-prefix=ERROR
 ! RUN: not %flang-new  -fc1 -emit-obj %s 2>&1 | FileCheck %s --check-prefix=ERROR-FC1
 
 ! REQUIRES: new-flang-driver
@@ -8,10 +8,7 @@
 ! creates a job that corresponds to `-emit-obj`. This option/action is
 ! not yet supported. Verify that this is correctly reported as error.
 
-! ERROR-IMPLICIT: error: unknown argument: '-triple'
-! ERROR-IMPLICIT: error: unknown argument: '-emit-obj'
-! ERROR-IMPLICIT: error: unknown argument: '-o'
-
-! ERROR-EXPLICIT: error: unknown argument: '-o'
+! ERROR: error: unknown argument: '-triple'
+! ERROR: error: unknown argument: '-emit-obj'
 
 ! ERROR-FC1: error: unknown argument: '-emit-obj'

diff  --git a/flang/test/Frontend/Inputs/hello-world.f90 b/flang/test/Frontend/Inputs/hello-world.f90
new file mode 100644
index 000000000000..b06ad6a5c29c
--- /dev/null
+++ b/flang/test/Frontend/Inputs/hello-world.f90
@@ -0,0 +1,5 @@
+!This is a test file with a hello world in Fortran
+program hello
+  implicit none
+  write(*,*) 'Hello world!'
+end program hello

diff  --git a/flang/test/Frontend/input-output-file.f90 b/flang/test/Frontend/input-output-file.f90
new file mode 100644
index 000000000000..15705f21edb0
--- /dev/null
+++ b/flang/test/Frontend/input-output-file.f90
@@ -0,0 +1,35 @@
+! RUN: rm -rf %S/input-output-file.txt
+
+! REQUIRES: new-flang-driver
+
+!--------------------------
+! FLANG DRIVER (flang-new)
+!--------------------------
+! TEST 1: Print to stdout (implicit)
+! RUN: %flang-new -test-io %s  2>&1 | FileCheck %s
+! TEST 2: Print to stdout (explicit)
+! RUN: %flang-new -test-io -o - %s  2>&1 | FileCheck %s
+! TEST 3: Print to a file
+! RUN: %flang-new -test-io -o %t %s 2>&1 && FileCheck %s --input-file=%t
+
+!----------------------------------------
+! FLANG FRONTEND DRIVER (flang-new -fc1)
+!----------------------------------------
+! TEST 4: Write to a file (implicit)
+! RUN: %flang-new -fc1 -test-io  %s 2>&1 && FileCheck %s --input-file=%S/input-output-file.txt
+! TEST 5: Write to a file (explicit)
+! RUN: %flang-new -fc1 -test-io  -o %t %s 2>&1 && FileCheck %s --input-file=%t
+
+
+!-----------------------
+! EXPECTED OUTPUT
+!-----------------------
+! CHECK-LABEL: Program arithmetic
+! CHECK-NEXT:    Integer :: i, j
+! CHECK-NEXT:    i = 2; j = 3; i= i * j;
+! CHECK-NEXT:  End Program arithmetic
+
+Program arithmetic
+  Integer :: i, j
+  i = 2; j = 3; i= i * j;
+End Program arithmetic
\ No newline at end of file

diff  --git a/flang/test/Frontend/multiple-input-files.f90 b/flang/test/Frontend/multiple-input-files.f90
new file mode 100644
index 000000000000..d091faa9b9b1
--- /dev/null
+++ b/flang/test/Frontend/multiple-input-files.f90
@@ -0,0 +1,62 @@
+! RUN: rm -rf %S/multiple-input-files.txt  %S/Inputs/hello-world.txt
+
+! REQUIRES: new-flang-driver
+
+!--------------------------
+! FLANG DRIVER (flang-new)
+!--------------------------
+! TEST 1: Both input files are processed (output is printed to stdout)
+! RUN: %flang-new -test-io %s %S/Inputs/hello-world.f90 | FileCheck %s  -check-prefix=flang-new
+
+! TEST 2: None of the files is processed (not possible to specify the output file when multiple input files are present)
+! RUN: not %flang-new -test-io -o - %S/Inputs/hello-world.f90 %s  2>&1 | FileCheck %s -check-prefix=ERROR
+! RUN: not %flang-new -test-io -o %t %S/Inputs/hello-world.f90 %s 2>&1 | FileCheck %s -check-prefix=ERROR
+
+!----------------------------------------
+! FLANG FRONTEND DRIVER (flang-new -fc1)
+!----------------------------------------
+! TEST 3: Both input files are processed
+! RUN: %flang-new -fc1 -test-io  %S/Inputs/hello-world.f90 %s 2>&1 \
+! RUN:  && FileCheck %s --input-file=%S/multiple-input-files.txt -check-prefix=flang-new-FC1-OUTPUT1 
+
+! TEST 4: Only the last input file is processed
+! RUN: %flang-new -fc1 -test-io  %S/Inputs/hello-world.f90 %s -o %t 2>&1 \
+! RUN:  && FileCheck %s --input-file=%t -check-prefix=flang-new-FC1-OUTPUT1
+
+!-----------------------
+! EXPECTED OUTPUT
+!-----------------------
+! TEST 1: By default, `flang-new` prints the output from all input files to
+! stdout
+! flang-new-LABEL: Program arithmetic
+! flang-new-NEXT:    Integer :: i, j
+! flang-new-NEXT:    i = 2; j = 3; i= i * j;
+! flang-new-NEXT:  End Program arithmetic
+! flang-new-NEXT: !This is a test file with a hello world in Fortran
+! flang-new-NEXT:program hello
+! flang-new-NEXT:  implicit none
+! flang-new-NEXT:  write(*,*) 'Hello world!'
+! flang-new-NEXT:end program hello
+
+
+! TEST 2: `-o` does not work for `flang-new` when multiple input files are present
+! ERROR:error: cannot specify -o when generating multiple output files
+
+
+! TEST 3 & TEST 4: Unless the output file is specified, `flang-new -fc1` generates one output file for every input file. If an
+! output file is specified (with `-o`), then only the last input file is processed.
+! flang-new-FC1-OUTPUT1-LABEL: Program arithmetic
+! flang-new-FC1-OUTPUT1-NEXT:    Integer :: i, j
+! flang-new-FC1-OUTPUT1-NEXT:    i = 2; j = 3; i= i * j;
+! flang-new-FC1-OUTPUT1-NEXT:  End Program arithmetic
+! flang-new-FC1-OUTPUT1-NEXT: !This is a test file with a hello world in Fortran
+! flang-new-FC1-OUTPUT1-NEXT:program hello
+! flang-new-FC1-OUTPUT1-NEXT:  implicit none
+! flang-new-FC1-OUTPUT1-NEXT:  write(*,*) 'Hello world!'
+! flang-new-FC1-OUTPUT1-NEXT:end program hello
+
+
+Program arithmetic
+  Integer :: i, j
+  i = 2; j = 3; i= i * j;
+End Program arithmetic

diff  --git a/flang/test/lit.cfg.py b/flang/test/lit.cfg.py
index 21d853043431..77b26711707c 100644
--- a/flang/test/lit.cfg.py
+++ b/flang/test/lit.cfg.py
@@ -43,7 +43,7 @@
 if config.include_flang_new_driver_test:
   config.available_features.add('new-flang-driver')
 else:
-  config.excludes.append('Flang-Driver')
+  config.excludes.append('Flang-Driver','Frontend')
 
 # test_source_root: The root path where tests are located.
 config.test_source_root = os.path.dirname(__file__)

diff  --git a/flang/tools/flang-driver/fc1_main.cpp b/flang/tools/flang-driver/fc1_main.cpp
index 5f7eeb1ea501..ba6552b453f0 100644
--- a/flang/tools/flang-driver/fc1_main.cpp
+++ b/flang/tools/flang-driver/fc1_main.cpp
@@ -56,5 +56,8 @@ int fc1_main(llvm::ArrayRef<const char *> argv, const char *argv0) {
   // Execute the frontend actions.
   success = ExecuteCompilerInvocation(flang.get());
 
+  // Delete output files to free Compiler Instance
+  flang->ClearOutputFiles(/*EraseFiles=*/false);
+
   return !success;
 }

diff  --git a/flang/unittests/Frontend/CMakeLists.txt b/flang/unittests/Frontend/CMakeLists.txt
index fdccd68fb0ea..4c22b6591287 100644
--- a/flang/unittests/Frontend/CMakeLists.txt
+++ b/flang/unittests/Frontend/CMakeLists.txt
@@ -1,5 +1,6 @@
 add_flang_unittest(FlangFrontendTests
   CompilerInstanceTest.cpp
+  InputOutputTest.cpp
 )
 
 target_link_libraries(FlangFrontendTests
@@ -7,4 +8,6 @@ target_link_libraries(FlangFrontendTests
   clangBasic
   clangFrontend
   flangFrontend
-  flangFrontendTool)
+  flangFrontendTool
+  FortranParser
+  )

diff  --git a/flang/unittests/Frontend/CompilerInstanceTest.cpp b/flang/unittests/Frontend/CompilerInstanceTest.cpp
index 04d581cd35cb..932cacd85df3 100644
--- a/flang/unittests/Frontend/CompilerInstanceTest.cpp
+++ b/flang/unittests/Frontend/CompilerInstanceTest.cpp
@@ -9,6 +9,7 @@
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Basic/DiagnosticOptions.h"
+#include "llvm/Support//FileSystem.h"
 
 #include "gtest/gtest.h"
 
@@ -17,6 +18,51 @@ using namespace Fortran::frontend;
 
 namespace {
 
+TEST(CompilerInstance, SanityCheckForFileManager) {
+  const char *inputSource = "InputSourceFile";
+  std::string inputFile = "buffer-file-test.f";
+  std::error_code ec;
+
+  // 1. Create the input file for the file manager
+  // AllSources (which is used to manage files inside every compiler instance),
+  // works with paths. This means that it requires a physical file. Create one.
+  std::unique_ptr<llvm::raw_fd_ostream> os{
+      new llvm::raw_fd_ostream(inputFile, ec, llvm::sys::fs::OF_None)};
+  if (ec)
+    FAIL() << "Failed to create the input file";
+
+  // Populate the input file with the pre-defined input and flush it.
+  *(os) << inputSource;
+  os.reset();
+
+  // Get the path of the input file
+  llvm::SmallString<64> cwd;
+  if (std::error_code ec = llvm::sys::fs::current_path(cwd))
+    FAIL() << "Failed to obtain the current working directory";
+  std::string testFilePath(cwd.c_str());
+  testFilePath += "/" + inputFile;
+
+  // 2. Set up CompilerInstance (i.e. specify the input file)
+  std::string buf;
+  llvm::raw_string_ostream error_stream{buf};
+  CompilerInstance compInst;
+  const Fortran::parser::SourceFile *sf =
+      compInst.GetAllSources().Open(testFilePath, error_stream);
+
+  // 3. Verify the content of the input file
+  // This is just a sanity check to make sure that CompilerInstance is capable
+  // of reading input files.
+  llvm::ArrayRef<char> fileContent = sf->content();
+  EXPECT_FALSE(fileContent.size() == 0);
+  EXPECT_TRUE(
+      llvm::StringRef(fileContent.data()).startswith("InputSourceFile"));
+
+  // 4. Delete the test file
+  ec = llvm::sys::fs::remove(inputFile);
+  if (ec)
+    FAIL() << "Failed to delete the test file";
+}
+
 TEST(CompilerInstance, AllowDiagnosticLogWithUnownedDiagnosticConsumer) {
   // 1. Set-up a basic DiagnosticConsumer
   std::string diagnosticOutput;

diff  --git a/flang/unittests/Frontend/InputOutputTest.cpp b/flang/unittests/Frontend/InputOutputTest.cpp
new file mode 100644
index 000000000000..f2f28ae68561
--- /dev/null
+++ b/flang/unittests/Frontend/InputOutputTest.cpp
@@ -0,0 +1,76 @@
+//===- unittests/Frontend/OutputStreamTest.cpp --- FrontendAction tests --===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "flang/Frontend/CompilerInstance.h"
+#include "flang/Frontend/CompilerInvocation.h"
+#include "flang/Frontend/FrontendOptions.h"
+#include "flang/FrontendTool/Utils.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace Fortran::frontend;
+
+namespace {
+
+TEST(FrontendAction, TestInputOutputTestAction) {
+  std::string inputFile = "io-file-test.f";
+  std::error_code ec;
+
+  // 1. Create the input file for the file manager
+  // AllSources (which is used to manage files inside every compiler instance),
+  // works with paths. This means that it requires a physical file. Create one.
+  std::unique_ptr<llvm::raw_fd_ostream> os{
+      new llvm::raw_fd_ostream(inputFile, ec, llvm::sys::fs::OF_None)};
+  if (ec)
+    FAIL() << "Failed to create the input file";
+
+  // Populate the input file with the pre-defined input and flush it.
+  *(os) << "End Program arithmetic";
+  os.reset();
+
+  // Get the path of the input file
+  llvm::SmallString<64> cwd;
+  if (std::error_code ec = llvm::sys::fs::current_path(cwd))
+    FAIL() << "Failed to obtain the current working directory";
+  std::string testFilePath(cwd.c_str());
+  testFilePath += "/" + inputFile;
+
+  // 2. Prepare the compiler (CompilerInvocation + CompilerInstance)
+  CompilerInstance compInst;
+  compInst.CreateDiagnostics();
+  auto invocation = std::make_shared<CompilerInvocation>();
+  invocation->GetFrontendOpts().programAction_ = InputOutputTest;
+  compInst.SetInvocation(std::move(invocation));
+  compInst.GetFrontendOpts().inputs_.push_back(
+      FrontendInputFile(/*File=*/testFilePath, Language::Fortran));
+
+  // 3. Set-up the output stream. Using output buffer wrapped as an output
+  // stream, as opposed to an actual file (or a file descriptor).
+  llvm::SmallVector<char, 256> outputFileBuffer;
+  std::unique_ptr<llvm::raw_pwrite_stream> outputFileStream(
+      new llvm::raw_svector_ostream(outputFileBuffer));
+  compInst.SetOutputStream(std::move(outputFileStream));
+
+  // 4. Run the earlier defined FrontendAction
+  bool success = ExecuteCompilerInvocation(&compInst);
+
+  EXPECT_TRUE(success);
+  EXPECT_TRUE(!outputFileBuffer.empty());
+  EXPECT_TRUE(llvm::StringRef(outputFileBuffer.data())
+                  .startswith("End Program arithmetic"));
+
+  // 5. Clear the input and the output files. Since we used an output buffer,
+  // there are no physical output files to delete.
+  ec = llvm::sys::fs::remove(inputFile);
+  if (ec)
+    FAIL() << "Failed to delete the test file";
+
+  compInst.ClearOutputFiles(/*EraseFiles=*/false);
+}
+} // namespace


        


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