Index: modularize/CMakeLists.txt
===================================================================
--- modularize/CMakeLists.txt
+++ modularize/CMakeLists.txt
@@ -7,6 +7,7 @@
 
 add_clang_executable(modularize
   Modularize.cpp
+  ModuleAssistant.cpp
   PreprocessorTracker.cpp
   )
 
Index: modularize/Modularize.h
===================================================================
--- modularize/Modularize.h
+++ modularize/Modularize.h
@@ -0,0 +1,52 @@
+//===--- Modularize.h - Common definitions for Modularize -*- C++ -*-----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===--------------------------------------------------------------------===//
+///
+/// \file
+/// \brief Common definitions for Modularize.
+///
+//===--------------------------------------------------------------------===//
+
+#ifndef MODULARIZE_H
+#define MODULARIZE_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringRef.h"
+#include <string>
+#include <vector>
+
+// Save the program name for error messages.
+extern const char *Argv0;
+// Save the command line for comments.
+extern std::string CommandLine;
+
+// Dependency types.
+typedef llvm::SmallVector<std::string, 4> DependentsVector;
+typedef llvm::StringMap<DependentsVector> DependencyMap;
+
+// Global function declarations.
+
+/// Create the module map file.
+/// \param ModuleMapPath The path to the module map file to be generated.
+/// \param HeaderFileNames The list of header files, absolute native paths.
+/// \param Dependencies Map of headers that depend on other headers.
+/// \param HeaderPrefix Tells the code where the headers are, if they
+///   aren's in the current directory, allowing the generator to strip
+///   the leading, non-relative beginning of the header paths.
+/// \brief RootModuleName If not empty, specifies that a root module
+///   should be created with this name.
+/// \returns True if successful.
+bool createModuleMap(llvm::StringRef ModuleMapPath,
+                     llvm::ArrayRef<std::string> HeaderFileNames,
+                     DependencyMap &Dependencies, llvm::StringRef HeaderPrefix,
+                     llvm::StringRef RootModuleName);
+
+#endif // MODULARIZE_H
Index: modularize/Modularize.cpp
===================================================================
--- modularize/Modularize.cpp
+++ modularize/Modularize.cpp
@@ -90,6 +90,25 @@
 //
 // See PreprocessorTracker.cpp for additional details.
 //
+// Modularize also has an option ("-module-map-path=module.map") that will
+// skip the checks, and instead act as a module.map generation assistant,
+// generating a module map file based on the header list.  An optional
+// "-root-module=(rootName)" argument can specify a root module to be
+// created in the generated module.map file.  Note that you will likely
+// need to edit this file to suit the needs of your headers.
+//
+// An example command line for generating a module.map file:
+//
+//   modularize -module-map-path=module.map -root-module=myroot headerlist.txt
+//
+// Note that if the headers in the header list have partial paths, sub-modules
+// will be created for the subdirectires involved, assuming that the
+// subdirectories contain headers to be grouped into a module, but still with
+// individual modules for the headers in the subdirectory.
+//
+// See the ModuleAssistant.cpp file comments for additional details about the
+// implementation of the assistant mode.
+//
 // Future directions:
 //
 // Basically, we want to add new checks for whatever we can check with respect
@@ -134,8 +153,6 @@
 #include "clang/Tooling/CompilationDatabase.h"
 #include "clang/Tooling/Tooling.h"
 #include "llvm/ADT/OwningPtr.h"
-#include "llvm/ADT/StringRef.h"
-#include "llvm/ADT/StringMap.h"
 #include "llvm/Config/config.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
@@ -150,6 +167,7 @@
 #include <iterator>
 #include <string>
 #include <vector>
+#include "Modularize.h"
 #include "PreprocessorTracker.h"
 
 using namespace clang;
@@ -178,8 +196,24 @@
         " If not specified,"
         " the files are considered to be relative to the header list file."));
 
-typedef SmallVector<std::string, 4> DependentsVector;
-typedef StringMap<DependentsVector> DependencyMap;
+// Option for assistant mode, telling modularize to output a module map
+// based on the headers list, and where to put it.
+cl::opt<std::string> ModuleMapPath(
+    "module-map-path", cl::init(""),
+    cl::desc("Turn on module map output and specify output path or file name."
+             " If no path is specified and if prefix option is specified,"
+             " use prefix for file path."));
+
+// Option for assistant mode, telling modularize to output a module map
+// based on the headers list, and where to put it.
+cl::opt<std::string>
+RootModule("root-module", cl::init(""),
+           cl::desc("Specify the name of the root module."));
+
+// Save the program name for error messages.
+const char *Argv0;
+// Save the command line for comments.
+std::string CommandLine;
 
 // Read the header list file and collect the header file names and
 // optional dependencies.
@@ -651,6 +685,16 @@
 
 int main(int Argc, const char **Argv) {
 
+  // Save program name for error messages.
+  Argv0 = Argv[0];
+
+  // Save program arguments for use in module.map comment.
+  CommandLine = sys::path::stem(sys::path::filename(Argv0));
+  for (int ArgIndex = 1; ArgIndex < Argc; ArgIndex++) {
+    CommandLine.append(" ");
+    CommandLine.append(Argv[ArgIndex]);
+  }
+
   // This causes options to be parsed.
   cl::ParseCommandLineOptions(Argc, Argv, "modularize.\n");
 
@@ -670,6 +714,14 @@
     return 1;
   }
 
+  // If we are in assistant mode, output the module map and quit.
+  if (ModuleMapPath[0]) {
+    if (!createModuleMap(ModuleMapPath, Headers, Dependencies, HeaderPrefix,
+                         RootModule))
+      return 1; // Failed.
+    return 0;   // Success - Skip checks in assistant mode.
+  }
+
   // Create the compilation database.
   SmallString<256> PathBuf;
   sys::fs::current_path(PathBuf);
Index: modularize/ModuleAssistant.cpp
===================================================================
--- modularize/ModuleAssistant.cpp
+++ modularize/ModuleAssistant.cpp
@@ -0,0 +1,287 @@
+//===--- ModuleAssistant.cpp - Module map generation manager -*- C++ -*---===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+//
+// This file defines the module generation entry point function,
+// createModuleMap, a Module class for representing a module,
+// and various implementation functions for doing the underlying
+// work, described below.
+//
+// The "Module" class represents a module, with members for storing the module
+// name, associated header file names, and sub-modules, and an "output"
+// function that recursively writes the module definitions.
+//
+// The "createModuleMap" function implements the top-level logic of the
+// assistant mode.  It calls a loadModuleDescriptions function to walk
+// the header list passed to it and creates a tree of Module objects
+// representing the module hierarchy, represented by a "Module" object,
+// the "RootModule".  This root module may or may not represent an actual
+// module in the module map, depending on the "--root-module" option passed
+// to modularize.  It then calls a writeModuleMap function to set up the
+// module map file output and walk the module tree, outputting the module
+// map file using a stream obtained and managed by an
+// llvm::tool_output_file object.
+//
+//===---------------------------------------------------------------------===//
+
+#include "Modularize.h"
+#include "llvm/ADT/OwningPtr.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/ToolOutputFile.h"
+#include <vector>
+
+// Local definitions:
+
+namespace {
+
+// Internal class definitions:
+
+// Represents a module.
+class Module {
+public:
+  Module(llvm::StringRef Name);
+  Module();
+  ~Module();
+  bool output(llvm::raw_fd_ostream &OS, int Indent);
+  Module *findSubModule(llvm::StringRef SubName);
+
+public:
+  std::string Name;
+  std::vector<std::string> HeaderFileNames;
+  std::vector<Module *> SubModules;
+};
+
+} // end anonymous namespace.
+
+// Module functions:
+
+// Constructors.
+Module::Module(llvm::StringRef Name) : Name(Name) {}
+Module::Module() {}
+
+// Destructor.
+Module::~Module() {
+  // Free submodules.
+  while (SubModules.size()) {
+    Module *last = SubModules.back();
+    SubModules.pop_back();
+    delete last;
+  }
+}
+
+// Write a module hierarchy to the given output stream.
+bool Module::output(llvm::raw_fd_ostream &OS, int Indent) {
+  // If this is not the nameless root module, start a module definition.
+  if (Name.size() != 0) {
+    OS.indent(Indent);
+    OS << "module " << Name << " {\n";
+    Indent += 2;
+  }
+
+  // Output submodules.
+  for (std::vector<Module *>::iterator I = SubModules.begin(),
+                                       E = SubModules.end();
+       I != E; ++I) {
+    if (!(*I)->output(OS, Indent))
+      return false;
+  }
+
+  // Output header files.
+  for (std::vector<std::string>::iterator I = HeaderFileNames.begin(),
+                                          E = HeaderFileNames.end();
+       I != E; ++I) {
+    OS.indent(Indent);
+    OS << "header \"" << *I << "\"\n";
+  }
+
+  // If this module has header files, output export directive.
+  if (HeaderFileNames.size() != 0) {
+    OS.indent(Indent);
+    OS << "export *\n";
+  }
+
+  // If this is not the nameless root module, close the module definition.
+  if (Name.size() != 0) {
+    Indent -= 2;
+    OS.indent(Indent);
+    OS << "}\n";
+  }
+
+  return true;
+}
+
+// Lookup a sub-module.
+Module *Module::findSubModule(llvm::StringRef SubName) {
+  for (std::vector<Module *>::iterator I = SubModules.begin(),
+                                       E = SubModules.end();
+       I != E; ++I) {
+    if ((*I)->Name == SubName)
+      return *I;
+  }
+  return 0;
+}
+
+// Implementation functions:
+
+// Reserved keywords in module.map syntax.
+// Keep in sync with keywords in module map parser in Lex/ModuleMap.cpp,
+// such as in ModuleMapParser::consumeToken().
+static const char *ReservedNames[] = {
+  "config_macros", "export",   "module", "conflict", "framework",
+  "requires",      "exclude",  "header", "private",  "explicit",
+  "link",          "umbrella", "extern", "use",      0 // Flag end.
+};
+
+// Convert module name to a non keyword.
+// Prepends a '_' to the name if and only if the name is a keyword.
+static std::string
+ensureNoCollisionWithReservedName(llvm::StringRef MightBeReservedName) {
+  std::string SafeName = MightBeReservedName;
+  for (int Index = 0; ReservedNames[Index] != 0; ++Index) {
+    if (MightBeReservedName == ReservedNames[Index]) {
+      SafeName.insert(0, "_");
+      break;
+    }
+  }
+  return SafeName;
+}
+
+// Add one module, given a header file path.
+static bool addModuleDescription(Module *RootModule,
+                                 llvm::StringRef HeaderFilePath,
+                                 llvm::StringRef HeaderPrefix,
+                                 DependencyMap &Dependencies) {
+  Module *CurrentModule = RootModule;
+  DependentsVector &FileDependents = Dependencies[HeaderFilePath];
+  std::string FilePath;
+  // Strip prefix.
+  if (HeaderFilePath.startswith(HeaderPrefix))
+    FilePath = HeaderFilePath.substr(HeaderPrefix.size() + 1);
+  else
+    FilePath = HeaderFilePath;
+  int Count = FileDependents.size();
+  // Headers that go into modules must not depend on other files being
+  // included first.  If there are any dependents, warn user and omit.
+  if (Count != 0) {
+    llvm::errs() << "warning: " << FilePath
+                 << " depends on other headers being included first,"
+                    " meaning the module.map won't compile."
+                    "  This header will be omitted from the module map.\n";
+    return true;
+  }
+  // Make canonical.
+  std::replace(FilePath.begin(), FilePath.end(), '\\', '/');
+  // Insert module into tree, using subdirectories as submodules.
+  for (llvm::sys::path::const_iterator I = llvm::sys::path::begin(FilePath),
+                                       E = llvm::sys::path::end(FilePath);
+       I != E; ++I) {
+    if ((*I)[0] == '.')
+      continue;
+    std::string Stem = llvm::sys::path::stem(*I);
+    Stem = ensureNoCollisionWithReservedName(Stem);
+    Module *SubModule = CurrentModule->findSubModule(Stem);
+    if (SubModule == 0) {
+      SubModule = new Module(Stem);
+      CurrentModule->SubModules.push_back(SubModule);
+    }
+    CurrentModule = SubModule;
+  }
+  // Add header file name to headers.
+  CurrentModule->HeaderFileNames.push_back(FilePath);
+  return true;
+}
+
+// Create the internal module tree representation.
+static Module *loadModuleDescriptions(
+    llvm::StringRef RootModuleName, llvm::ArrayRef<std::string> HeaderFileNames,
+    DependencyMap &Dependencies, llvm::StringRef HeaderPrefix) {
+
+  // Create root module.
+  Module *RootModule = new Module(RootModuleName);
+
+  llvm::SmallString<256> CurrentDirectory;
+  llvm::sys::fs::current_path(CurrentDirectory);
+
+  // If no header prefix, use current directory.
+  if (HeaderPrefix.size() == 0)
+    HeaderPrefix = CurrentDirectory;
+
+  // Walk the header file names and output the module map.
+  for (llvm::ArrayRef<std::string>::iterator I = HeaderFileNames.begin(),
+                                             E = HeaderFileNames.end();
+       I != E; ++I) {
+    // Add as a module.
+    if (!addModuleDescription(RootModule, *I, HeaderPrefix, Dependencies))
+      return false;
+  }
+
+  return RootModule;
+}
+
+// Kick off the writing of the module map.
+static bool writeModuleMap(llvm::StringRef ModuleMapPath,
+                           llvm::StringRef HeaderPrefix, Module *RootModule) {
+  llvm::SmallString<256> HeaderDirectory(ModuleMapPath);
+  llvm::sys::path::remove_filename(HeaderDirectory);
+  llvm::SmallString<256> FilePath;
+
+  // Get the module map file path to be used.
+  if ((HeaderDirectory.size() == 0) && (HeaderPrefix.size() != 0)) {
+    FilePath = HeaderPrefix;
+    // Prepend header file name prefix if it's not absolute.
+    llvm::sys::path::append(FilePath, ModuleMapPath);
+    llvm::sys::path::native(FilePath);
+  } else {
+    FilePath = ModuleMapPath;
+    llvm::sys::path::native(FilePath);
+  }
+
+  // Set up module map output file.
+  std::string Error;
+  llvm::tool_output_file Out(FilePath.c_str(), Error);
+  if (!Error.empty()) {
+    llvm::errs() << Argv0 << ": error opening " << FilePath << ":" << Error
+                 << "\n";
+    return false;
+  }
+
+  // Get output stream from tool output buffer/manager.
+  llvm::raw_fd_ostream &OS = Out.os();
+
+  // Output file comment.
+  OS << "// " << ModuleMapPath << "\n";
+  OS << "// Generated by: " << CommandLine << "\n\n";
+
+  // Write module hierarchy from internal representation.
+  if (!RootModule->output(OS, 0))
+    return false;
+
+  // Tell tool_output_file that we want to keep the file.
+  Out.keep();
+
+  return true;
+}
+
+// Global functions:
+
+// Module map generation entry point.
+bool createModuleMap(llvm::StringRef ModuleMapPath,
+                     llvm::ArrayRef<std::string> HeaderFileNames,
+                     DependencyMap &Dependencies, llvm::StringRef HeaderPrefix,
+                     llvm::StringRef RootModuleName) {
+  // Load internal representation of modules.
+  llvm::OwningPtr<Module> RootModule(loadModuleDescriptions(
+      RootModuleName, HeaderFileNames, Dependencies, HeaderPrefix));
+  if (!RootModule.get())
+    return false;
+
+  // Write module map file.
+  return writeModuleMap(ModuleMapPath, HeaderPrefix, RootModule.get());
+}
Index: test/modularize/Inputs/SubModule1/Header1.h
===================================================================
--- test/modularize/Inputs/SubModule1/Header1.h
+++ test/modularize/Inputs/SubModule1/Header1.h
@@ -0,0 +1 @@
+// Header1.h - Empty.
Index: test/modularize/Inputs/SubModule1/Header2.h
===================================================================
--- test/modularize/Inputs/SubModule1/Header2.h
+++ test/modularize/Inputs/SubModule1/Header2.h
@@ -0,0 +1 @@
+// Header2.h - Empty.
Index: test/modularize/Inputs/SubModule2/Header3.h
===================================================================
--- test/modularize/Inputs/SubModule2/Header3.h
+++ test/modularize/Inputs/SubModule2/Header3.h
@@ -0,0 +1 @@
+// Header3.h - Empty.
Index: test/modularize/Inputs/SubModule2/Header4.h
===================================================================
--- test/modularize/Inputs/SubModule2/Header4.h
+++ test/modularize/Inputs/SubModule2/Header4.h
@@ -0,0 +1 @@
+// Header4.h - Empty.
Index: test/modularize/NoProblemsAssistant.modularize
===================================================================
--- test/modularize/NoProblemsAssistant.modularize
+++ test/modularize/NoProblemsAssistant.modularize
@@ -0,0 +1,45 @@
+# RUN: modularize -module-map-path=Output/NoProblemsAssistant.txt -root-module=Root -prefix=%S/Input %s
+# RUN: FileCheck --input-file=%T/NoProblemsAssistant.txt %s
+
+SomeTypes.h
+SomeDecls.h
+SubModule1/Header1.h
+SubModule1/Header2.h
+SubModule2/Header3.h
+SubModule2/Header4.h
+SubModule2.h
+
+# CHECK: // Output/NoProblemsAssistant.txt
+# CHECK-NEXT: // Generated by: modularize -module-map-path=Output/NoProblemsAssistant.txt -root-module=Root -prefix={{.*}}{{[/\\]}}{{.*}} {{.*}}{{[/\\]}}NoProblemsAssistant.modularize
+# CHECK: module Root {
+# CHECK-NEXT:   module SomeTypes {
+# CHECK-NEXT:     header "SomeTypes.h"
+# CHECK-NEXT:     export *
+# CHECK-NEXT:   }
+# CHECK-NEXT:   module SomeDecls {
+# CHECK-NEXT:     header "SomeDecls.h"
+# CHECK-NEXT:     export *
+# CHECK-NEXT:   }
+# CHECK-NEXT:   module SubModule1 {
+# CHECK-NEXT:     module Header1 {
+# CHECK-NEXT:       header "SubModule1/Header1.h"
+# CHECK-NEXT:       export *
+# CHECK-NEXT:     }
+# CHECK-NEXT:     module Header2 {
+# CHECK-NEXT:       header "SubModule1/Header2.h"
+# CHECK-NEXT:       export *
+# CHECK-NEXT:     }
+# CHECK-NEXT:   }
+# CHECK-NEXT:   module SubModule2 {
+# CHECK-NEXT:     module Header3 {
+# CHECK-NEXT:       header "SubModule2/Header3.h"
+# CHECK-NEXT:       export *
+# CHECK-NEXT:     }
+# CHECK-NEXT:     module Header4 {
+# CHECK-NEXT:       header "SubModule2/Header4.h"
+# CHECK-NEXT:       export *
+# CHECK-NEXT:     }
+# CHECK-NEXT:     header "SubModule2.h"
+# CHECK-NEXT:     export *
+# CHECK-NEXT:   }
+# CHECK-NEXT: }
Index: test/modularize/SubModule2.h
===================================================================
--- test/modularize/SubModule2.h
+++ test/modularize/SubModule2.h
@@ -0,0 +1,3 @@
+// SubModule2.h - Master header with same name as directory.
+#include "SubModule2/Header3.h"
+#include "SubModule2/Header4.h"