diff --git a/include/swift/Frontend/FrontendInputsAndOutputs.h b/include/swift/Frontend/FrontendInputsAndOutputs.h
index 42211e96c59..4205b1965a1 100644
--- a/include/swift/Frontend/FrontendInputsAndOutputs.h
+++ b/include/swift/Frontend/FrontendInputsAndOutputs.h
@@ -33,8 +33,8 @@ class FrontendInputsAndOutputs {
   friend class ArgsToFrontendInputsConverter;
 
   std::vector<InputFile> AllInputs;
-  typedef llvm::StringMap<unsigned> InputFileMap;
-  InputFileMap PrimaryInputs;
+
+  llvm::StringMap<unsigned> PrimaryInputs;
 
 public:
   FrontendInputsAndOutputs() = default;
@@ -43,6 +43,8 @@ public:
 
   // Readers:
 
+  // All inputs:
+
   ArrayRef<InputFile> getAllInputs() const { return AllInputs; }
 
   std::vector<std::string> getInputFilenames() const;
@@ -53,22 +55,25 @@ public:
 
   bool hasSingleInput() const { return inputCount() == 1; }
 
+  const InputFile &firstInput() const { return AllInputs[0]; }
+  InputFile &firstInput() { return AllInputs[0]; }
+
+  const InputFile &lastInput() const { return AllInputs.back(); }
+
   StringRef getFilenameOfFirstInput() const;
 
   bool isReadingFromStdin() const;
 
-  // If we have exactly one input filename, and its extension is "bc" or "ll",
-  // treat the input as LLVM_IR.
-  bool shouldTreatAsLLVM() const;
+  void forEachInput(llvm::function_ref<void(const InputFile &)> fn) const;
 
-  // Primary input readers
+  // Primaries:
 
-private:
-  void assertMustNotBeMoreThanOnePrimaryInput() const;
+  const InputFile &firstPrimaryInput() const;
+  const InputFile &lastPrimaryInput() const;
 
-  bool areAllNonPrimariesSIB() const;
+  void
+  forEachPrimaryInput(llvm::function_ref<void(const InputFile &)> fn) const;
 
-public:
   unsigned primaryInputCount() const { return PrimaryInputs.size(); }
 
   // Primary count readers:
@@ -79,6 +84,11 @@ public:
 
   bool isWholeModule() const { return !hasPrimaryInputs(); }
 
+  /// Fails an assertion if there is more than one primary input.
+  /// Used in situations where only one primary input can be handled
+  /// and where batch mode has not been implemented yet.
+  void assertMustNotBeMoreThanOnePrimaryInput() const;
+
   // Count-dependend readers:
 
   /// \return the unique primary input, if one exists.
@@ -96,28 +106,25 @@ public:
 
   // Multi-facet readers
 
+  // If we have exactly one input filename, and its extension is "bc" or "ll",
+  // treat the input as LLVM_IR.
+  bool shouldTreatAsLLVM() const;
   bool shouldTreatAsSIL() const;
 
+  bool areAllNonPrimariesSIB() const;
+
   /// \return true for error
   bool verifyInputs(DiagnosticEngine &diags, bool treatAsSIL,
                     bool isREPLRequested, bool isNoneRequested) const;
 
-  // Writers
-
-  void addInputFile(StringRef file, llvm::MemoryBuffer *buffer = nullptr) {
-    addInput(InputFile(file, false, buffer));
-  }
-  void addPrimaryInputFile(StringRef file,
-                           llvm::MemoryBuffer *buffer = nullptr) {
-    addInput(InputFile(file, true, buffer));
-  }
+  // Changing inputs
 
+public:
+  void clearInputs();
   void addInput(const InputFile &input);
-
-  void clearInputs() {
-    AllInputs.clear();
-    PrimaryInputs.clear();
-  }
+  void addInputFile(StringRef file, llvm::MemoryBuffer *buffer = nullptr);
+  void addPrimaryInputFile(StringRef file,
+                           llvm::MemoryBuffer *buffer = nullptr);
 };
 
 } // namespace swift
diff --git a/lib/Frontend/FrontendInputsAndOutputs.cpp b/lib/Frontend/FrontendInputsAndOutputs.cpp
index e2c2ab34046..f4229f4e0b6 100644
--- a/lib/Frontend/FrontendInputsAndOutputs.cpp
+++ b/lib/Frontend/FrontendInputsAndOutputs.cpp
@@ -58,6 +58,45 @@ bool FrontendInputsAndOutputs::isReadingFromStdin() const {
   return hasSingleInput() && getFilenameOfFirstInput() == "-";
 }
 
+StringRef FrontendInputsAndOutputs::getFilenameOfFirstInput() const {
+  assert(hasInputs());
+  const InputFile &inp = AllInputs[0];
+  StringRef f = inp.file();
+  assert(!f.empty());
+  return f;
+}
+
+void FrontendInputsAndOutputs::forEachInput(
+    llvm::function_ref<void(const InputFile &)> fn) const {
+  for (const InputFile &input : AllInputs)
+    fn(input);
+}
+
+// Primaries:
+
+const InputFile &FrontendInputsAndOutputs::firstPrimaryInput() const {
+  assert(!PrimaryInputs.empty());
+  for (const auto &f : AllInputs)
+    if (f.isPrimary())
+      return f;
+  llvm_unreachable("no first primary?!");
+}
+
+const InputFile &FrontendInputsAndOutputs::lastPrimaryInput() const {
+  assert(!PrimaryInputs.empty());
+  for (const auto &f : reversed(AllInputs))
+    if (f.isPrimary())
+      return f;
+  llvm_unreachable("no last primary?!");
+}
+
+void FrontendInputsAndOutputs::forEachPrimaryInput(
+    llvm::function_ref<void(const InputFile &)> fn) const {
+  for (auto &f : AllInputs)
+    if (f.isPrimary())
+      fn(f);
+}
+
 void FrontendInputsAndOutputs::assertMustNotBeMoreThanOnePrimaryInput() const {
   assert(primaryInputCount() < 2 &&
          "have not implemented >1 primary input yet");
@@ -87,14 +126,18 @@ bool FrontendInputsAndOutputs::isInputPrimary(StringRef file) const {
          AllInputs[iterator->second].isPrimary();
 }
 
-StringRef FrontendInputsAndOutputs::getFilenameOfFirstInput() const {
-  assert(hasInputs());
-  const InputFile &inp = AllInputs[0];
-  StringRef f = inp.file();
-  assert(!f.empty());
-  return f;
+unsigned FrontendInputsAndOutputs::numberOfPrimaryInputsEndingWith(
+    const char *extension) const {
+  return count_if(
+      PrimaryInputs, [&](const llvm::StringMapEntry<unsigned> &elem) -> bool {
+        StringRef filename = AllInputs[elem.second].file();
+
+        return llvm::sys::path::extension(filename).endswith(extension);
+      });
 }
 
+// Input queries
+
 bool FrontendInputsAndOutputs::shouldTreatAsLLVM() const {
   if (hasSingleInput()) {
     StringRef Input(getFilenameOfFirstInput());
@@ -124,19 +167,15 @@ bool FrontendInputsAndOutputs::shouldTreatAsSIL() const {
   llvm_unreachable("Either all primaries or none must end with .sil");
 }
 
-void FrontendInputsAndOutputs::addInput(const InputFile &input) {
-  if (!input.file().empty() && input.isPrimary())
-    PrimaryInputs.insert(std::make_pair(input.file(), AllInputs.size()));
-  AllInputs.push_back(input);
-}
-
-unsigned FrontendInputsAndOutputs::numberOfPrimaryInputsEndingWith(
-    const char *extension) const {
-  return count_if(
-      PrimaryInputs, [&](const llvm::StringMapEntry<unsigned> &elem) -> bool {
-        StringRef filename = AllInputs[elem.second].file();
-        return llvm::sys::path::extension(filename).endswith(extension);
-      });
+bool FrontendInputsAndOutputs::areAllNonPrimariesSIB() const {
+  for (const InputFile &input : AllInputs) {
+    if (input.isPrimary())
+      continue;
+    if (!llvm::sys::path::extension(input.file()).endswith(SIB_EXTENSION)) {
+      return false;
+    }
+  }
+  return true;
 }
 
 bool FrontendInputsAndOutputs::verifyInputs(DiagnosticEngine &diags,
@@ -171,13 +210,25 @@ bool FrontendInputsAndOutputs::verifyInputs(DiagnosticEngine &diags,
   return false;
 }
 
-bool FrontendInputsAndOutputs::areAllNonPrimariesSIB() const {
-  for (const InputFile &input : AllInputs) {
-    if (input.isPrimary())
-      continue;
-    if (!llvm::sys::path::extension(input.file()).endswith(SIB_EXTENSION)) {
-      return false;
-    }
-  }
-  return true;
+// Changing inputs
+
+void FrontendInputsAndOutputs::clearInputs() {
+  AllInputs.clear();
+  PrimaryInputs.clear();
+}
+
+void FrontendInputsAndOutputs::addInput(const InputFile &input) {
+  if (!input.file().empty() && input.isPrimary())
+    PrimaryInputs.insert(std::make_pair(input.file(), AllInputs.size()));
+  AllInputs.push_back(input);
+}
+
+void FrontendInputsAndOutputs::addInputFile(StringRef file,
+                                            llvm::MemoryBuffer *buffer) {
+  addInput(InputFile(file, false, buffer));
+}
+
+void FrontendInputsAndOutputs::addPrimaryInputFile(StringRef file,
+                                                   llvm::MemoryBuffer *buffer) {
+  addInput(InputFile(file, true, buffer));
 }