diff --git a/include/clang/Basic/DiagnosticFrontendKinds.td b/include/clang/Basic/DiagnosticFrontendKinds.td
index 230a6d3..111622e 100644
--- a/include/clang/Basic/DiagnosticFrontendKinds.td
+++ b/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -137,5 +137,11 @@
 def err_module_map_temp_file : Error<
   "unable to write temporary module map file '%0'">, DefaultFatal;
 def err_module_unavailable : Error<"module '%0' requires feature '%1'">;
-
+def warn_module_config_macro_undef : Warning<
+  "%select{definition|#undef}0 of configuration macro '%1' has no effect on "
+  "the import of '%2'; pass '%select{-D%1=...|-U%1}0' on the command line "
+  "to configure the module">,
+  InGroup<ConfigMacros>;
+def note_module_def_undef_here : Note<
+  "macro was %select{defined|#undef'd}0 here">;
 }
diff --git a/include/clang/Basic/DiagnosticGroups.td b/include/clang/Basic/DiagnosticGroups.td
index 0f127bf..9af83b1 100644
--- a/include/clang/Basic/DiagnosticGroups.td
+++ b/include/clang/Basic/DiagnosticGroups.td
@@ -47,6 +47,7 @@
 def : DiagGroup<"cast-qual">;
 def : DiagGroup<"char-align">;
 def Comment : DiagGroup<"comment">;
+def ConfigMacros : DiagGroup<"config-macros">;
 def : DiagGroup<"ctor-dtor-privacy">;
 def GNUDesignator : DiagGroup<"gnu-designator">;
 
diff --git a/include/clang/Basic/DiagnosticLexKinds.td b/include/clang/Basic/DiagnosticLexKinds.td
index 7f66a87..80c5249 100644
--- a/include/clang/Basic/DiagnosticLexKinds.td
+++ b/include/clang/Basic/DiagnosticLexKinds.td
@@ -532,6 +532,10 @@
   "expected an exported module name or '*'">;
 def err_mmap_expected_library_name : Error<
   "expected %select{library|framework}0 name as a string">;
+def err_mmap_config_macro_submodule : Error<
+  "configuration macros are only allowed on top-level modules">;
+def err_mmap_expected_config_macro : Error<
+  "expected configuration macro name after ','">;
 def err_mmap_missing_module_unqualified : Error<
   "no module named '%0' visible from '%1'">;
 def err_mmap_missing_module_qualified : Error<
diff --git a/include/clang/Basic/Module.h b/include/clang/Basic/Module.h
index 5f081e0..2add580 100644
--- a/include/clang/Basic/Module.h
+++ b/include/clang/Basic/Module.h
@@ -119,7 +119,14 @@
   /// \brief Whether, when inferring submodules, the inferr submodules should
   /// export all modules they import (e.g., the equivalent of "export *").
   unsigned InferExportWildcard : 1;
-  
+
+  /// \brief Whether the set of configuration macros is exhaustive.
+  ///
+  /// When the set of configuration macros is exhaustive, meaning
+  /// that no identifier not in this list should affect how the module is
+  /// built.
+  unsigned ConfigMacrosExhaustive : 1;
+
   /// \brief Describes the visibility of the various names within a
   /// particular module.
   enum NameVisibilityKind {
@@ -190,6 +197,10 @@
   /// an entity from this module is used.
   llvm::SmallVector<LinkLibrary, 2> LinkLibraries;
 
+  /// \brief The set of "configuration macros", which are macros that
+  /// (intentionally) change how this module is built.
+  std::vector<std::string> ConfigMacros;
+
   /// \brief Construct a top-level module.
   explicit Module(StringRef Name, SourceLocation DefinitionLoc,
                   bool IsFramework)
@@ -197,7 +208,8 @@
       IsAvailable(true), IsFromModuleFile(false), IsFramework(IsFramework), 
       IsExplicit(false), IsSystem(false),
       InferSubmodules(false), InferExplicitSubmodules(false),
-      InferExportWildcard(false), NameVisibility(Hidden) { }
+      InferExportWildcard(false), ConfigMacrosExhaustive(false),
+      NameVisibility(Hidden) { }
   
   /// \brief Construct a new module or submodule.
   Module(StringRef Name, SourceLocation DefinitionLoc, Module *Parent, 
@@ -328,7 +340,7 @@
   ///
   /// \returns The submodule if found, or NULL otherwise.
   Module *findSubmodule(StringRef Name) const;
-  
+
   typedef std::vector<Module *>::iterator submodule_iterator;
   typedef std::vector<Module *>::const_iterator submodule_const_iterator;
   
diff --git a/include/clang/Serialization/ASTBitCodes.h b/include/clang/Serialization/ASTBitCodes.h
index 93c5c07..2669d8e 100644
--- a/include/clang/Serialization/ASTBitCodes.h
+++ b/include/clang/Serialization/ASTBitCodes.h
@@ -609,7 +609,9 @@
       /// from this submodule.
       SUBMODULE_EXCLUDED_HEADER = 9,
       /// \brief Specifies a library or framework to link against.
-      SUBMODULE_LINK_LIBRARY = 10
+      SUBMODULE_LINK_LIBRARY = 10,
+      /// \brief Specifies a configuration macro for this module.
+      SUBMODULE_CONFIG_MACRO = 11
     };
 
     /// \brief Record types used within a comments block.
diff --git a/lib/Basic/Module.cpp b/lib/Basic/Module.cpp
index f074391..019d047 100644
--- a/lib/Basic/Module.cpp
+++ b/lib/Basic/Module.cpp
@@ -279,7 +279,19 @@
     OS.write_escaped(UmbrellaDir->getName());
     OS << "\"\n";    
   }
-  
+
+  if (!ConfigMacros.empty() || ConfigMacrosExhaustive) {
+    OS.indent(Indent + 2);
+    OS << "config_macros ";
+    if (ConfigMacrosExhaustive)
+      OS << "[exhausive]";
+    for (unsigned I = 0, N = ConfigMacros.size(); I != N; ++I) {
+      if (I)
+        OS << ", ";
+      OS << ConfigMacros[I];
+    }
+  }
+
   for (unsigned I = 0, N = Headers.size(); I != N; ++I) {
     OS.indent(Indent + 2);
     OS << "header \"";
diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index d5c1a27..a20e7d7 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -905,6 +905,96 @@
   }
 }
 
+/// \brief Diagnose differences between the current definition of the given
+/// configuration macro and the definition provided on the command line.
+static void checkConfigMacro(Preprocessor &PP, StringRef ConfigMacro,
+                             Module *Mod, SourceLocation ImportLoc) {
+  IdentifierInfo *Id = PP.getIdentifierInfo(ConfigMacro);
+  SourceManager &SourceMgr = PP.getSourceManager();
+  
+  // If this identifier has never had a macro definition, then it could
+  // not have changed.
+  if (!Id->hadMacroDefinition())
+    return;
+
+  // If this identifier does not currently have a macro definition,
+  // check whether it had one on the command line.
+  if (!Id->hasMacroDefinition()) {
+    MacroDirective *UndefMD = PP.getMacroDirectiveHistory(Id);
+    for (MacroDirective *MD = UndefMD; MD; MD = MD->getPrevious()) {
+
+      FileID FID = SourceMgr.getFileID(MD->getLocation());
+      if (FID.isInvalid())
+        continue;
+
+      const llvm::MemoryBuffer *Buffer = SourceMgr.getBuffer(FID);
+      if (!Buffer)
+        continue;
+
+      // We only care about the predefines buffer.
+      if (!StringRef(Buffer->getBufferIdentifier()).equals("<built-in>"))
+        continue;
+
+      // This macro was defined on the command line, then #undef'd later.
+      // Complain.
+      PP.Diag(ImportLoc, diag::warn_module_config_macro_undef)
+        << true << ConfigMacro << Mod->getFullModuleName();
+      if (UndefMD->getUndefLoc().isValid())
+        PP.Diag(UndefMD->getUndefLoc(), diag::note_module_def_undef_here)
+          << true;
+      return;
+    }
+
+    // Okay: no definition in the predefines buffer.
+    return;
+  }
+
+  // This identifier has a macro definition. Check whether we had a definition
+  // on the command line.
+  MacroDirective *DefMD = PP.getMacroDirective(Id);
+  MacroDirective *PredefinedMD = 0;
+  for (MacroDirective *MD = DefMD; MD; MD = MD->getPrevious()) {
+    FileID FID = SourceMgr.getFileID(MD->getLocation());
+    if (FID.isInvalid())
+      continue;
+
+    const llvm::MemoryBuffer *Buffer = SourceMgr.getBuffer(FID);
+    if (!Buffer)
+      continue;
+
+    // We only care about the predefines buffer.
+    if (!StringRef(Buffer->getBufferIdentifier()).equals("<built-in>"))
+      continue;
+
+    PredefinedMD = MD;
+    break;
+  }
+
+  // If there was no definition for this macro in the predefines buffer,
+  // complain.
+  if (!PredefinedMD ||
+      (!PredefinedMD->getLocation().isValid() &&
+       PredefinedMD->getUndefLoc().isValid())) {
+    PP.Diag(ImportLoc, diag::warn_module_config_macro_undef)
+      << false << ConfigMacro << Mod->getFullModuleName();
+    PP.Diag(DefMD->getLocation(), diag::note_module_def_undef_here)
+      << false;
+    return;
+  }
+
+  // If the current macro definition is the same as the predefined macro
+  // definition, it's okay.
+  if (DefMD == PredefinedMD ||
+      DefMD->getInfo()->isIdenticalTo(*PredefinedMD->getInfo(), PP))
+    return;
+
+  // The macro definitions differ.
+  PP.Diag(ImportLoc, diag::warn_module_config_macro_undef)
+    << false << ConfigMacro << Mod->getFullModuleName();
+  PP.Diag(DefMD->getLocation(), diag::note_module_def_undef_here)
+    << false;
+}
+
 ModuleLoadResult
 CompilerInstance::loadModule(SourceLocation ImportLoc,
                              ModuleIdPath Path,
@@ -1177,7 +1267,14 @@
 
     ModuleManager->makeModuleVisible(Module, Visibility, ImportLoc);
   }
-  
+
+  // Check for any configuration macros that have changed.
+  clang::Module *TopModule = Module->getTopLevelModule();
+  for (unsigned I = 0, N = TopModule->ConfigMacros.size(); I != N; ++I) {
+    checkConfigMacro(getPreprocessor(), TopModule->ConfigMacros[I],
+                     Module, ImportLoc);
+  }
+
   // If this module import was due to an inclusion directive, create an 
   // implicit import declaration to capture it in the AST.
   if (IsInclusionDirective && hasASTContext()) {
diff --git a/lib/Lex/ModuleMap.cpp b/lib/Lex/ModuleMap.cpp
index 71a98e2..9cea5aa 100644
--- a/lib/Lex/ModuleMap.cpp
+++ b/lib/Lex/ModuleMap.cpp
@@ -643,6 +643,7 @@
   struct MMToken {
     enum TokenKind {
       Comma,
+      ConfigMacros,
       EndOfFile,
       HeaderKeyword,
       Identifier,
@@ -687,10 +688,13 @@
 
   /// \brief The set of attributes that can be attached to a module.
   struct Attributes {
-    Attributes() : IsSystem() { }
+    Attributes() : IsSystem(), IsExhaustive() { }
 
     /// \brief Whether this is a system module.
     unsigned IsSystem : 1;
+
+    /// \brief Whether this is an exhaustive set of configuration macros.
+    unsigned IsExhaustive : 1;
   };
   
 
@@ -739,6 +743,7 @@
     void parseUmbrellaDirDecl(SourceLocation UmbrellaLoc);
     void parseExportDecl();
     void parseLinkDecl();
+    void parseConfigMacros();
     void parseInferredModuleDecl(bool Framework, bool Explicit);
     bool parseOptionalAttributes(Attributes &Attrs);
 
@@ -776,11 +781,12 @@
     Tok.StringData = LToken.getRawIdentifierData();
     Tok.StringLength = LToken.getLength();
     Tok.Kind = llvm::StringSwitch<MMToken::TokenKind>(Tok.getString())
-                 .Case("header", MMToken::HeaderKeyword)
+                 .Case("config_macros", MMToken::ConfigMacros)
                  .Case("exclude", MMToken::ExcludeKeyword)
                  .Case("explicit", MMToken::ExplicitKeyword)
                  .Case("export", MMToken::ExportKeyword)
                  .Case("framework", MMToken::FrameworkKeyword)
+                 .Case("header", MMToken::HeaderKeyword)
                  .Case("link", MMToken::LinkKeyword)
                  .Case("module", MMToken::ModuleKeyword)
                  .Case("requires", MMToken::RequiresKeyword)
@@ -937,7 +943,9 @@
     /// \brief An unknown attribute.
     AT_unknown,
     /// \brief The 'system' attribute.
-    AT_system
+    AT_system,
+    /// \brief The 'exhaustive' attribute.
+    AT_exhaustive
   };
 }
 
@@ -1094,7 +1102,11 @@
     case MMToken::RBrace:
       Done = true;
       break;
-        
+
+    case MMToken::ConfigMacros:
+      parseConfigMacros();
+      break;
+
     case MMToken::ExplicitKeyword:
     case MMToken::FrameworkKeyword:
     case MMToken::ModuleKeyword:
@@ -1489,6 +1501,59 @@
                                                             IsFramework));
 }
 
+/// \brief Parse a configuration macro declaration.
+///
+///   module-declaration:
+///     'config_macros' attributes[opt] config-macro-list?
+///
+///   config-macro-list:
+///     identifier (',' identifier)?
+void ModuleMapParser::parseConfigMacros() {
+  assert(Tok.is(MMToken::ConfigMacros));
+  SourceLocation ConfigMacrosLoc = consumeToken();
+
+  // Only top-level modules can have configuration macros.
+  if (ActiveModule->Parent) {
+    Diags.Report(ConfigMacrosLoc, diag::err_mmap_config_macro_submodule);
+  }
+
+  // Parse the optional attributes.
+  Attributes Attrs;
+  parseOptionalAttributes(Attrs);
+  if (Attrs.IsExhaustive && !ActiveModule->Parent) {
+    ActiveModule->ConfigMacrosExhaustive = true;
+  }
+
+  // If we don't have an identifier, we're done.
+  if (!Tok.is(MMToken::Identifier))
+    return;
+
+  // Consume the first identifier.
+  if (!ActiveModule->Parent) {
+    ActiveModule->ConfigMacros.push_back(Tok.getString().str());
+  }
+  consumeToken();
+
+  do {
+    // If there's a comma, consume it.
+    if (!Tok.is(MMToken::Comma))
+      break;
+    consumeToken();
+
+    // We expect to see a macro name here.
+    if (!Tok.is(MMToken::Identifier)) {
+      Diags.Report(Tok.getLocation(), diag::err_mmap_expected_config_macro);
+      break;
+    }
+
+    // Consume the macro name.
+    if (!ActiveModule->Parent) {
+      ActiveModule->ConfigMacros.push_back(Tok.getString().str());
+    }
+    consumeToken();
+  } while (true);
+}
+
 /// \brief Parse an inferred module declaration (wildcard modules).
 ///
 ///   module-declaration:
@@ -1668,6 +1733,7 @@
     // Decode the attribute name.
     AttributeKind Attribute
       = llvm::StringSwitch<AttributeKind>(Tok.getString())
+          .Case("exhaustive", AT_exhaustive)
           .Case("system", AT_system)
           .Default(AT_unknown);
     switch (Attribute) {
@@ -1679,6 +1745,10 @@
     case AT_system:
       Attrs.IsSystem = true;
       break;
+
+    case AT_exhaustive:
+      Attrs.IsExhaustive = true;
+      break;
     }
     consumeToken();
 
@@ -1730,6 +1800,7 @@
       break;
 
     case MMToken::Comma:
+    case MMToken::ConfigMacros:
     case MMToken::ExcludeKeyword:
     case MMToken::ExportKeyword:
     case MMToken::HeaderKeyword:
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 42b5038..00e5ce9 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -3718,6 +3718,18 @@
       CurrentModule->LinkLibraries.push_back(
                                          Module::LinkLibrary(Blob, Record[0]));
       break;
+
+    case SUBMODULE_CONFIG_MACRO:
+      if (First) {
+        Error("missing submodule metadata record at beginning of block");
+        return true;
+      }
+
+      if (!CurrentModule)
+        break;
+
+      CurrentModule->ConfigMacros.push_back(Blob.str());
+      break;
     }
   }
 }
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index 724c8cf..08e8afd 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -2135,6 +2135,7 @@
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // InferSubmodules...
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // InferExplicit...
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // InferExportWild...
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // ConfigMacrosExh...
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob)); // Name
   unsigned DefinitionAbbrev = Stream.EmitAbbrev(Abbrev);
 
@@ -2174,6 +2175,11 @@
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));     // Name
   unsigned LinkLibraryAbbrev = Stream.EmitAbbrev(Abbrev);
 
+  Abbrev = new BitCodeAbbrev();
+  Abbrev->Add(BitCodeAbbrevOp(SUBMODULE_CONFIG_MACRO));
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));    // Macro name
+  unsigned ConfigMacroAbbrev = Stream.EmitAbbrev(Abbrev);
+
   // Write the submodule metadata block.
   RecordData Record;
   Record.push_back(getNumberOfModules(WritingModule));
@@ -2204,6 +2210,7 @@
     Record.push_back(Mod->InferSubmodules);
     Record.push_back(Mod->InferExplicitSubmodules);
     Record.push_back(Mod->InferExportWildcard);
+    Record.push_back(Mod->ConfigMacrosExhaustive);
     Stream.EmitRecordWithBlob(DefinitionAbbrev, Record, Mod->Name);
     
     // Emit the requirements.
@@ -2288,6 +2295,14 @@
                                 Mod->LinkLibraries[I].Library);
     }
 
+    // Emit the configuration macros.
+    for (unsigned I = 0, N =  Mod->ConfigMacros.size(); I != N; ++I) {
+      Record.clear();
+      Record.push_back(SUBMODULE_CONFIG_MACRO);
+      Stream.EmitRecordWithBlob(ConfigMacroAbbrev, Record,
+                                Mod->ConfigMacros[I]);
+    }
+
     // Queue up the submodules of this module.
     for (Module::submodule_iterator Sub = Mod->submodule_begin(),
                                  SubEnd = Mod->submodule_end();
diff --git a/test/Modules/Inputs/config.h b/test/Modules/Inputs/config.h
new file mode 100644
index 0000000..f2dfda6
--- /dev/null
+++ b/test/Modules/Inputs/config.h
@@ -0,0 +1,7 @@
+#ifdef WANT_FOO
+int* foo();
+#endif
+
+#ifdef WANT_BAR
+char *bar();
+#endif
diff --git a/test/Modules/Inputs/module.map b/test/Modules/Inputs/module.map
index 53f2fd6..479bfcf 100644
--- a/test/Modules/Inputs/module.map
+++ b/test/Modules/Inputs/module.map
@@ -183,3 +183,8 @@
 module cxx_linkage_cache {
   header "cxx-linkage-cache.h"
 }
+
+module config {
+  header "config.h"
+  config_macros [exhaustive] WANT_FOO, WANT_BAR
+}
diff --git a/test/Modules/config_macros.m b/test/Modules/config_macros.m
new file mode 100644
index 0000000..200744d
--- /dev/null
+++ b/test/Modules/config_macros.m
@@ -0,0 +1,28 @@
+@import config;
+
+int *test_foo() {
+  return foo();
+}
+
+char *test_bar() {
+  return bar(); // expected-warning{{implicit declaration of function 'bar' is invalid in C99}} \
+                // expected-warning{{incompatible integer to pointer conversion}}
+}
+
+#undef WANT_FOO // expected-note{{macro was #undef'd here}}
+@import config; // expected-warning{{#undef of configuration macro 'WANT_FOO' has no effect on the import of 'config'; pass '-UWANT_FOO' on the command line to configure the module}}
+
+#define WANT_FOO 2 // expected-note{{macro was defined here}}
+@import config; // expected-warning{{definition of configuration macro 'WANT_FOO' has no effect on the import of 'config'; pass '-DWANT_FOO=...' on the command line to configure the module}}
+
+#undef WANT_FOO
+#define WANT_FOO 1
+@import config; // okay
+
+#define WANT_BAR 1 // expected-note{{macro was defined here}}
+@import config; // expected-warning{{definition of configuration macro 'WANT_BAR' has no effect on the import of 'config'; pass '-DWANT_BAR=...' on the command line to configure the module}}
+
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -x objective-c -fmodules-cache-path=%t -DWANT_FOO=1 -emit-module -fmodule-name=config %S/Inputs/module.map
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t -DWANT_FOO=1 %s -verify
+