Modules: Use hash of PCM content for SIGNATURE

Change ASTFileSignature from a random 32-bit number to the hash of the
PCM content.

  - Move definition ASTFileSignature to Basic/Module.h so Module and
    ASTSourceDescriptor can use it.

  - Change the signature from uint64_t to std::array<uint32_t,5>.

  - Stop using (saving/reading) the size and modification time of PCM
    files when there is a valid SIGNATURE.

  - Add UNHASHED_CONTROL_BLOCK, and use it to store the SIGNATURE record
    and other records that shouldn't affect the hash.  Because implicit
    modules reuses the same file for multiple levels of -Werror, this
    includes DIAGNOSTIC_OPTIONS and DIAG_PRAGMA_MAPPINGS.

This helps to solve a PCH + implicit Modules dependency issue: PCH files
are handled by the external build system, whereas implicit modules are
handled by internal compiler build system.  This prevents invalidating a
PCH when the compiler overwrites a PCM file with the same content
(modulo the diagnostic differences).

Design and original patch by Manman Ren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297655 91177308-0d34-0410-b5e6-96231b3b80d8

22 files changed, 436 insertions, 160 deletions

diff --git a/include/clang/AST/ExternalASTSource.h b/include/clang/AST/ExternalASTSource.h
index 9e48a6a2f3..40c54b2e8d 100644
--- a/include/clang/AST/ExternalASTSource.h
+++ b/include/clang/AST/ExternalASTSource.h
@@ -150,20 +150,20 @@ public:
     StringRef PCHModuleName;
     StringRef Path;
     StringRef ASTFile;
-    uint64_t Signature = 0;
+    ASTFileSignature Signature;
     const Module *ClangModule = nullptr;
 
   public:
     ASTSourceDescriptor(){};
     ASTSourceDescriptor(StringRef Name, StringRef Path, StringRef ASTFile,
-                        uint64_t Signature)
+                        ASTFileSignature Signature)
         : PCHModuleName(std::move(Name)), Path(std::move(Path)),
           ASTFile(std::move(ASTFile)), Signature(Signature){};
     ASTSourceDescriptor(const Module &M);
     std::string getModuleName() const;
     StringRef getPath() const { return Path; }
     StringRef getASTFile() const { return ASTFile; }
-    uint64_t getSignature() const { return Signature; }
+    ASTFileSignature getSignature() const { return Signature; }
     const Module *getModuleOrNull() const { return ClangModule; }
   };
 
diff --git a/include/clang/Basic/Module.h b/include/clang/Basic/Module.h
index da74d0be86..51ad109088 100644
--- a/include/clang/Basic/Module.h
+++ b/include/clang/Basic/Module.h
@@ -42,7 +42,17 @@ class IdentifierInfo;
   
 /// \brief Describes the name of a module.
 typedef SmallVector<std::pair<std::string, SourceLocation>, 2> ModuleId;
-  
+
+/// The signature of a module, which is a hash of the AST content.
+struct ASTFileSignature : std::array<uint32_t, 5> {
+  ASTFileSignature(std::array<uint32_t, 5> S = {{0}})
+      : std::array<uint32_t, 5>(std::move(S)) {}
+
+  explicit operator bool() const {
+    return *this != std::array<uint32_t, 5>({{0}});
+  }
+};
+
 /// \brief Describes a module or submodule.
 class Module {
 public:
@@ -65,7 +75,7 @@ public:
   llvm::PointerUnion<const DirectoryEntry *, const FileEntry *> Umbrella;
 
   /// \brief The module signature.
-  uint64_t Signature;
+  ASTFileSignature Signature;
 
   /// \brief The name of the umbrella entry, as written in the module map.
   std::string UmbrellaAsWritten;
diff --git a/include/clang/Driver/CC1Options.td b/include/clang/Driver/CC1Options.td
index ab90a64fdb..acc76f6aa6 100644
--- a/include/clang/Driver/CC1Options.td
+++ b/include/clang/Driver/CC1Options.td
@@ -671,6 +671,8 @@ def nostdsysteminc : Flag<["-"], "nostdsysteminc">,
   HelpText<"Disable standard system #include directories">;
 def fdisable_module_hash : Flag<["-"], "fdisable-module-hash">,
   HelpText<"Disable the module hash">;
+def fmodules_hash_content : Flag<["-"], "fmodules-hash-content">,
+  HelpText<"Enable hashing the content of a module file">;
 def c_isystem : JoinedOrSeparate<["-"], "c-isystem">, MetaVarName<"<directory>">,
   HelpText<"Add directory to the C SYSTEM include search path">;
 def objc_isystem : JoinedOrSeparate<["-"], "objc-isystem">,
diff --git a/include/clang/Frontend/PCHContainerOperations.h b/include/clang/Frontend/PCHContainerOperations.h
index d323fb3e8b..f9a73508d7 100644
--- a/include/clang/Frontend/PCHContainerOperations.h
+++ b/include/clang/Frontend/PCHContainerOperations.h
@@ -10,6 +10,7 @@
 #ifndef LLVM_CLANG_PCH_CONTAINER_OPERATIONS_H
 #define LLVM_CLANG_PCH_CONTAINER_OPERATIONS_H
 
+#include "clang/Basic/Module.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/MemoryBuffer.h"
@@ -29,7 +30,7 @@ class DiagnosticsEngine;
 class CompilerInstance;
 
 struct PCHBuffer {
-  uint64_t Signature;
+  ASTFileSignature Signature;
   llvm::SmallVector<char, 0> Data;
   bool IsComplete;
 };
diff --git a/include/clang/Lex/HeaderSearchOptions.h b/include/clang/Lex/HeaderSearchOptions.h
index e999805373..ca3a84e75e 100644
--- a/include/clang/Lex/HeaderSearchOptions.h
+++ b/include/clang/Lex/HeaderSearchOptions.h
@@ -178,6 +178,8 @@ public:
 
   unsigned ModulesValidateDiagnosticOptions : 1;
 
+  unsigned ModulesHashContent : 1;
+
   HeaderSearchOptions(StringRef _Sysroot = "/")
       : Sysroot(_Sysroot), ModuleFormat("raw"), DisableModuleHash(0),
         ImplicitModuleMaps(0), ModuleMapFileHomeIsCwd(0),
@@ -186,8 +188,8 @@ public:
         UseBuiltinIncludes(true), UseStandardSystemIncludes(true),
         UseStandardCXXIncludes(true), UseLibcxx(false), Verbose(false),
         ModulesValidateOncePerBuildSession(false),
-        ModulesValidateSystemHeaders(false),
-        UseDebugInfo(false), ModulesValidateDiagnosticOptions(true) {}
+        ModulesValidateSystemHeaders(false), UseDebugInfo(false),
+        ModulesValidateDiagnosticOptions(true), ModulesHashContent(false) {}
 
   /// AddPath - Add the \p Path path to the specified \p Group list.
   void AddPath(StringRef Path, frontend::IncludeDirGroup Group,
diff --git a/include/clang/Serialization/ASTBitCodes.h b/include/clang/Serialization/ASTBitCodes.h
index 91e5734451..de8e2a8183 100644
--- a/include/clang/Serialization/ASTBitCodes.h
+++ b/include/clang/Serialization/ASTBitCodes.h
@@ -226,7 +226,7 @@ namespace clang {
 
       /// \brief The block containing the detailed preprocessing record.
       PREPROCESSOR_DETAIL_BLOCK_ID,
-      
+
       /// \brief The block containing the submodule structure.
       SUBMODULE_BLOCK_ID,
 
@@ -253,6 +253,12 @@ namespace clang {
 
       /// \brief A block containing a module file extension.
       EXTENSION_BLOCK_ID,
+
+      /// A block with unhashed content.
+      ///
+      /// These records should not change the \a ASTFileSignature.  See \a
+      /// UnhashedControlBlockRecordTypes for the list of records.
+      UNHASHED_CONTROL_BLOCK_ID,
     };
 
     /// \brief Record types that occur within the control block.
@@ -288,9 +294,6 @@ namespace clang {
       /// AST file.
       MODULE_MAP_FILE,
 
-      /// \brief Record code for the signature that identifiers this AST file.
-      SIGNATURE,
-
       /// \brief Record code for the module build directory.
       MODULE_DIRECTORY,
     };
@@ -309,9 +312,6 @@ namespace clang {
       /// \brief Record code for the target options table.
       TARGET_OPTIONS,
 
-      /// \brief Record code for the diagnostic options table.
-      DIAGNOSTIC_OPTIONS,
-
       /// \brief Record code for the filesystem options table.
       FILE_SYSTEM_OPTIONS,
 
@@ -322,6 +322,18 @@ namespace clang {
       PREPROCESSOR_OPTIONS,
     };
 
+    /// Record codes for the unhashed control block.
+    enum UnhashedControlBlockRecordTypes {
+      /// Record code for the signature that identifiers this AST file.
+      SIGNATURE = 1,
+
+      /// Record code for the diagnostic options table.
+      DIAGNOSTIC_OPTIONS,
+
+      /// Record code for \#pragma diagnostic mappings.
+      DIAG_PRAGMA_MAPPINGS,
+    };
+
     /// \brief Record code for extension blocks.
     enum ExtensionBlockRecordTypes {
       /// Metadata describing this particular extension.
@@ -493,8 +505,7 @@ namespace clang {
 
       // ID 31 used to be a list of offsets to DECL_CXX_BASE_SPECIFIERS records.
 
-      /// \brief Record code for \#pragma diagnostic mappings.
-      DIAG_PRAGMA_MAPPINGS = 32,
+      // ID 32 used to be the code for \#pragma diagnostic mappings.
 
       /// \brief Record code for special CUDA declarations.
       CUDA_SPECIAL_DECL_REFS = 33,
diff --git a/include/clang/Serialization/ASTReader.h b/include/clang/Serialization/ASTReader.h
index edceb64847..5a1514ad80 100644
--- a/include/clang/Serialization/ASTReader.h
+++ b/include/clang/Serialization/ASTReader.h
@@ -1174,7 +1174,7 @@ private:
                             SourceLocation ImportLoc, ModuleFile *ImportedBy,
                             SmallVectorImpl<ImportedModule> &Loaded,
                             off_t ExpectedSize, time_t ExpectedModTime,
-                            serialization::ASTFileSignature ExpectedSignature,
+                            ASTFileSignature ExpectedSignature,
                             unsigned ClientLoadCapabilities);
   ASTReadResult ReadControlBlock(ModuleFile &F,
                                  SmallVectorImpl<ImportedModule> &Loaded,
@@ -1183,7 +1183,22 @@ private:
   static ASTReadResult ReadOptionsBlock(
       llvm::BitstreamCursor &Stream, unsigned ClientLoadCapabilities,
       bool AllowCompatibleConfigurationMismatch, ASTReaderListener &Listener,
-      std::string &SuggestedPredefines, bool ValidateDiagnosticOptions);
+      std::string &SuggestedPredefines);
+
+  /// Read the unhashed control block.
+  ///
+  /// This has no effect on \c F.Stream, instead creating a fresh cursor from
+  /// \c F.Data and reading ahead.
+  ASTReadResult readUnhashedControlBlock(ModuleFile &F, bool WasImportedBy,
+                                         unsigned ClientLoadCapabilities);
+
+  static ASTReadResult
+  readUnhashedControlBlockImpl(ModuleFile *F, llvm::StringRef StreamData,
+                               unsigned ClientLoadCapabilities,
+                               bool AllowCompatibleConfigurationMismatch,
+                               ASTReaderListener *Listener,
+                               bool ValidateDiagnosticOptions);
+
   ASTReadResult ReadASTBlock(ModuleFile &F, unsigned ClientLoadCapabilities);
   ASTReadResult ReadExtensionBlock(ModuleFile &F);
   void ReadModuleOffsetMap(ModuleFile &F) const;
diff --git a/include/clang/Serialization/ASTWriter.h b/include/clang/Serialization/ASTWriter.h
index 23afa639de..f3c644b9e2 100644
--- a/include/clang/Serialization/ASTWriter.h
+++ b/include/clang/Serialization/ASTWriter.h
@@ -106,6 +106,9 @@ private:
   /// \brief The bitstream writer used to emit this precompiled header.
   llvm::BitstreamWriter &Stream;
 
+  /// The buffer associated with the bitstream.
+  const SmallVectorImpl<char> &Buffer;
+
   /// \brief The ASTContext we're writing.
   ASTContext *Context = nullptr;
 
@@ -425,8 +428,16 @@ private:
   void WriteSubStmt(Stmt *S);
 
   void WriteBlockInfoBlock();
-  uint64_t WriteControlBlock(Preprocessor &PP, ASTContext &Context,
-                             StringRef isysroot, const std::string &OutputFile);
+  void WriteControlBlock(Preprocessor &PP, ASTContext &Context,
+                         StringRef isysroot, const std::string &OutputFile);
+
+  /// Write out the signature and diagnostic options, and return the signature.
+  ASTFileSignature writeUnhashedControlBlock(Preprocessor &PP,
+                                             ASTContext &Context);
+
+  /// Calculate hash of the pcm content.
+  static ASTFileSignature createSignature(StringRef Bytes);
+
   void WriteInputFiles(SourceManager &SourceMgr, HeaderSearchOptions &HSOpts,
                        bool Modules);
   void WriteSourceManagerBlock(SourceManager &SourceMgr,
@@ -493,14 +504,14 @@ private:
   void WriteDeclAbbrevs();
   void WriteDecl(ASTContext &Context, Decl *D);
 
-  uint64_t WriteASTCore(Sema &SemaRef,
-                        StringRef isysroot, const std::string &OutputFile,
-                        Module *WritingModule);
+  ASTFileSignature WriteASTCore(Sema &SemaRef, StringRef isysroot,
+                                const std::string &OutputFile,
+                                Module *WritingModule);
 
 public:
   /// \brief Create a new precompiled header writer that outputs to
   /// the given bitstream.
-  ASTWriter(llvm::BitstreamWriter &Stream,
+  ASTWriter(llvm::BitstreamWriter &Stream, SmallVectorImpl<char> &Buffer,
             ArrayRef<std::shared_ptr<ModuleFileExtension>> Extensions,
             bool IncludeTimestamps = true);
   ~ASTWriter() override;
@@ -526,9 +537,9 @@ public:
   ///
   /// \return the module signature, which eventually will be a hash of
   /// the module but currently is merely a random 32-bit number.
-  uint64_t WriteAST(Sema &SemaRef, const std::string &OutputFile,
-                    Module *WritingModule, StringRef isysroot,
-                    bool hasErrors = false);
+  ASTFileSignature WriteAST(Sema &SemaRef, const std::string &OutputFile,
+                            Module *WritingModule, StringRef isysroot,
+                            bool hasErrors = false);
 
   /// \brief Emit a token.
   void AddToken(const Token &Tok, RecordDataImpl &Record);
diff --git a/include/clang/Serialization/Module.h b/include/clang/Serialization/Module.h
index 2fd156ceb4..c1d4a9fd2d 100644
--- a/include/clang/Serialization/Module.h
+++ b/include/clang/Serialization/Module.h
@@ -16,6 +16,7 @@
 #define LLVM_CLANG_SERIALIZATION_MODULE_H
 
 #include "clang/Basic/FileManager.h"
+#include "clang/Basic/Module.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Serialization/ContinuousRangeMap.h"
@@ -89,8 +90,6 @@ public:
   bool isNotFound() const { return Val.getInt() == NotFound; }
 };
 
-typedef unsigned ASTFileSignature;
-
 /// \brief Information about a module that has been loaded by the ASTReader.
 ///
 /// Each instance of the Module class corresponds to a single AST file, which
@@ -153,9 +152,9 @@ public:
   /// \brief The file entry for the module file.
   const FileEntry *File = nullptr;
 
-  /// \brief The signature of the module file, which may be used along with size
+  /// The signature of the module file, which may be used instead of the size
   /// and modification time to identify this particular file.
-  ASTFileSignature Signature = 0;
+  ASTFileSignature Signature;
 
   /// \brief Whether this module has been directly imported by the
   /// user.
diff --git a/lib/Basic/Module.cpp b/lib/Basic/Module.cpp
index 719ff6709f..ad814fda9a 100644
--- a/lib/Basic/Module.cpp
+++ b/lib/Basic/Module.cpp
@@ -27,7 +27,7 @@ using namespace clang;
 Module::Module(StringRef Name, SourceLocation DefinitionLoc, Module *Parent,
                bool IsFramework, bool IsExplicit, unsigned VisibilityID)
     : Name(Name), DefinitionLoc(DefinitionLoc), Parent(Parent), Directory(),
-      Umbrella(), Signature(0), ASTFile(nullptr), VisibilityID(VisibilityID),
+      Umbrella(), ASTFile(nullptr), VisibilityID(VisibilityID),
       IsMissingRequirement(false), HasIncompatibleModuleFile(false),
       IsAvailable(true), IsFromModuleFile(false), IsFramework(IsFramework),
       IsExplicit(IsExplicit), IsSystem(false), IsExternC(false),
diff --git a/lib/CodeGen/CGDebugInfo.cpp b/lib/CodeGen/CGDebugInfo.cpp
index d8638c6f8d..05987be845 100644
--- a/lib/CodeGen/CGDebugInfo.cpp
+++ b/lib/CodeGen/CGDebugInfo.cpp
@@ -2053,7 +2053,11 @@ CGDebugInfo::getOrCreateModuleRef(ExternalASTSource::ASTSourceDescriptor Mod,
   if (CreateSkeletonCU && IsRootModule) {
     // PCH files don't have a signature field in the control block,
     // but LLVM detects skeleton CUs by looking for a non-zero DWO id.
-    uint64_t Signature = Mod.getSignature() ? Mod.getSignature() : ~1ULL;
+    // We use the lower 64 bits for debug info.
+    uint64_t Signature =
+        Mod.getSignature()
+            ? (uint64_t)Mod.getSignature()[1] << 32 | Mod.getSignature()[0]
+            : ~1ULL;
     llvm::DIBuilder DIB(CGM.getModule());
     DIB.createCompileUnit(TheCU->getSourceLanguage(),
                           DIB.createFile(Mod.getModuleName(), Mod.getPath()),
diff --git a/lib/CodeGen/ObjectFilePCHContainerOperations.cpp b/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
index 754f9968b6..37ecc05aa1 100644
--- a/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
+++ b/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
@@ -171,7 +171,8 @@ public:
     // Prepare CGDebugInfo to emit debug info for a clang module.
     auto *DI = Builder->getModuleDebugInfo();
     StringRef ModuleName = llvm::sys::path::filename(MainFileName);
-    DI->setPCHDescriptor({ModuleName, "", OutputFileName, ~1ULL});
+    DI->setPCHDescriptor({ModuleName, "", OutputFileName,
+                          ASTFileSignature{{{~0U, ~0U, ~0U, ~0U, ~1U}}}});
     DI->setModuleMap(MMap);
   }
 
@@ -241,7 +242,11 @@ public:
 
     // PCH files don't have a signature field in the control block,
     // but LLVM detects DWO CUs by looking for a non-zero DWO id.
-    uint64_t Signature = Buffer->Signature ? Buffer->Signature : ~1ULL;
+    // We use the lower 64 bits for debug info.
+    uint64_t Signature =
+        Buffer->Signature
+            ? (uint64_t)Buffer->Signature[1] << 32 | Buffer->Signature[0]
+            : ~1ULL;
     Builder->getModuleDebugInfo()->setDwoId(Signature);
 
     // Finalize the Builder.
diff --git a/lib/Frontend/ASTUnit.cpp b/lib/Frontend/ASTUnit.cpp
index db19804d5f..ac5c7ca3be 100644
--- a/lib/Frontend/ASTUnit.cpp
+++ b/lib/Frontend/ASTUnit.cpp
@@ -185,7 +185,7 @@ struct ASTUnit::ASTWriterData {
   llvm::BitstreamWriter Stream;
   ASTWriter Writer;
 
-  ASTWriterData() : Stream(Buffer), Writer(Stream, { }) { }
+  ASTWriterData() : Stream(Buffer), Writer(Stream, Buffer, {}) {}
 };
 
 void ASTUnit::clearFileLevelDecls() {
@@ -2523,7 +2523,7 @@ bool ASTUnit::serialize(raw_ostream &OS) {
 
   SmallString<128> Buffer;
   llvm::BitstreamWriter Stream(Buffer);
-  ASTWriter Writer(Stream, { });
+  ASTWriter Writer(Stream, Buffer, {});
   return serializeUnit(Writer, Buffer, getSema(), hasErrors, OS);
 }
 
diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index 39fe7771a6..a0e81f01f9 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -1029,7 +1029,7 @@ static bool compileModuleImpl(CompilerInstance &ImportingInstance,
 
   // Remove any macro definitions that are explicitly ignored by the module.
   // They aren't supposed to affect how the module is built anyway.
-  const HeaderSearchOptions &HSOpts = Invocation->getHeaderSearchOpts();
+  HeaderSearchOptions &HSOpts = Invocation->getHeaderSearchOpts();
   PPOpts.Macros.erase(
       std::remove_if(PPOpts.Macros.begin(), PPOpts.Macros.end(),
                      [&HSOpts](const std::pair<std::string, bool> &def) {
@@ -1060,6 +1060,8 @@ static bool compileModuleImpl(CompilerInstance &ImportingInstance,
   FrontendOpts.DisableFree = false;
   FrontendOpts.GenerateGlobalModuleIndex = false;
   FrontendOpts.BuildingImplicitModule = true;
+  // Force implicitly-built modules to hash the content of the module file.
+  HSOpts.ModulesHashContent = true;
   FrontendOpts.Inputs.clear();
   InputKind IK = getSourceInputKindFromOptions(*Invocation->getLangOpts());
 
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 2352974909..d5ad3df35d 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1434,6 +1434,7 @@ static void ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args) {
   for (const Arg *A : Args.filtered(OPT_fprebuilt_module_path))
     Opts.AddPrebuiltModulePath(A->getValue());
   Opts.DisableModuleHash = Args.hasArg(OPT_fdisable_module_hash);
+  Opts.ModulesHashContent = Args.hasArg(OPT_fmodules_hash_content);
   Opts.ModulesValidateDiagnosticOptions =
       !Args.hasArg(OPT_fmodules_disable_diagnostic_validation);
   Opts.ImplicitModuleMaps = Args.hasArg(OPT_fimplicit_module_maps);
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 648c7b48ed..be0cc9ab1a 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -2163,7 +2163,7 @@ static bool isDiagnosedResult(ASTReader::ASTReadResult ARR, unsigned Caps) {
 ASTReader::ASTReadResult ASTReader::ReadOptionsBlock(
     BitstreamCursor &Stream, unsigned ClientLoadCapabilities,
     bool AllowCompatibleConfigurationMismatch, ASTReaderListener &Listener,
-    std::string &SuggestedPredefines, bool ValidateDiagnosticOptions) {
+    std::string &SuggestedPredefines) {
   if (Stream.EnterSubBlock(OPTIONS_BLOCK_ID))
     return Failure;
 
@@ -2205,15 +2205,6 @@ ASTReader::ASTReadResult ASTReader::ReadOptionsBlock(
       break;
     }
 
-    case DIAGNOSTIC_OPTIONS: {
-      bool Complain = (ClientLoadCapabilities & ARR_OutOfDate) == 0;
-      if (ValidateDiagnosticOptions &&
-          !AllowCompatibleConfigurationMismatch &&
-          ParseDiagnosticOptions(Record, Complain, Listener))
-        return OutOfDate;
-      break;
-    }
-
     case FILE_SYSTEM_OPTIONS: {
       bool Complain = (ClientLoadCapabilities & ARR_ConfigurationMismatch) == 0;
       if (!AllowCompatibleConfigurationMismatch &&
@@ -2254,6 +2245,23 @@ ASTReader::ReadControlBlock(ModuleFile &F,
     return Failure;
   }
 
+  // Lambda to read the unhashed control block the first time it's called.
+  //
+  // For PCM files, the unhashed control block cannot be read until after the
+  // MODULE_NAME record.  However, PCH files have no MODULE_NAME, and yet still
+  // need to look ahead before reading the IMPORTS record.  For consistency,
+  // this block is always read somehow (see BitstreamEntry::EndBlock).
+  bool HasReadUnhashedControlBlock = false;
+  auto readUnhashedControlBlockOnce = [&]() {
+    if (!HasReadUnhashedControlBlock) {
+      HasReadUnhashedControlBlock = true;
+      if (ASTReadResult Result =
+              readUnhashedControlBlock(F, ImportedBy, ClientLoadCapabilities))
+        return Result;
+    }
+    return Success;
+  };
+
   // Read all of the records and blocks in the control block.
   RecordData Record;
   unsigned NumInputs = 0;
@@ -2266,6 +2274,11 @@ ASTReader::ReadControlBlock(ModuleFile &F,
       Error("malformed block record in AST file");
       return Failure;
     case llvm::BitstreamEntry::EndBlock: {
+      // Validate the module before returning.  This call catches an AST with
+      // no module name and no imports.
+      if (ASTReadResult Result = readUnhashedControlBlockOnce())
+        return Result;
+
       // Validate input files.
       const HeaderSearchOptions &HSOpts =
           PP.getHeaderSearchInfo().getHeaderSearchOpts();
@@ -2337,13 +2350,10 @@ ASTReader::ReadControlBlock(ModuleFile &F,
           // FIXME: Allow this for files explicitly specified with -include-pch.
           bool AllowCompatibleConfigurationMismatch =
               F.Kind == MK_ExplicitModule || F.Kind == MK_PrebuiltModule;
-          const HeaderSearchOptions &HSOpts =
-              PP.getHeaderSearchInfo().getHeaderSearchOpts();
 
           Result = ReadOptionsBlock(Stream, ClientLoadCapabilities,
                                     AllowCompatibleConfigurationMismatch,
-                                    *Listener, SuggestedPredefines,
-                                    HSOpts.ModulesValidateDiagnosticOptions);
+                                    *Listener, SuggestedPredefines);
           if (Result == Failure) {
             Error("malformed block record in AST file");
             return Result;
@@ -2417,12 +2427,13 @@ ASTReader::ReadControlBlock(ModuleFile &F,
       break;
     }
 
-    case SIGNATURE:
-      assert((!F.Signature || F.Signature == Record[0]) && "signature changed");
-      F.Signature = Record[0];
-      break;
-
     case IMPORTS: {
+      // Validate the AST before processing any imports (otherwise, untangling
+      // them can be error-prone and expensive).  A module will have a name and
+      // will already have been validated, but this catches the PCH case.
+      if (ASTReadResult Result = readUnhashedControlBlockOnce())
+        return Result;
+
       // Load each of the imported PCH files.
       unsigned Idx = 0, N = Record.size();
       while (Idx < N) {
@@ -2435,7 +2446,10 @@ ASTReader::ReadControlBlock(ModuleFile &F,
             ReadUntranslatedSourceLocation(Record[Idx++]);
         off_t StoredSize = (off_t)Record[Idx++];
         time_t StoredModTime = (time_t)Record[Idx++];
-        ASTFileSignature StoredSignature = Record[Idx++];
+        ASTFileSignature StoredSignature = {
+            {{(uint32_t)Record[Idx++], (uint32_t)Record[Idx++],
+              (uint32_t)Record[Idx++], (uint32_t)Record[Idx++],
+              (uint32_t)Record[Idx++]}}};
         auto ImportedFile = ReadPath(F, Record, Idx);
 
         // If our client can't cope with us being out of date, we can't cope with
@@ -2487,6 +2501,12 @@ ASTReader::ReadControlBlock(ModuleFile &F,
       F.ModuleName = Blob;
       if (Listener)
         Listener->ReadModuleName(F.ModuleName);
+
+      // Validate the AST as soon as we have a name so we can exit early on
+      // failure.
+      if (ASTReadResult Result = readUnhashedControlBlockOnce())
+        return Result;
+
       break;
 
     case MODULE_DIRECTORY: {
@@ -3076,14 +3096,6 @@ ASTReader::ReadASTBlock(ModuleFile &F, unsigned ClientLoadCapabilities) {
       F.ObjCCategories.swap(Record);
       break;
 
-    case DIAG_PRAGMA_MAPPINGS:
-      if (F.PragmaDiagMappings.empty())
-        F.PragmaDiagMappings.swap(Record);
-      else
-        F.PragmaDiagMappings.insert(F.PragmaDiagMappings.end(),
-                                    Record.begin(), Record.end());
-      break;
-
     case CUDA_SPECIAL_DECL_REFS:
       // Later tables overwrite earlier ones.
       // FIXME: Modules will have trouble with this.
@@ -3657,10 +3669,10 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName,
 
   unsigned NumModules = ModuleMgr.size();
   SmallVector<ImportedModule, 4> Loaded;
-  switch(ASTReadResult ReadResult = ReadASTCore(FileName, Type, ImportLoc,
-                                                /*ImportedBy=*/nullptr, Loaded,
-                                                0, 0, 0,
-                                                ClientLoadCapabilities)) {
+  switch (ASTReadResult ReadResult =
+              ReadASTCore(FileName, Type, ImportLoc,
+                          /*ImportedBy=*/nullptr, Loaded, 0, 0,
+                          ASTFileSignature(), ClientLoadCapabilities)) {
   case Failure:
   case Missing:
   case OutOfDate:
@@ -4021,6 +4033,12 @@ ASTReader::ReadASTCore(StringRef FileName,
       Loaded.push_back(ImportedModule(M, ImportedBy, ImportLoc));
       return Success;
 
+    case UNHASHED_CONTROL_BLOCK_ID:
+      // This block is handled using look-ahead during ReadControlBlock.  We
+      // shouldn't get here!
+      Error("malformed block record in AST file");
+      return Failure;
+
     default:
       if (Stream.SkipBlock()) {
         Error("malformed block record in AST file");
@@ -4033,6 +4051,93 @@ ASTReader::ReadASTCore(StringRef FileName,
   return Success;
 }
 
+ASTReader::ASTReadResult
+ASTReader::readUnhashedControlBlock(ModuleFile &F, bool WasImportedBy,
+                                    unsigned ClientLoadCapabilities) {
+  const HeaderSearchOptions &HSOpts =
+      PP.getHeaderSearchInfo().getHeaderSearchOpts();
+  bool AllowCompatibleConfigurationMismatch =
+      F.Kind == MK_ExplicitModule || F.Kind == MK_PrebuiltModule;
+
+  ASTReadResult Result = readUnhashedControlBlockImpl(
+      &F, F.Data, ClientLoadCapabilities, AllowCompatibleConfigurationMismatch,
+      Listener.get(),
+      WasImportedBy ? false : HSOpts.ModulesValidateDiagnosticOptions);
+
+  if (DisableValidation || WasImportedBy ||
+      (AllowConfigurationMismatch && Result == ConfigurationMismatch))
+    return Success;
+
+  if (Result == Failure)
+    Error("malformed block record in AST file");
+
+  return Result;
+}
+
+ASTReader::ASTReadResult ASTReader::readUnhashedControlBlockImpl(
+    ModuleFile *F, llvm::StringRef StreamData, unsigned ClientLoadCapabilities,
+    bool AllowCompatibleConfigurationMismatch, ASTReaderListener *Listener,
+    bool ValidateDiagnosticOptions) {
+  // Initialize a stream.
+  BitstreamCursor Stream(StreamData);
+
+  // Sniff for the signature.
+  if (!startsWithASTFileMagic(Stream))
+    return Failure;
+
+  // Scan for the UNHASHED_CONTROL_BLOCK_ID block.
+  if (SkipCursorToBlock(Stream, UNHASHED_CONTROL_BLOCK_ID))
+    return Failure;
+
+  // Read all of the records in the options block.
+  RecordData Record;
+  ASTReadResult Result = Success;
+  while (1) {
+    llvm::BitstreamEntry Entry = Stream.advance();
+
+    switch (Entry.Kind) {
+    case llvm::BitstreamEntry::Error:
+    case llvm::BitstreamEntry::SubBlock:
+      return Failure;
+
+    case llvm::BitstreamEntry::EndBlock:
+      return Result;
+
+    case llvm::BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
+    // Read and process a record.
+    Record.clear();
+    switch (
+        (UnhashedControlBlockRecordTypes)Stream.readRecord(Entry.ID, Record)) {
+    case SIGNATURE: {
+      if (F)
+        std::copy(Record.begin(), Record.end(), F->Signature.data());
+      break;
+    }
+    case DIAGNOSTIC_OPTIONS: {
+      bool Complain = (ClientLoadCapabilities & ARR_OutOfDate) == 0;
+      if (Listener && ValidateDiagnosticOptions &&
+          !AllowCompatibleConfigurationMismatch &&
+          ParseDiagnosticOptions(Record, Complain, *Listener))
+        return OutOfDate;
+      break;
+    }
+    case DIAG_PRAGMA_MAPPINGS:
+      if (!F)
+        break;
+      if (F->PragmaDiagMappings.empty())
+        F->PragmaDiagMappings.swap(Record);
+      else
+        F->PragmaDiagMappings.insert(F->PragmaDiagMappings.end(),
+                                     Record.begin(), Record.end());
+      break;
+    }
+  }
+}
+
 /// Parse a record and blob containing module file extension metadata.
 static bool parseModuleFileExtensionMetadata(
               const SmallVectorImpl<uint64_t> &Record,
@@ -4249,23 +4354,24 @@ void ASTReader::finalizeForWriting() {
 static ASTFileSignature readASTFileSignature(StringRef PCH) {
   BitstreamCursor Stream(PCH);
   if (!startsWithASTFileMagic(Stream))
-    return 0;
+    return ASTFileSignature();
 
-  // Scan for the CONTROL_BLOCK_ID block.
-  if (SkipCursorToBlock(Stream, CONTROL_BLOCK_ID))
-    return 0;
+  // Scan for the UNHASHED_CONTROL_BLOCK_ID block.
+  if (SkipCursorToBlock(Stream, UNHASHED_CONTROL_BLOCK_ID))
+    return ASTFileSignature();
 
-  // Scan for SIGNATURE inside the control block.
+  // Scan for SIGNATURE inside the diagnostic options block.
   ASTReader::RecordData Record;
   while (true) {
     llvm::BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
     if (Entry.Kind != llvm::BitstreamEntry::Record)
-      return 0;
+      return ASTFileSignature();
 
     Record.clear();
     StringRef Blob;
     if (SIGNATURE == Stream.readRecord(Entry.ID, Record, &Blob))
-      return Record[0];
+      return {{{(uint32_t)Record[0], (uint32_t)Record[1], (uint32_t)Record[2],
+                (uint32_t)Record[3], (uint32_t)Record[4]}}};
   }
 }
 
@@ -4384,7 +4490,8 @@ bool ASTReader::readASTFileControlBlock(
   }
 
   // Initialize the stream
-  BitstreamCursor Stream(PCHContainerRdr.ExtractPCH(**Buffer));
+  StringRef Bytes = PCHContainerRdr.ExtractPCH(**Buffer);
+  BitstreamCursor Stream(Bytes);
 
   // Sniff for the signature.
   if (!startsWithASTFileMagic(Stream))
@@ -4412,8 +4519,7 @@ bool ASTReader::readASTFileControlBlock(
         std::string IgnoredSuggestedPredefines;
         if (ReadOptionsBlock(Stream, ARR_ConfigurationMismatch | ARR_OutOfDate,
                              /*AllowCompatibleConfigurationMismatch*/ false,
-                             Listener, IgnoredSuggestedPredefines,
-                             ValidateDiagnosticOptions) != Success)
+                             Listener, IgnoredSuggestedPredefines) != Success)
           return true;
         break;
       }
@@ -4534,6 +4640,7 @@ bool ASTReader::readASTFileControlBlock(
 
   // Look for module file extension blocks, if requested.
   if (FindModuleFileExtensions) {
+    BitstreamCursor SavedStream = Stream;
     while (!SkipCursorToBlock(Stream, EXTENSION_BLOCK_ID)) {
       bool DoneWithExtensionBlock = false;
       while (!DoneWithExtensionBlock) {
@@ -4572,8 +4679,16 @@ bool ASTReader::readASTFileControlBlock(
        }
       }
     }
+    Stream = SavedStream;
   }
 
+  // Scan for the UNHASHED_CONTROL_BLOCK_ID block.
+  if (readUnhashedControlBlockImpl(
+          nullptr, Bytes, ARR_ConfigurationMismatch | ARR_OutOfDate,
+          /*AllowCompatibleConfigurationMismatch*/ false, &Listener,
+          ValidateDiagnosticOptions) != Success)
+    return true;
+
   return false;
 }
 
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index cdc1cf5bcb..b13a4e1ff4 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -18,8 +18,8 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTUnresolvedSet.h"
 #include "clang/AST/Decl.h"
-#include "clang/AST/DeclContextInternals.h"
 #include "clang/AST/DeclCXX.h"
+#include "clang/AST/DeclContextInternals.h"
 #include "clang/AST/DeclFriend.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
@@ -33,8 +33,8 @@
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/FileManager.h"
 #include "clang/Basic/FileSystemOptions.h"
-#include "clang/Basic/LangOptions.h"
 #include "clang/Basic/LLVM.h"
+#include "clang/Basic/LangOptions.h"
 #include "clang/Basic/Module.h"
 #include "clang/Basic/ObjCRuntime.h"
 #include "clang/Basic/SourceManager.h"
@@ -64,9 +64,9 @@
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallString.h"
-#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Bitcode/BitCodes.h"
 #include "llvm/Bitcode/BitstreamWriter.h"
@@ -79,6 +79,7 @@
 #include "llvm/Support/OnDiskHashTable.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
+#include "llvm/Support/SHA1.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -1016,7 +1017,6 @@ void ASTWriter::WriteBlockInfoBlock() {
   // Control Block.
   BLOCK(CONTROL_BLOCK);
   RECORD(METADATA);
-  RECORD(SIGNATURE);
   RECORD(MODULE_NAME);
   RECORD(MODULE_DIRECTORY);
   RECORD(MODULE_MAP_FILE);
@@ -1029,7 +1029,6 @@ void ASTWriter::WriteBlockInfoBlock() {
   BLOCK(OPTIONS_BLOCK);
   RECORD(LANGUAGE_OPTIONS);
   RECORD(TARGET_OPTIONS);
-  RECORD(DIAGNOSTIC_OPTIONS);
   RECORD(FILE_SYSTEM_OPTIONS);
   RECORD(HEADER_SEARCH_OPTIONS);
   RECORD(PREPROCESSOR_OPTIONS);
@@ -1065,7 +1064,6 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(UPDATE_VISIBLE);
   RECORD(DECL_UPDATE_OFFSETS);
   RECORD(DECL_UPDATES);
-  RECORD(DIAG_PRAGMA_MAPPINGS);
   RECORD(CUDA_SPECIAL_DECL_REFS);
   RECORD(HEADER_SEARCH_TABLE);
   RECORD(FP_PRAGMA_OPTIONS);
@@ -1258,6 +1256,11 @@ void ASTWriter::WriteBlockInfoBlock() {
   BLOCK(EXTENSION_BLOCK);
   RECORD(EXTENSION_METADATA);
 
+  BLOCK(UNHASHED_CONTROL_BLOCK);
+  RECORD(SIGNATURE);
+  RECORD(DIAGNOSTIC_OPTIONS);
+  RECORD(DIAG_PRAGMA_MAPPINGS);
+
 #undef RECORD
 #undef BLOCK
   Stream.ExitBlock();
@@ -1320,21 +1323,73 @@ adjustFilenameForRelocatableAST(const char *Filename, StringRef BaseDir) {
   return Filename + Pos;
 }
 
-static ASTFileSignature getSignature() {
-  while (true) {
-    if (ASTFileSignature S = llvm::sys::Process::GetRandomNumber())
-      return S;
-    // Rely on GetRandomNumber to eventually return non-zero...
+ASTFileSignature ASTWriter::createSignature(StringRef Bytes) {
+  // Calculate the hash till start of UNHASHED_CONTROL_BLOCK.
+  llvm::SHA1 Hasher;
+  Hasher.update(ArrayRef<uint8_t>(Bytes.bytes_begin(), Bytes.size()));
+  auto Hash = Hasher.result();
+
+  // Convert to an array [5*i32].
+  ASTFileSignature Signature;
+  auto LShift = [&](unsigned char Val, unsigned Shift) {
+    return (uint32_t)Val << Shift;
+  };
+  for (int I = 0; I != 5; ++I)
+    Signature[I] = LShift(Hash[I * 4 + 0], 24) | LShift(Hash[I * 4 + 1], 16) |
+                   LShift(Hash[I * 4 + 2], 8) | LShift(Hash[I * 4 + 3], 0);
+
+  return Signature;
+}
+
+ASTFileSignature ASTWriter::writeUnhashedControlBlock(Preprocessor &PP,
+                                                      ASTContext &Context) {
+  // Flush first to prepare the PCM hash (signature).
+  Stream.FlushToWord();
+  auto StartOfUnhashedControl = Stream.GetCurrentBitNo() >> 3;
+
+  // Enter the block and prepare to write records.
+  RecordData Record;
+  Stream.EnterSubblock(UNHASHED_CONTROL_BLOCK_ID, 5);
+
+  // For implicit modules, write the hash of the PCM as its signature.
+  ASTFileSignature Signature;
+  if (WritingModule &&
+      PP.getHeaderSearchInfo().getHeaderSearchOpts().ModulesHashContent) {
+    Signature = createSignature(StringRef(Buffer.begin(), StartOfUnhashedControl));
+    Record.append(Signature.begin(), Signature.end());
+    Stream.EmitRecord(SIGNATURE, Record);
+    Record.clear();
   }
+
+  // Diagnostic options.
+  const auto &Diags = Context.getDiagnostics();
+  const DiagnosticOptions &DiagOpts = Diags.getDiagnosticOptions();
+#define DIAGOPT(Name, Bits, Default) Record.push_back(DiagOpts.Name);
+#define ENUM_DIAGOPT(Name, Type, Bits, Default)                                \
+  Record.push_back(static_cast<unsigned>(DiagOpts.get##Name()));
+#include "clang/Basic/DiagnosticOptions.def"
+  Record.push_back(DiagOpts.Warnings.size());
+  for (unsigned I = 0, N = DiagOpts.Warnings.size(); I != N; ++I)
+    AddString(DiagOpts.Warnings[I], Record);
+  Record.push_back(DiagOpts.Remarks.size());
+  for (unsigned I = 0, N = DiagOpts.Remarks.size(); I != N; ++I)
+    AddString(DiagOpts.Remarks[I], Record);
+  // Note: we don't serialize the log or serialization file names, because they
+  // are generally transient files and will almost always be overridden.
+  Stream.EmitRecord(DIAGNOSTIC_OPTIONS, Record);
+
+  // Write out the diagnostic/pragma mappings.
+  WritePragmaDiagnosticMappings(Diags, /* IsModule = */ WritingModule);
+
+  // Leave the options block.
+  Stream.ExitBlock();
+  return Signature;
 }
 
 /// \brief Write the control block.
-uint64_t ASTWriter::WriteControlBlock(Preprocessor &PP,
-                                      ASTContext &Context,
-                                      StringRef isysroot,
-                                      const std::string &OutputFile) {
-  ASTFileSignature Signature = 0;
-
+void ASTWriter::WriteControlBlock(Preprocessor &PP, ASTContext &Context,
+                                  StringRef isysroot,
+                                  const std::string &OutputFile) {
   using namespace llvm;
   Stream.EnterSubblock(CONTROL_BLOCK_ID, 5);
   RecordData Record;
@@ -1362,15 +1417,6 @@ uint64_t ASTWriter::WriteControlBlock(Preprocessor &PP,
                               getClangFullRepositoryVersion());
   }
   if (WritingModule) {
-    // For implicit modules we output a signature that we can use to ensure
-    // duplicate module builds don't collide in the cache as their output order
-    // is non-deterministic.
-    // FIXME: Remove this when output is deterministic.
-    if (Context.getLangOpts().ImplicitModules) {
-      Signature = getSignature();
-      RecordData::value_type Record[] = {Signature};
-      Stream.EmitRecord(SIGNATURE, Record);
-    }
 
     // Module name
     auto Abbrev = std::make_shared<BitCodeAbbrev>();
@@ -1443,9 +1489,15 @@ uint64_t ASTWriter::WriteControlBlock(Preprocessor &PP,
 
       Record.push_back((unsigned)M.Kind); // FIXME: Stable encoding
       AddSourceLocation(M.ImportLoc, Record);
-      Record.push_back(M.File->getSize());
-      Record.push_back(getTimestampForOutput(M.File));
-      Record.push_back(M.Signature);
+
+      // If we have calculated signature, there is no need to store
+      // the size or timestamp.
+      Record.push_back(M.Signature ? 0 : M.File->getSize());
+      Record.push_back(M.Signature ? 0 : getTimestampForOutput(M.File));
+
+      for (auto I : M.Signature)
+        Record.push_back(I);
+
       AddPath(M.FileName, Record);
     }
     Stream.EmitRecord(IMPORTS, Record);
@@ -1508,24 +1560,6 @@ uint64_t ASTWriter::WriteControlBlock(Preprocessor &PP,
   }
   Stream.EmitRecord(TARGET_OPTIONS, Record);
 
-  // Diagnostic options.
-  Record.clear();
-  const DiagnosticOptions &DiagOpts
-    = Context.getDiagnostics().getDiagnosticOptions();
-#define DIAGOPT(Name, Bits, Default) Record.push_back(DiagOpts.Name);
-#define ENUM_DIAGOPT(Name, Type, Bits, Default) \
-  Record.push_back(static_cast<unsigned>(DiagOpts.get##Name()));
-#include "clang/Basic/DiagnosticOptions.def"
-  Record.push_back(DiagOpts.Warnings.size());
-  for (unsigned I = 0, N = DiagOpts.Warnings.size(); I != N; ++I)
-    AddString(DiagOpts.Warnings[I], Record);
-  Record.push_back(DiagOpts.Remarks.size());
-  for (unsigned I = 0, N = DiagOpts.Remarks.size(); I != N; ++I)
-    AddString(DiagOpts.Remarks[I], Record);
-  // Note: we don't serialize the log or serialization file names, because they
-  // are generally transient files and will almost always be overridden.
-  Stream.EmitRecord(DIAGNOSTIC_OPTIONS, Record);
-
   // File system options.
   Record.clear();
   const FileSystemOptions &FSOpts =
@@ -1639,7 +1673,6 @@ uint64_t ASTWriter::WriteControlBlock(Preprocessor &PP,
                   PP.getHeaderSearchInfo().getHeaderSearchOpts(),
                   PP.getLangOpts().Modules);
   Stream.ExitBlock();
-  return Signature;
 }
 
 namespace  {
@@ -4267,9 +4300,10 @@ void ASTWriter::SetSelectorOffset(Selector Sel, uint32_t Offset) {
 }
 
 ASTWriter::ASTWriter(llvm::BitstreamWriter &Stream,
+                     SmallVectorImpl<char> &Buffer,
                      ArrayRef<std::shared_ptr<ModuleFileExtension>> Extensions,
                      bool IncludeTimestamps)
-    : Stream(Stream), IncludeTimestamps(IncludeTimestamps) {
+    : Stream(Stream), Buffer(Buffer), IncludeTimestamps(IncludeTimestamps) {
   for (const auto &Ext : Extensions) {
     if (auto Writer = Ext->createExtensionWriter(*this))
       ModuleFileExtensionWriters.push_back(std::move(Writer));
@@ -4289,9 +4323,10 @@ time_t ASTWriter::getTimestampForOutput(const FileEntry *E) const {
   return IncludeTimestamps ? E->getModificationTime() : 0;
 }
 
-uint64_t ASTWriter::WriteAST(Sema &SemaRef, const std::string &OutputFile,
-                             Module *WritingModule, StringRef isysroot,
-                             bool hasErrors) {
+ASTFileSignature ASTWriter::WriteAST(Sema &SemaRef,
+                                     const std::string &OutputFile,
+                                     Module *WritingModule, StringRef isysroot,
+                                     bool hasErrors) {
   WritingAST = true;
 
   ASTHasCompilerErrors = hasErrors;
@@ -4327,9 +4362,9 @@ static void AddLazyVectorDecls(ASTWriter &Writer, Vector &Vec,
   }
 }
 
-uint64_t ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
-                                 const std::string &OutputFile,
-                                 Module *WritingModule) {
+ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
+                                         const std::string &OutputFile,
+                                         Module *WritingModule) {
   using namespace llvm;
 
   bool isModule = WritingModule != nullptr;
@@ -4477,7 +4512,7 @@ uint64_t ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   }
 
   // Write the control block
-  uint64_t Signature = WriteControlBlock(PP, Context, isysroot, OutputFile);
+  WriteControlBlock(PP, Context, isysroot, OutputFile);
 
   // Write the remaining AST contents.
   Stream.EnterSubblock(AST_BLOCK_ID, 5);
@@ -4694,7 +4729,6 @@ uint64_t ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   WriteOpenCLExtensionTypes(SemaRef);
   WriteOpenCLExtensionDecls(SemaRef);
   WriteCUDAPragmas(SemaRef);
-  WritePragmaDiagnosticMappings(Context.getDiagnostics(), isModule);
 
   // If we're emitting a module, write out the submodule information.  
   if (WritingModule)
@@ -4823,7 +4857,7 @@ uint64_t ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   for (const auto &ExtWriter : ModuleFileExtensionWriters)
     WriteModuleFileExtension(SemaRef, *ExtWriter);
 
-  return Signature;
+  return writeUnhashedControlBlock(PP, Context);
 }
 
 void ASTWriter::WriteDeclUpdatesBlocks(RecordDataImpl &OffsetsRecord) {
diff --git a/lib/Serialization/GeneratePCH.cpp b/lib/Serialization/GeneratePCH.cpp
index 7f1b75055b..141a5594ae 100644
--- a/lib/Serialization/GeneratePCH.cpp
+++ b/lib/Serialization/GeneratePCH.cpp
@@ -28,7 +28,7 @@ PCHGenerator::PCHGenerator(
     bool AllowASTWithErrors, bool IncludeTimestamps)
     : PP(PP), OutputFile(OutputFile), isysroot(isysroot.str()),
       SemaPtr(nullptr), Buffer(Buffer), Stream(Buffer->Data),
-      Writer(Stream, Extensions, IncludeTimestamps),
+      Writer(Stream, Buffer->Data, Extensions, IncludeTimestamps),
       AllowASTWithErrors(AllowASTWithErrors) {
   Buffer->IsComplete = false;
 }
diff --git a/lib/Serialization/GlobalModuleIndex.cpp b/lib/Serialization/GlobalModuleIndex.cpp
index ae5796ede1..6978e7e097 100644
--- a/lib/Serialization/GlobalModuleIndex.cpp
+++ b/lib/Serialization/GlobalModuleIndex.cpp
@@ -376,6 +376,15 @@ namespace {
     /// \brief The set of modules on which this module depends. Each entry is
     /// a module ID.
     SmallVector<unsigned, 4> Dependencies;
+    ASTFileSignature Signature;
+  };
+
+  struct ImportedModuleFileInfo {
+    off_t StoredSize;
+    time_t StoredModTime;
+    ASTFileSignature StoredSignature;
+    ImportedModuleFileInfo(off_t Size, time_t ModTime, ASTFileSignature Sig)
+        : StoredSize(Size), StoredModTime(ModTime), StoredSignature(Sig) {}
   };
 
   /// \brief Builder that generates the global module index file.
@@ -383,12 +392,20 @@ namespace {
     FileManager &FileMgr;
     const PCHContainerReader &PCHContainerRdr;
 
-    /// \brief Mapping from files to module file information.
+    /// Mapping from files to module file information.
     typedef llvm::MapVector<const FileEntry *, ModuleFileInfo> ModuleFilesMap;
 
-    /// \brief Information about each of the known module files.
+    /// Information about each of the known module files.
     ModuleFilesMap ModuleFiles;
 
+    /// \brief Mapping from the imported module file to the imported
+    /// information.
+    typedef std::multimap<const FileEntry *, ImportedModuleFileInfo>
+        ImportedModuleFilesMap;
+
+    /// \brief Information about each importing of a module file.
+    ImportedModuleFilesMap ImportedModuleFiles;
+
     /// \brief Mapping from identifiers to the list of module file IDs that
     /// consider this identifier to be interesting.
     typedef llvm::StringMap<SmallVector<unsigned, 2> > InterestingIdentifierMap;
@@ -424,7 +441,8 @@ namespace {
     bool loadModuleFile(const FileEntry *File);
 
     /// \brief Write the index to the given bitstream.
-    void writeIndex(llvm::BitstreamWriter &Stream);
+    /// \returns true if an error occurred, false otherwise.
+    bool writeIndex(llvm::BitstreamWriter &Stream);
   };
 }
 
@@ -515,7 +533,7 @@ bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
   unsigned ID = getModuleFileInfo(File).ID;
 
   // Search for the blocks and records we care about.
-  enum { Other, ControlBlock, ASTBlock } State = Other;
+  enum { Other, ControlBlock, ASTBlock, DiagnosticOptionsBlock } State = Other;
   bool Done = false;
   while (!Done) {
     llvm::BitstreamEntry Entry = InStream.advance();
@@ -553,6 +571,15 @@ bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
         continue;
       }
 
+      if (Entry.ID == UNHASHED_CONTROL_BLOCK_ID) {
+        if (InStream.EnterSubBlock(UNHASHED_CONTROL_BLOCK_ID))
+          return true;
+
+        // Found the Diagnostic Options block.
+        State = DiagnosticOptionsBlock;
+        continue;
+      }
+
       if (InStream.SkipBlock())
         return true;
 
@@ -587,7 +614,10 @@ bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
 
         // Skip the stored signature.
         // FIXME: we could read the signature out of the import and validate it.
-        Idx++;
+        ASTFileSignature StoredSignature = {
+            {{(uint32_t)Record[Idx++], (uint32_t)Record[Idx++],
+              (uint32_t)Record[Idx++], (uint32_t)Record[Idx++],
+              (uint32_t)Record[Idx++]}}};
 
         // Retrieve the imported file name.
         unsigned Length = Record[Idx++];
@@ -599,11 +629,16 @@ bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
         const FileEntry *DependsOnFile
           = FileMgr.getFile(ImportedFile, /*openFile=*/false,
                             /*cacheFailure=*/false);
-        if (!DependsOnFile ||
-            (StoredSize != DependsOnFile->getSize()) ||
-            (StoredModTime != DependsOnFile->getModificationTime()))
+
+        if (!DependsOnFile)
           return true;
 
+        // Save the information in ImportedModuleFileInfo so we can verify after
+        // loading all pcms.
+        ImportedModuleFiles.insert(std::make_pair(
+            DependsOnFile, ImportedModuleFileInfo(StoredSize, StoredModTime,
+                                                  StoredSignature)));
+
         // Record the dependency.
         unsigned DependsOnID = getModuleFileInfo(DependsOnFile).ID;
         getModuleFileInfo(File).Dependencies.push_back(DependsOnID);
@@ -632,6 +667,12 @@ bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
       }
     }
 
+    // Get Signature.
+    if (State == DiagnosticOptionsBlock && Code == SIGNATURE)
+      getModuleFileInfo(File).Signature = {
+          {{(uint32_t)Record[0], (uint32_t)Record[1], (uint32_t)Record[2],
+            (uint32_t)Record[3], (uint32_t)Record[4]}}};
+
     // We don't care about this record.
   }
 
@@ -680,7 +721,20 @@ public:
 
 }
 
-void GlobalModuleIndexBuilder::writeIndex(llvm::BitstreamWriter &Stream) {
+bool GlobalModuleIndexBuilder::writeIndex(llvm::BitstreamWriter &Stream) {
+  for (auto MapEntry : ImportedModuleFiles) {
+    auto *File = MapEntry.first;
+    ImportedModuleFileInfo &Info = MapEntry.second;
+    if (getModuleFileInfo(File).Signature) {
+      if (getModuleFileInfo(File).Signature != Info.StoredSignature)
+        // Verify Signature.
+        return true;
+    } else if (Info.StoredSize != File->getSize() ||
+               Info.StoredModTime != File->getModificationTime())
+      // Verify Size and ModTime.
+      return true;
+  }
+
   using namespace llvm;
   
   // Emit the file header.
@@ -756,6 +810,7 @@ void GlobalModuleIndexBuilder::writeIndex(llvm::BitstreamWriter &Stream) {
   }
 
   Stream.ExitBlock();
+  return false;
 }
 
 GlobalModuleIndex::ErrorCode
@@ -816,7 +871,8 @@ GlobalModuleIndex::writeIndex(FileManager &FileMgr,
   SmallVector<char, 16> OutputBuffer;
   {
     llvm::BitstreamWriter OutputStream(OutputBuffer);
-    Builder.writeIndex(OutputStream);
+    if (Builder.writeIndex(OutputStream))
+      return EC_IOError;
   }
 
   // Write the global index file to a temporary file.
diff --git a/test/Modules/diagnostic-options-out-of-date.m b/test/Modules/diagnostic-options-out-of-date.m
index ed9e8e1fe1..fd98a8f99d 100644
--- a/test/Modules/diagnostic-options-out-of-date.m
+++ b/test/Modules/diagnostic-options-out-of-date.m
@@ -7,6 +7,16 @@
 // RUN: %clang_cc1 -Werror -Wconversion -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs %s -fmodules-disable-diagnostic-validation
 // Make sure we don't error out when using the pch
 // RUN: %clang_cc1 -Werror -Wno-conversion -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -fsyntax-only -I %S/Inputs -include-pch %t.pch %s -verify -fmodules-disable-diagnostic-validation
+
+// Build A.pcm
+// RUN: %clang_cc1 -Werror -Wno-conversion -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs %s
+// Build pch that imports A.pcm
+// RUN: %clang_cc1 -Werror -Wno-conversion -emit-pch -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -o %t.pch -I %S/Inputs -x objective-c-header %S/Inputs/pch-import-module-out-of-date.pch
+// We will rebuild A.pcm and overwrite the original A.pcm that the pch imports, but the two versions have the same hash.
+// RUN: %clang_cc1 -Werror -Wconversion -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs %s
+// Make sure we don't error out when using the pch
+// RUN: %clang_cc1 -Werror -Wno-conversion -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -fsyntax-only -I %S/Inputs -include-pch %t.pch %s -verify
+
 // expected-no-diagnostics
 
 @import DiagOutOfDate;
diff --git a/test/Modules/module_file_info.m b/test/Modules/module_file_info.m
index 2c1021afac..f2967be6f8 100644
--- a/test/Modules/module_file_info.m
+++ b/test/Modules/module_file_info.m
@@ -29,11 +29,6 @@
 // CHECK:     CPU:
 // CHECK:     ABI:
 
-// CHECK: Diagnostic options:
-// CHECK:   IgnoreWarnings: Yes
-// CHECK:   Diagnostic flags:
-// CHECK:     -Wunused
-
 // CHECK: Header search options:
 // CHECK:   System root [-isysroot=]: '/'
 // CHECK:   Resource dir [ -resource-dir=]: '{{.*}}clang{{.*}}'
@@ -48,3 +43,8 @@
 // CHECK:   Predefined macros:
 // CHECK:     -DBLARG
 // CHECK:     -DWIBBLE=WOBBLE
+
+// CHECK: Diagnostic options:
+// CHECK:   IgnoreWarnings: Yes
+// CHECK:   Diagnostic flags:
+// CHECK:     -Wunused
diff --git a/test/Modules/rebuild.m b/test/Modules/rebuild.m
index 40f2d47e09..150c2ce266 100644
--- a/test/Modules/rebuild.m
+++ b/test/Modules/rebuild.m
@@ -19,11 +19,10 @@
 // RUN: diff %t/Module.size %t/Module.size.saved
 // RUN: cp %t/Module.pcm %t/Module.pcm.saved.2
 
-// But the signature at least is expected to change, so we rebuild DependsOnModule.
-// NOTE: if we change how the signature is created, this test may need updating.
+// The signature is the hash of the PCM content, we will not rebuild rebuild DependsOnModule.
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fdisable-module-hash -fsyntax-only -F %S/Inputs %s
 // RUN: diff %t/Module.pcm %t/Module.pcm.saved.2
-// RUN: not diff %t/DependsOnModule.pcm %t/DependsOnModule.pcm.saved
+// RUN: diff %t/DependsOnModule.pcm %t/DependsOnModule.pcm.saved
 
 // Rebuild Module, reset its timestamp, and verify its size hasn't changed
 // RUN: rm %t/Module.pcm
@@ -34,10 +33,9 @@
 // RUN: cp %t/Module.pcm %t/Module.pcm.saved.2
 
 // Verify again with Module pre-imported.
-// NOTE: if we change how the signature is created, this test may need updating.
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -fdisable-module-hash -fsyntax-only -F %S/Inputs %s
 // RUN: diff %t/Module.pcm %t/Module.pcm.saved.2
-// RUN: not diff %t/DependsOnModule.pcm %t/DependsOnModule.pcm.saved
+// RUN: diff %t/DependsOnModule.pcm %t/DependsOnModule.pcm.saved
 
 #ifdef PREIMPORT
 @import Module;