diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -394,6 +394,9 @@
       /// Record code for the signature that identifiers this AST file.
       SIGNATURE = 1,
 
+      /// Record code for the signature of the AST block.
+      AST_SIGNATURE,
+
       /// Record code for the diagnostic options table.
       DIAGNOSTIC_OPTIONS,
 
diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -472,7 +472,8 @@
 
   /// Write out the signature and diagnostic options, and return the signature.
   ASTFileSignature writeUnhashedControlBlock(Preprocessor &PP,
-                                             ASTContext &Context);
+                                             ASTContext &Context,
+                                             StringRef ASTBlockBytes);
 
   /// Calculate hash of the pcm content.
   static ASTFileSignature createSignature(StringRef Bytes);
diff --git a/clang/include/clang/Serialization/ModuleFile.h b/clang/include/clang/Serialization/ModuleFile.h
--- a/clang/include/clang/Serialization/ModuleFile.h
+++ b/clang/include/clang/Serialization/ModuleFile.h
@@ -168,6 +168,10 @@
   /// and modification time to identify this particular file.
   ASTFileSignature Signature;
 
+  /// The signature of the AST block of the module file, this can be used to
+  /// unique module files based on AST contents.
+  ASTFileSignature ASTSignature;
+
   /// Whether this module has been directly imported by the
   /// user.
   bool DirectlyImported = false;
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3832,17 +3832,18 @@
 
   while (Data < DataEnd) {
     // FIXME: Looking up dependency modules by filename is horrible. Let's
-    // start fixing this with prebuilt and explicit modules and see how it
-    // goes...
+    // start fixing this with prebuilt, explicit and implicit modules and see
+    // how it goes...
     using namespace llvm::support;
     ModuleKind Kind = static_cast<ModuleKind>(
       endian::readNext<uint8_t, little, unaligned>(Data));
     uint16_t Len = endian::readNext<uint16_t, little, unaligned>(Data);
     StringRef Name = StringRef((const char*)Data, Len);
     Data += Len;
-    ModuleFile *OM = (Kind == MK_PrebuiltModule || Kind == MK_ExplicitModule
-                      ? ModuleMgr.lookupByModuleName(Name)
-                      : ModuleMgr.lookupByFileName(Name));
+    ModuleFile *OM = (Kind == MK_PrebuiltModule || Kind == MK_ExplicitModule ||
+                              Kind == MK_ImplicitModule
+                          ? ModuleMgr.lookupByModuleName(Name)
+                          : ModuleMgr.lookupByFileName(Name));
     if (!OM) {
       std::string Msg =
           "SourceLocation remap refers to unknown module, cannot find ";
@@ -4734,6 +4735,10 @@
       if (F)
         std::copy(Record.begin(), Record.end(), F->Signature.data());
       break;
+    case AST_SIGNATURE:
+      if (F)
+        std::copy(Record.begin(), Record.end(), F->ASTSignature.data());
+      break;
     case DIAGNOSTIC_OPTIONS: {
       bool Complain = (ClientLoadCapabilities & ARR_OutOfDate) == 0;
       if (Listener && ValidateDiagnosticOptions &&
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -64,6 +64,7 @@
 #include "clang/Sema/ObjCMethodList.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/Weak.h"
+#include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Serialization/ASTReader.h"
 #include "clang/Serialization/ASTRecordWriter.h"
 #include "clang/Serialization/InMemoryModuleCache.h"
@@ -961,6 +962,7 @@
 
   BLOCK(UNHASHED_CONTROL_BLOCK);
   RECORD(SIGNATURE);
+  RECORD(AST_SIGNATURE);
   RECORD(DIAGNOSTIC_OPTIONS);
   RECORD(DIAG_PRAGMA_MAPPINGS);
 
@@ -1045,7 +1047,8 @@
 }
 
 ASTFileSignature ASTWriter::writeUnhashedControlBlock(Preprocessor &PP,
-                                                      ASTContext &Context) {
+                                                      ASTContext &Context,
+                                                      StringRef ASTBlockBytes) {
   // Flush first to prepare the PCM hash (signature).
   Stream.FlushToWord();
   auto StartOfUnhashedControl = Stream.GetCurrentBitNo() >> 3;
@@ -1062,6 +1065,10 @@
     Record.append(Signature.begin(), Signature.end());
     Stream.EmitRecord(SIGNATURE, Record);
     Record.clear();
+    ASTFileSignature ASTSignature = createSignature(ASTBlockBytes);
+    Record.append(ASTSignature.begin(), ASTSignature.end());
+    Stream.EmitRecord(AST_SIGNATURE, Record);
+    Record.clear();
   }
 
   // Diagnostic options.
@@ -4548,6 +4555,8 @@
   populateInputFileIDs(Context.SourceMgr);
 
   // Write the remaining AST contents.
+  Stream.FlushToWord();
+  auto StartOfASTBlock = Stream.GetCurrentBitNo() >> 3;
   Stream.EnterSubblock(AST_BLOCK_ID, 5);
 
   // This is so that older clang versions, before the introduction
@@ -4679,9 +4688,9 @@
     //   c++-base-specifiers-id:i32
     //   type-id:i32)
     //
-    // module-kind is the ModuleKind enum value. If it is MK_PrebuiltModule or
-    // MK_ExplicitModule, then the module-name is the module name. Otherwise,
-    // it is the module file name.
+    // module-kind is the ModuleKind enum value. If it is MK_PrebuiltModule,
+    // MK_ExplicitModule or MK_ImplicitModule, then the module-name is the
+    // module name. Otherwise, it is the module file name.
     auto Abbrev = std::make_shared<BitCodeAbbrev>();
     Abbrev->Add(BitCodeAbbrevOp(MODULE_OFFSET_MAP));
     Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));
@@ -4694,10 +4703,11 @@
 
         endian::Writer LE(Out, little);
         LE.write<uint8_t>(static_cast<uint8_t>(M.Kind));
-        StringRef Name =
-          M.Kind == MK_PrebuiltModule || M.Kind == MK_ExplicitModule
-          ? M.ModuleName
-          : M.FileName;
+        StringRef Name = M.Kind == MK_PrebuiltModule ||
+                                 M.Kind == MK_ExplicitModule ||
+                                 M.Kind == MK_ImplicitModule
+                             ? M.ModuleName
+                             : M.FileName;
         LE.write<uint16_t>(Name.size());
         Out.write(Name.data(), Name.size());
 
@@ -4909,6 +4919,11 @@
       NumStatements, NumMacros, NumLexicalDeclContexts, NumVisibleDeclContexts};
   Stream.EmitRecord(STATISTICS, Record);
   Stream.ExitBlock();
+  Stream.FlushToWord();
+  auto EndOfASTBlock = Stream.GetCurrentBitNo() >> 3;
+
+  StringRef ASTBlockBytes(Buffer.begin() + StartOfASTBlock,
+                          EndOfASTBlock - StartOfASTBlock);
 
   // Write the control block
   WriteControlBlock(PP, Context, isysroot, OutputFile);
@@ -4917,7 +4932,7 @@
   for (const auto &ExtWriter : ModuleFileExtensionWriters)
     WriteModuleFileExtension(SemaRef, *ExtWriter);
 
-  return writeUnhashedControlBlock(PP, Context);
+  return writeUnhashedControlBlock(PP, Context, ASTBlockBytes);
 }
 
 void ASTWriter::WriteDeclUpdatesBlocks(RecordDataImpl &OffsetsRecord) {