Index: clang/include/clang/Serialization/ASTBitCodes.h
===================================================================
--- clang/include/clang/Serialization/ASTBitCodes.h
+++ clang/include/clang/Serialization/ASTBitCodes.h
@@ -782,7 +782,16 @@
 
     /// Record types used within a comments block.
     enum CommentRecordTypes {
-      COMMENTS_RAW_COMMENT = 0
+      COMMENTS_RAW_COMMENT = 0,
+
+      /// Maps from a decl to the comment that documents it.
+      COMMENTS_DECL_TO_COMMENT = 1,
+
+      /// Maps from a canonical decl to a redecl in the chain that has comment attached.
+      COMMENTS_CANON_DECL_TO_COMMENTED_DECL = 2,
+
+      /// For redecl chains with no comment attached maps from a canonical decl to the last redecl in the chain that has no comment attached.
+      COMMENTS_CANON_DECL_TO_LAST_COMMENLESS_REDECL = 3
     };
 
     /// \defgroup ASTAST AST file AST constants
Index: clang/lib/Serialization/ASTReader.cpp
===================================================================
--- clang/lib/Serialization/ASTReader.cpp
+++ clang/lib/Serialization/ASTReader.cpp
@@ -9728,6 +9728,28 @@
             SR, Kind, IsTrailingComment, IsAlmostTrailingComment));
         break;
       }
+      case COMMENTS_DECL_TO_COMMENT: {
+        unsigned Idx = 0;
+        const Decl *D = GetDecl(Record[Idx++]);
+        RawComment *RC = Comments[Record[Idx++]];
+        RC->setAttached();
+        Context.DeclRawComments[D] = RC;
+        break;
+      }
+      case COMMENTS_CANON_DECL_TO_COMMENTED_DECL : {
+        unsigned Idx = 0;
+        const Decl *CanonD = GetDecl(Record[Idx++]);
+        const Decl *CommentedD = GetDecl(Record[Idx++]);
+        Context.RedeclChainComments[CanonD] = CommentedD;
+        break;
+      }
+      case COMMENTS_CANON_DECL_TO_LAST_COMMENLESS_REDECL : {
+        unsigned Idx = 0;
+        const Decl *CanonD = GetDecl(Record[Idx++]);
+        const Decl *LastCommentlessRedecl = GetDecl(Record[Idx++]);
+        Context.RedeclChainComments[CanonD] = LastCommentlessRedecl;
+        break;
+      }
       }
     }
   NextCursor:
Index: clang/lib/Serialization/ASTWriter.cpp
===================================================================
--- clang/lib/Serialization/ASTWriter.cpp
+++ clang/lib/Serialization/ASTWriter.cpp
@@ -3266,9 +3266,13 @@
   auto _ = llvm::make_scope_exit([this] { Stream.ExitBlock(); });
   if (!PP->getPreprocessorOpts().WriteCommentListToPCH)
     return;
+
+  llvm::DenseMap<const RawComment *, uint32_t> RawCommentIndex;
+  uint32_t NextCommentIndex = 0;
+
   RecordData Record;
-  for (const auto &FO : Context->Comments.OrderedComments) {
-    for (const auto &OC : FO.second) {
+  for (const auto& FO : Context->Comments.OrderedComments) {
+    for (const auto& OC : FO.second) {
       const RawComment *I = OC.second;
       Record.clear();
       AddSourceRange(I->getSourceRange(), Record);
@@ -3276,8 +3280,30 @@
       Record.push_back(I->isTrailingComment());
       Record.push_back(I->isAlmostTrailingComment());
       Stream.EmitRecord(COMMENTS_RAW_COMMENT, Record);
+      RawCommentIndex[I] = NextCommentIndex++;
     }
   }
+
+  for (const auto I : Context->DeclRawComments) {
+    Record.clear();
+    AddDeclRef(I.getFirst(), Record);
+    Record.push_back(RawCommentIndex[I.getSecond()]);
+    Stream.EmitRecord(COMMENTS_DECL_TO_COMMENT, Record);
+  }
+
+  for (const auto I : Context->RedeclChainComments) {
+    Record.clear();
+    AddDeclRef(I.getFirst(), Record);
+    AddDeclRef(I.getSecond(), Record);
+    Stream.EmitRecord(COMMENTS_CANON_DECL_TO_COMMENTED_DECL, Record);
+  }
+
+  for (const auto I : Context->CommentlessRedeclChains) {
+    Record.clear();
+    AddDeclRef(I.getFirst(), Record);
+    AddDeclRef(I.getSecond(), Record);
+    Stream.EmitRecord(COMMENTS_CANON_DECL_TO_LAST_COMMENLESS_REDECL, Record);
+  }
 }
 
 //===----------------------------------------------------------------------===//
Index: clang/test/Modules/Inputs/documentation.h
===================================================================
--- /dev/null
+++ clang/test/Modules/Inputs/documentation.h
@@ -0,0 +1,9 @@
+/// This function is really cool
+void foo() {
+
+}
+
+/// But this one is at least 20% cooler
+void bar() {
+
+}
Index: clang/test/Modules/Inputs/module.map
===================================================================
--- clang/test/Modules/Inputs/module.map
+++ clang/test/Modules/Inputs/module.map
@@ -14,6 +14,7 @@
   header "diamond_bottom.h" 
   export *
 }
+module documentation { header "documentation.h" }
 module irgen { header "irgen.h" }
 module cxx_irgen_top { header "cxx-irgen-top.h" }
 module cxx_irgen_left { header "cxx-irgen-left.h" }
Index: clang/test/Modules/documentation.cpp
===================================================================
--- /dev/null
+++ clang/test/Modules/documentation.cpp
@@ -0,0 +1,17 @@
+// Note: the run lines follow their respective tests, since line/column matter
+// in this test.
+
+#include "documentation.h"
+
+void other() {
+    
+}
+
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fmodule-map-file=%S/Inputs/module.map \
+// RUN:     -fmodules-cache-path=%t -code-completion-brief-comments \
+// RUN:     -code-completion-at=%s:7:1 -o - -Wdocumentation -I%S/Inputs %s \
+// RUN:   | FileCheck %s
+
+// CHECK: COMPLETION: bar : [#void#]bar() : But this one is at least 20% cooler
+// CHECK: COMPLETION: foo : [#void#]foo() : This function is really cool