Index: include/llvm/Bitcode/LLVMBitCodes.h
===================================================================
--- include/llvm/Bitcode/LLVMBitCodes.h
+++ include/llvm/Bitcode/LLVMBitCodes.h
@@ -256,6 +256,10 @@
   // strings in strtab.
   // [n * name]
   FS_CFI_FUNCTION_DECLS = 18,
+  // PERMODULE_RELBF: [valueid, flags, instcount, numrefs,
+  //                   numrefs x valueid,
+  //                   n x (valueid, relblockfreq)]
+  FS_PERMODULE_RELBF = 19,
 };
 
 enum MetadataCodes {
Index: include/llvm/IR/ModuleSummaryIndex.h
===================================================================
--- include/llvm/IR/ModuleSummaryIndex.h
+++ include/llvm/IR/ModuleSummaryIndex.h
@@ -55,13 +55,19 @@
     Critical = 4
   };
   HotnessType Hotness = HotnessType::Unknown;
+  uint64_t RelBlockFreq = 0;
 
   CalleeInfo() = default;
-  explicit CalleeInfo(HotnessType Hotness) : Hotness(Hotness) {}
+  explicit CalleeInfo(HotnessType Hotness, uint64_t RelBF)
+      : Hotness(Hotness), RelBlockFreq(RelBF) {}
 
   void updateHotness(const HotnessType OtherHotness) {
     Hotness = std::max(Hotness, OtherHotness);
   }
+
+  // When there are multiple edges between the same (caller, callee) pair, the
+  // relative block frequencies are summed up.
+  void updateRelBlockFreq(uint64_t RBF) { RelBlockFreq += RBF; }
 };
 
 class GlobalValueSummary;
Index: lib/Analysis/ModuleSummaryAnalysis.cpp
===================================================================
--- lib/Analysis/ModuleSummaryAnalysis.cpp
+++ lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -49,6 +49,7 @@
 #include "llvm/Object/SymbolicFile.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
@@ -58,6 +59,9 @@
 
 #define DEBUG_TYPE "module-summary-analysis"
 
+cl::opt<bool> WriteRelBFToSummary(
+    "write-relbf-to-summary", cl::Hidden, cl::init(false),
+    cl::desc("Write relative block frequency to function summary "));
 // Walk through the operands of a given User via worklist iteration and populate
 // the set of GlobalValue references encountered. Invoked either on an
 // Instruction or a GlobalVariable (which walks its initializer).
@@ -276,6 +280,18 @@
         CallGraphEdges[Index.getOrInsertValueInfo(
                            cast<GlobalValue>(CalledValue))]
             .updateHotness(Hotness);
+        // Add the relative block frequency to CalleeInfo if there is no profile
+        // information and the -write-relbf-to-summary flag is set.
+        if (WriteRelBFToSummary &&
+            Hotness == CalleeInfo::HotnessType::Unknown) {
+          auto BBFreq = BFI->getBlockFreq(CI->getParent()).getFrequency();
+          // FIXME: This might need some scaling to prevent BBFreq values from
+          // being rounded down to 0.
+          BBFreq /= BFI->getEntryFreq();
+          CallGraphEdges[Index.getOrInsertValueInfo(
+                             cast<GlobalValue>(CalledValue))]
+              .updateRelBlockFreq(BBFreq);
+        }
       } else {
         // Skip inline assembly calls.
         if (CI && CI->isInlineAsm())
Index: lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- lib/Bitcode/Reader/BitcodeReader.cpp
+++ lib/Bitcode/Reader/BitcodeReader.cpp
@@ -743,7 +743,8 @@
   std::vector<ValueInfo> makeRefList(ArrayRef<uint64_t> Record);
   std::vector<FunctionSummary::EdgeTy> makeCallList(ArrayRef<uint64_t> Record,
                                                     bool IsOldProfileFormat,
-                                                    bool HasProfile);
+                                                    bool HasProfile,
+                                                    bool HasRelBF);
   Error parseEntireSummary(unsigned ID);
   Error parseModuleStringTable();
 
@@ -5043,12 +5044,15 @@
   return Ret;
 }
 
-std::vector<FunctionSummary::EdgeTy> ModuleSummaryIndexBitcodeReader::makeCallList(
-    ArrayRef<uint64_t> Record, bool IsOldProfileFormat, bool HasProfile) {
+std::vector<FunctionSummary::EdgeTy>
+ModuleSummaryIndexBitcodeReader::makeCallList(ArrayRef<uint64_t> Record,
+                                              bool IsOldProfileFormat,
+                                              bool HasProfile, bool HasRelBF) {
   std::vector<FunctionSummary::EdgeTy> Ret;
   Ret.reserve(Record.size());
   for (unsigned I = 0, E = Record.size(); I != E; ++I) {
     CalleeInfo::HotnessType Hotness = CalleeInfo::HotnessType::Unknown;
+    uint64_t RelBF = 0;
     ValueInfo Callee = getValueInfoFromValueId(Record[I]).first;
     if (IsOldProfileFormat) {
       I += 1; // Skip old callsitecount field
@@ -5056,7 +5060,9 @@
         I += 1; // Skip old profilecount field
     } else if (HasProfile)
       Hotness = static_cast<CalleeInfo::HotnessType>(Record[++I]);
-    Ret.push_back(FunctionSummary::EdgeTy{Callee, CalleeInfo{Hotness}});
+    else if (HasRelBF)
+      RelBF = Record[++I];
+    Ret.push_back(FunctionSummary::EdgeTy{Callee, CalleeInfo(Hotness, RelBF)});
   }
   return Ret;
 }
@@ -5135,7 +5141,11 @@
     // FS_PERMODULE_PROFILE: [valueid, flags, instcount, fflags, numrefs,
     //                        numrefs x valueid,
     //                        n x (valueid, hotness)]
+    // FS_PERMODULE_RELBF: [valueid, flags, instcount, fflags, numrefs,
+    //                      numrefs x valueid,
+    //                      n x (valueid, relblockfreq)]
     case bitc::FS_PERMODULE:
+    case bitc::FS_PERMODULE_RELBF:
     case bitc::FS_PERMODULE_PROFILE: {
       unsigned ValueID = Record[0];
       uint64_t RawFlags = Record[1];
@@ -5161,9 +5171,10 @@
       std::vector<ValueInfo> Refs = makeRefList(
           ArrayRef<uint64_t>(Record).slice(RefListStartIndex, NumRefs));
       bool HasProfile = (BitCode == bitc::FS_PERMODULE_PROFILE);
+      bool HasRelBF = (BitCode == bitc::FS_PERMODULE_RELBF);
       std::vector<FunctionSummary::EdgeTy> Calls = makeCallList(
           ArrayRef<uint64_t>(Record).slice(CallGraphEdgeStartIndex),
-          IsOldProfileFormat, HasProfile);
+          IsOldProfileFormat, HasProfile, HasRelBF);
       auto FS = llvm::make_unique<FunctionSummary>(
           Flags, InstCount, getDecodedFFlags(RawFunFlags), std::move(Refs),
           std::move(Calls), std::move(PendingTypeTests),
@@ -5255,7 +5266,7 @@
       bool HasProfile = (BitCode == bitc::FS_COMBINED_PROFILE);
       std::vector<FunctionSummary::EdgeTy> Edges = makeCallList(
           ArrayRef<uint64_t>(Record).slice(CallGraphEdgeStartIndex),
-          IsOldProfileFormat, HasProfile);
+          IsOldProfileFormat, HasProfile, false);
       ValueInfo VI = getValueInfoFromValueId(ValueID).first;
       auto FS = llvm::make_unique<FunctionSummary>(
           Flags, InstCount, getDecodedFFlags(RawFunFlags), std::move(Refs),
Index: lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitcodeWriter.cpp
+++ lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -86,6 +86,7 @@
                    cl::desc("Number of metadatas above which we emit an index "
                             "to enable lazy-loading"));
 
+extern cl::opt<bool> WriteRelBFToSummary;
 namespace {
 
 /// These are manifest constants used by the bitcode writer. They do not need to
@@ -3377,11 +3378,15 @@
     NameVals.push_back(getValueId(ECI.first));
     if (HasProfileData)
       NameVals.push_back(static_cast<uint8_t>(ECI.second.Hotness));
+    else if (WriteRelBFToSummary)
+      NameVals.push_back(ECI.second.RelBlockFreq);
   }
 
   unsigned FSAbbrev = (HasProfileData ? FSCallsProfileAbbrev : FSCallsAbbrev);
   unsigned Code =
-      (HasProfileData ? bitc::FS_PERMODULE_PROFILE : bitc::FS_PERMODULE);
+      (HasProfileData ? bitc::FS_PERMODULE_PROFILE
+                      : (WriteRelBFToSummary ? bitc::FS_PERMODULE_RELBF
+                                             : bitc::FS_PERMODULE));
 
   // Emit the finished record.
   Stream.EmitRecord(Code, NameVals, FSAbbrev);
@@ -3447,31 +3452,34 @@
                       ArrayRef<uint64_t>{GVI.second, GVI.first});
   }
 
-  // Abbrev for FS_PERMODULE.
+  // Abbrev for FS_PERMODULE_PROFILE.
   auto Abbv = std::make_shared<BitCodeAbbrev>();
-  Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE));
+  Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_PROFILE));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs
-  // numrefs x valueid, n x (valueid)
+  // numrefs x valueid, n x (valueid, hotness)
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
-  unsigned FSCallsAbbrev = Stream.EmitAbbrev(std::move(Abbv));
+  unsigned FSCallsProfileAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
-  // Abbrev for FS_PERMODULE_PROFILE.
+  // Abbrev for FS_PERMODULE or FS_PERMODULE_RELBF.
   Abbv = std::make_shared<BitCodeAbbrev>();
-  Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_PROFILE));
+  if (WriteRelBFToSummary)
+    Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_RELBF));
+  else
+    Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs
-  // numrefs x valueid, n x (valueid, hotness)
+  // numrefs x valueid, n x (valueid [, rel_block_freq])
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
-  unsigned FSCallsProfileAbbrev = Stream.EmitAbbrev(std::move(Abbv));
+  unsigned FSCallsAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
   // Abbrev for FS_PERMODULE_GLOBALVAR_INIT_REFS.
   Abbv = std::make_shared<BitCodeAbbrev>();
Index: test/Bitcode/thinlto-function-summary-callgraph-relbf.ll
===================================================================
--- /dev/null
+++ test/Bitcode/thinlto-function-summary-callgraph-relbf.ll
@@ -0,0 +1,35 @@
+; Test to check the callgraph in summary
+; RUN: opt -write-relbf-to-summary -module-summary %s -o %t.o
+; RUN: llvm-bcanalyzer -dump %t.o | FileCheck %s
+
+
+; CHECK: <SOURCE_FILENAME
+; CHECK-NEXT: <GLOBALVAR
+; CHECK-NEXT: <FUNCTION
+; "func"
+; CHECK-NEXT: <FUNCTION op0=17 op1=4
+; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
+; CHECK-NEXT:    <VERSION
+; See if the call to func is registered.
+; CHECK-NEXT:    <PERMODULE_RELBF {{.*}} op4=1 {{.*}} op7=1
+; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
+; CHECK: <STRTAB_BLOCK
+; CHECK-NEXT: blob data = 'undefinedglobmainfunc{{.*}}'
+
+
+; ModuleID = 'thinlto-function-summary-callgraph.ll'
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind uwtable
+define i32 @main() #0 {
+entry:
+    call void (...) @func()
+    %u = load i32, i32* @undefinedglob
+    ret i32 %u
+}
+
+declare void @func(...) #1
+@undefinedglob = external global i32
+
+; OLD: Index {{.*}} contains 1 nodes (1 functions, 0 alias, 0 globals) and 1 edges (0 refs and 1 calls)
Index: tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
===================================================================
--- tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
+++ tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
@@ -306,6 +306,7 @@
       return nullptr;
       STRINGIFY_CODE(FS, PERMODULE)
       STRINGIFY_CODE(FS, PERMODULE_PROFILE)
+      STRINGIFY_CODE(FS, PERMODULE_RELBF)
       STRINGIFY_CODE(FS, PERMODULE_GLOBALVAR_INIT_REFS)
       STRINGIFY_CODE(FS, COMBINED)
       STRINGIFY_CODE(FS, COMBINED_PROFILE)