Index: include/llvm/IR/ModuleSummaryIndex.h
===================================================================
--- include/llvm/IR/ModuleSummaryIndex.h
+++ include/llvm/IR/ModuleSummaryIndex.h
@@ -36,7 +36,6 @@
 #include <string>
 #include <utility>
 #include <vector>
-#include <llvm/Support/raw_ostream.h>
 
 namespace llvm {
 
@@ -107,11 +106,15 @@
   }
 };
 
-static bool operator==(ValueInfo A, ValueInfo B) {
-  assert(A.Ref && B.Ref);
+inline bool operator==(const ValueInfo &A, const ValueInfo &B) {
+  assert(A.Ref && B.Ref && "Need ValueInfo with non-null Ref to compare GUIDs");
   return A.getGUID() == B.getGUID();
 }
 
+inline bool operator!=(const ValueInfo &A, const ValueInfo &B) {
+  assert(A.Ref && B.Ref && "Need ValueInfo with non-null Ref to compare GUIDs");
+  return A.getGUID() != B.getGUID();
+}
 
 inline bool operator<(const ValueInfo &A, const ValueInfo &B) {
   assert(A.Ref && B.Ref && "Need ValueInfo with non-null Ref to compare GUIDs");
@@ -446,6 +449,8 @@
       TIdInfo = llvm::make_unique<TypeIdInfo>();
     TIdInfo->TypeTests.push_back(Guid);
   }
+
+  friend struct GraphTraits<ValueInfo>;
 };
 
 template <> struct DenseMapInfo<FunctionSummary::VFuncId> {
@@ -631,6 +636,7 @@
 
       for (auto &C : F->calls()) {
         auto S = FunctionHasParent.find(C.first);
+        // Skip nodes that we're sure have parents
         if (S != FunctionHasParent.end() && S->second)
           continue;
         FunctionHasParent[C.first] = true;
@@ -843,6 +849,55 @@
   /// Summary).
   void collectDefinedGVSummariesPerModule(
       StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries) const;
+
+  void dumpSCCs(raw_ostream &O);
+};
+
+/// GraphTraits definition to build SCC for the index
+template <> struct GraphTraits<ValueInfo> {
+  typedef ValueInfo NodeRef;
+
+  static NodeRef valueInfoFromEdge(FunctionSummary::EdgeTy &P) {
+    return P.first;
+  }
+  using ChildIteratorType =
+      mapped_iterator<std::vector<FunctionSummary::EdgeTy>::iterator,
+                      decltype(&valueInfoFromEdge)>;
+
+  static NodeRef getEntryNode(ValueInfo V) { return V; }
+
+  static ChildIteratorType child_begin(NodeRef N) {
+    if (!N.getSummaryList().size()) // handle external function
+      return ChildIteratorType(
+          FunctionSummary::ExternalNode.CallGraphEdgeList.begin(),
+          &valueInfoFromEdge);
+    FunctionSummary *F =
+        cast<FunctionSummary>(N.getSummaryList().front().get());
+    return ChildIteratorType(F->CallGraphEdgeList.begin(), &valueInfoFromEdge);
+  }
+
+  static ChildIteratorType child_end(NodeRef N) {
+    if (!N.getSummaryList().size()) // handle external function
+      return ChildIteratorType(
+          FunctionSummary::ExternalNode.CallGraphEdgeList.end(),
+          &valueInfoFromEdge);
+    FunctionSummary *F =
+        cast<FunctionSummary>(N.getSummaryList().front().get());
+    return ChildIteratorType(F->CallGraphEdgeList.end(), &valueInfoFromEdge);
+  }
+};
+
+template <>
+struct GraphTraits<ModuleSummaryIndex *> : public GraphTraits<ValueInfo> {
+  static NodeRef getEntryNode(ModuleSummaryIndex *I) {
+    std::unique_ptr<GlobalValueSummary> Root =
+        make_unique<FunctionSummary>(I->calculateCallGraphRoot());
+    GlobalValueSummaryInfo G;
+    G.SummaryList.push_back(std::move(Root));
+    static auto P =
+        GlobalValueSummaryMapTy::value_type(GlobalValue::GUID(0), std::move(G));
+    return ValueInfo(&P);
+  }
 };
 
 } // end namespace llvm
Index: lib/IR/ModuleSummaryIndex.cpp
===================================================================
--- lib/IR/ModuleSummaryIndex.cpp
+++ lib/IR/ModuleSummaryIndex.cpp
@@ -13,9 +13,14 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/ModuleSummaryIndex.h"
+#include "llvm/ADT/SCCIterator.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
+FunctionSummary FunctionSummary::ExternalNode =
+    FunctionSummary::makeDummyFunctionSummary({});
+
 // Collect for the given module the list of function it defines
 // (GUID -> Summary).
 void ModuleSummaryIndex::collectDefinedFunctionsForModule(
@@ -69,3 +74,21 @@
       return true;
   return false;
 }
+
+LLVM_DUMP_METHOD
+void ModuleSummaryIndex::dumpSCCs(raw_ostream &O) {
+  for (scc_iterator<ModuleSummaryIndex *> I =
+           scc_begin<ModuleSummaryIndex *>(this);
+       !I.isAtEnd(); ++I) {
+    O << "SCC (" << utostr(I->size()) << " node" << (I->size() == 1 ? "" : "s")
+      << ") {\n";
+    for (const ValueInfo V : *I) {
+      FunctionSummary *F = nullptr;
+      if (V.getSummaryList().size())
+        F = cast<FunctionSummary>(V.getSummaryList().front().get());
+      O << " " << (F == nullptr ? "External" : "") << " " << utostr(V.getGUID())
+        << (I.hasLoop() ? " (has loop)" : "") << "\n";
+    }
+    O << "}\n";
+  }
+}
Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -50,6 +50,10 @@
 
 #define DEBUG_TYPE "lto"
 
+static cl::opt<bool>
+    DumpThinCGSCCs("dump-thin-cg-sccs", cl::init(false), cl::Hidden,
+                   cl::desc("Dump the SCCs in the ThinLTO index's callgraph"));
+
 // The values are (type identifier, summary) pairs.
 typedef DenseMap<
     GlobalValue::GUID,
@@ -1054,6 +1058,9 @@
       ThinLTO.ModuleMap.size());
   StringMap<std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>> ResolvedODR;
 
+  if (DumpThinCGSCCs)
+    ThinLTO.CombinedIndex.dumpSCCs(outs());
+
   if (Conf.OptLevel > 0) {
     ComputeCrossModuleImport(ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
                              ImportLists, ExportLists);
Index: test/ThinLTO/X86/module_summary_graph_traits.ll
===================================================================
--- /dev/null
+++ test/ThinLTO/X86/module_summary_graph_traits.ll
@@ -0,0 +1,58 @@
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: llvm-lto2 run -print-summary-global-ids -dump-thin-cg-sccs %t1.bc -o %t.index.bc \
+; RUN:     -r %t1.bc,external,px -r %t1.bc,l2,pl -r %t1.bc,l1,pl \
+; RUN:     -r %t1.bc,simple,pl -r %t1.bc,root,pl 2>&1 | FileCheck %s
+
+; CHECK: 5224464028922159466{{.*}} is external
+; CHECK: 765152853862302398{{.*}} is l2
+; CHECK: 17000277804057984823{{.*}} is l1
+; CHECK: 15440740835768581517{{.*}} is simple
+; CHECK: 5800840261926955363{{.*}} is root
+
+; CHECK: SCC (2 nodes) {
+; CHECK-NEXT: {{^}} 765152853862302398 (has loop)
+; CHECK-NEXT: {{^}} 17000277804057984823 (has loop)
+; CHECK-NEXT: }
+
+; CHECK: SCC (1 node) {
+; CHECK-NEXT: {{^}} 15440740835768581517{{$}}
+; CHECK-NEXT: }
+
+; CHECK: SCC (1 node) {
+; CHECK-NEXT: External 5224464028922159466{{$}}
+; CHECK-NEXT: }
+
+; CHECK: SCC (1 node) {
+; CHECK-NEXT: {{^}} 5800840261926955363{{$}}
+; CHECK-NEXT: }
+
+; Dummy call graph root that points at all roots of the callgraph.
+; CHECK: SCC (1 node) {
+; CHECK-NEXT: {{^}} 0{{$}}
+; CHECK-NEXT: }
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @external()
+
+define void @l2() {
+  call void @l1()
+  ret void
+}
+
+define void @l1() {
+  call void @l2()
+  ret void
+}
+
+define i32 @simple() {
+  ret i32 23
+}
+
+define void @root() {
+  call void @l1()
+  call i32 @simple()
+  call void @external()
+  ret void
+}