diff --git a/lib/Analysis/ModuleSummaryAnalysis.cpp b/lib/Analysis/ModuleSummaryAnalysis.cpp
index 76a0b048493..58bf469493b 100644
--- a/lib/Analysis/ModuleSummaryAnalysis.cpp
+++ b/lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -70,6 +70,11 @@ cl::opt<FunctionSummary::ForceSummaryHotnessType, true> FSEC(
                           "all-non-critical", "All non-critical edges."),
                clEnumValN(FunctionSummary::FSHT_All, "all", "All edges.")));
 
+cl::opt<std::string> ModuleSummaryDotFile(
+    "module-summary-dot-file", cl::init(""), cl::Hidden,
+    cl::value_desc("filename"),
+    cl::desc("File to emit dot graph of new summary into."));
+
 // 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).
@@ -625,6 +630,15 @@ ModuleSummaryIndex llvm::buildModuleSummaryIndex(
     }
   }
 
+  if (!ModuleSummaryDotFile.empty()) {
+    std::error_code EC;
+    raw_fd_ostream OSDot(ModuleSummaryDotFile, EC, sys::fs::OpenFlags::F_None);
+    if (EC)
+      report_fatal_error(Twine("Failed to open dot file ") +
+                         ModuleSummaryDotFile + ": " + EC.message() + "\n");
+    Index.exportToDot(OSDot);
+  }
+
   return Index;
 }
 
diff --git a/test/ThinLTO/X86/dot-dumper.ll b/test/ThinLTO/X86/dot-dumper.ll
index 7578212ec44..16bf475f2b9 100644
--- a/test/ThinLTO/X86/dot-dumper.ll
+++ b/test/ThinLTO/X86/dot-dumper.ll
@@ -1,5 +1,5 @@
-; RUN: opt -module-summary %s -o %t1.bc
-; RUN: opt -module-summary %p/Inputs/dot-dumper.ll -o %t2.bc
+; RUN: opt -module-summary %s -o %t1.bc -module-summary-dot-file=%t1.dot
+; RUN: opt -module-summary %p/Inputs/dot-dumper.ll -o %t2.bc -module-summary-dot-file=%t2.dot
 ; RUN: llvm-lto2 run -save-temps %t1.bc %t2.bc -o %t3 \
 ; RUN:  -r=%t1.bc,main,px \
 ; RUN:  -r=%t1.bc,main_alias,p \
@@ -11,10 +11,33 @@
 ; RUN:  -r=%t2.bc,B,p
 
 ; Never assume specific order of clusters, nodes or edges
+; RUN: cat %t1.dot | FileCheck --check-prefix=STRUCTURE1 %s
+; RUN: cat %t1.dot | FileCheck --check-prefix=CLUSTER0 %s
+; RUN: cat %t2.dot | FileCheck --check-prefix=STRUCTURE2 %s
+; RUN: cat %t2.dot | FileCheck --check-prefix=CLUSTER1 %s
 ; RUN: cat %t3.index.dot | FileCheck --check-prefix=STRUCTURE %s
-; RUN: cat %t3.index.dot | FileCheck --check-prefix=CLUSTER0 %s
-; RUN: cat %t3.index.dot | FileCheck --check-prefix=CLUSTER1 %s
+; RUN: cat %t3.index.dot | FileCheck --check-prefix=CLUSTER0 --check-prefix=COMBINED0 %s
+; RUN: cat %t3.index.dot | FileCheck --check-prefix=CLUSTER1 --check-prefix=COMBINED1 %s
 
+; %t1 index
+; STRUCTURE1:        digraph Summary {
+; STRUCTURE1:          subgraph cluster_0
+; STRUCTURE1:          // Cross-module edges:
+; STRUCTURE1:          0 [label="@0"]; // defined externally
+; STRUCTURE1:          M0_{{[0-9]+}} -> 0 [style=dotted]; // alias
+; STRUCTURE1-DAG:      [[A:[0-9]+]] [label="A"]; // defined externally
+; STRUCTURE1-DAG:      [[FOO:[0-9]+]] [label="foo"]; // defined externally
+; STRUCTURE1-DAG:      M0_{{[0-9]+}} -> [[FOO]] // call
+; STRUCTURE1-DAG:      M0_{{[0-9]+}} -> [[A]] [{{.*}}]; // const-ref
+; STRUCTURE1-NEXT:   }
+
+; %t2 index
+; STRUCTURE2:        digraph Summary {
+; STRUCTURE2:          subgraph cluster_0
+; STRUCTURE2:          // Cross-module edges:
+; STRUCTURE2-NEXT:   }
+
+; Combined index
 ; STRUCTURE:        digraph Summary {
 ; STRUCTURE-DAG:      subgraph cluster_0
 ; STRUCTURE-DAG:      subgraph cluster_1
@@ -23,23 +46,28 @@
 ; STRUCTURE-DAG:      M0_{{[0-9]+}} -> M1_{{[0-9]+}} [{{.*}}]; // const-ref
 ; STRUCTURE-NEXT:   }
 
-; CLUSTER0:         // Module: {{.*}}1.bc
-; CLUSTER0-NEXT:    subgraph cluster_0 {
-; CLUSTER0-DAG:       M0_[[MAIN_ALIAS:[0-9]+]] [{{.*}}main_alias{{.*}}]; // alias, dead
-; CLUSTER0-DAG:       M0_[[MAIN:[0-9]+]] [{{.*}}main|extern{{.*}}]; // function
+; CLUSTER0:         // Module: {{.*}}
+; CLUSTER0-NEXT:    subgraph cluster_[[ID0:[0-1]]] {
+; CLUSTER0-DAG:       M[[ID0]]_[[MAIN_ALIAS:[0-9]+]] [{{.*}}main_alias{{.*}}]; // alias, dead
+; CLUSTER0-DAG:       M[[ID0]]_[[MAIN:[0-9]+]] [{{.*}}main|extern{{.*}}]; // function
 ; CLUSTER0-NEXT:      // Edges:
-; CLUSTER0-NEXT:      M0_[[MAIN_ALIAS]] -> M0_[[MAIN]] [{{.*}}]; // alias
+; COMBINED0-NEXT:     M[[ID0]]_[[MAIN_ALIAS]] -> M[[ID0]]_[[MAIN]] [{{.*}}]; // alias
 ; CLUSTER0-NEXT:    }
 
-; CLUSTER1:         // Module: {{.*}}2.bc
-; CLUSTER1-NEXT:    subgraph cluster_1 {
-; CLUSTER1-DAG:       M1_[[A:[0-9]+]] [{{.*}}A|extern{{.*}}]; // variable, immutable
-; CLUSTER1-DAG:       M1_[[FOO:[0-9]+]] [{{.*}}foo|extern{{.*}} ffl: 00001{{.*}}]; // function
-; CLUSTER1-DAG:       M1_[[B:[0-9]+]] [{{.*}}B|extern{{.*}}]; // variable, immutable
-; CLUSTER1-DAG:       M1_[[BAR:[0-9]+]] [{{.*}}bar|extern{{.*}}]; // function, dead
+; For the combined index make sure we match the second cluster.
+; COMBINED1:	    // Module: {{.*}}1.bc
+; CLUSTER1:         // Module:
+; COMBINED1-SAME:	{{.*}}2.bc
+; CLUSTER1-NEXT:    subgraph cluster_[[ID1:[0-1]]] {
+; CLUSTER1-DAG:       M[[ID1]]_[[A:[0-9]+]] [{{.*}}A|extern{{.*}}]; // variable
+; COMBINED1-SAME:	, immutable
+; CLUSTER1-DAG:       M[[ID1]]_[[FOO:[0-9]+]] [{{.*}}foo|extern{{.*}} ffl: 00001{{.*}}]; // function
+; CLUSTER1-DAG:       M[[ID1]]_[[B:[0-9]+]] [{{.*}}B|extern{{.*}}]; // variable
+; COMBINED1-SAME:	, immutable
+; CLUSTER1-DAG:       M[[ID1]]_[[BAR:[0-9]+]] [{{.*}}bar|extern{{.*}}]; // function, dead
 ; CLUSTER1-NEXT:      // Edges:
-; CLUSTER1-DAG:       M1_[[FOO]] -> M1_[[B]] [{{.*}}]; // const-ref
-; CLUSTER1-DAG:       M1_[[FOO]] -> M1_[[A]] [{{.*}}]; // const-ref
+; CLUSTER1-DAG:       M[[ID1]]_[[FOO]] -> M[[ID1]]_[[B]] [{{.*}}]; // const-ref
+; CLUSTER1-DAG:       M[[ID1]]_[[FOO]] -> M[[ID1]]_[[A]] [{{.*}}]; // const-ref
 ; CLUSTER1-DAG:     }
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"