Index: lib/Analysis/ModuleSummaryAnalysis.cpp
===================================================================
--- lib/Analysis/ModuleSummaryAnalysis.cpp
+++ lib/Analysis/ModuleSummaryAnalysis.cpp
@@ -246,10 +246,15 @@
   findRefEdges(Index, &F, RefEdges, Visited);
 
   bool HasInlineAsmMaybeReferencingInternal = false;
+  bool InitsVarArgs = false;
   for (const BasicBlock &BB : F)
     for (const Instruction &I : BB) {
       if (isa<DbgInfoIntrinsic>(I))
         continue;
+      if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I)) {
+        if (II->getIntrinsicID() == Intrinsic::vastart)
+          InitsVarArgs = true;
+      }
       ++NumInsts;
       findRefEdges(Index, &I, RefEdges, Visited);
       auto CS = ImmutableCallSite(&I);
@@ -357,9 +362,9 @@
       F.hasFnAttribute(Attribute::ReadNone),
       F.hasFnAttribute(Attribute::ReadOnly),
       F.hasFnAttribute(Attribute::NoRecurse), F.returnDoesNotAlias(),
-      // Inliner doesn't handle variadic functions.
+      // Inliner doesn't handle variadic functions with va_start calls.
       // FIXME: refactor this to use the same code that inliner is using.
-      F.isVarArg() ||
+      InitsVarArgs ||
           // Don't try to import functions with noinline attribute.
           F.getAttributes().hasFnAttribute(Attribute::NoInline)};
   auto FuncSummary = llvm::make_unique<FunctionSummary>(
Index: test/Bitcode/thinlto-function-summary.ll
===================================================================
--- test/Bitcode/thinlto-function-summary.ll
+++ test/Bitcode/thinlto-function-summary.ll
@@ -13,18 +13,23 @@
 ; BC-NEXT: <FUNCTION op0=7 op1=39
 ; "variadic"
 ; BC-NEXT: <FUNCTION op0=46 op1=8
+; "llvm.va_start"
+; BC-NEXT: <FUNCTION op0=54 op1=13
 ; "f"
-; BC-NEXT: <ALIAS op0=54 op1=1
+; BC-NEXT: <ALIAS op0=67 op1=1
 ; BC: <GLOBALVAL_SUMMARY_BLOCK
 ; BC-NEXT: <VERSION
 ; BC-NEXT: <PERMODULE {{.*}} op0=1 op1=0
 ; BC-NEXT: <PERMODULE {{.*}} op0=2 op1=0
 ; BC-NEXT: <PERMODULE {{.*}} op0=3 op1=7
-; BC-NEXT: <PERMODULE {{.*}} op0=4 op1=0 op2=1 op3=16
-; BC-NEXT: <ALIAS {{.*}} op0=5 op1=0 op2=3
+; Summary for @variadic has flags (op3) = 16 since non-inlinable owing to
+; va_start call.
+; flag is set due to va_start call.
+; BC-NEXT: <PERMODULE {{.*}} op0=4 op1=0 op2=4 op3=16
+; BC-NEXT: <ALIAS {{.*}} op0=6 op1=0 op2=3
 ; BC-NEXT: </GLOBALVAL_SUMMARY_BLOCK
 ; BC: <STRTAB_BLOCK
-; BC-NEXT: blob data = 'hfoobaranon.{{................................}}.0variadicf{{.*}}'
+; BC-NEXT: blob data = 'hfoobaranon.{{................................}}.0variadicllvm.va_startf{{.*}}'
 
 
 ; RUN: opt -name-anon-globals -module-summary < %s | llvm-dis | FileCheck %s
@@ -68,5 +73,10 @@
 }
 
 define i32 @variadic(...) {
+    %ap = alloca i8*, align 8
+    %ap.0 = bitcast i8** %ap to i8*
+    call void @llvm.va_start(i8* %ap.0)
     ret i32 42
 }
+
+declare void @llvm.va_start(i8*) nounwind
Index: test/Transforms/FunctionImport/Inputs/funcimport.ll
===================================================================
--- test/Transforms/FunctionImport/Inputs/funcimport.ll
+++ test/Transforms/FunctionImport/Inputs/funcimport.ll
@@ -11,7 +11,7 @@
 define void @globalfunc1() #0 {
 entry:
   call void @funcwithpersonality()
-  call void (...) @variadic()
+  call void (...) @variadic_va_start()
   ret void
 }
 
@@ -147,8 +147,19 @@
   ret void
 }
 
-; Variadic function should not be imported because inliner doesn't handle it.
-define void @variadic(...) {
+; Variadic function without va_start can be imported because inliner
+; can handle it.
+define void @variadic_no_va_start(...) {
     ret void
 }
 
+; Variadic function with va_start should not be imported because inliner
+; doesn't handle it.
+define void @variadic_va_start(...) {
+    %ap = alloca i8*, align 8
+    %ap.0 = bitcast i8** %ap to i8*
+    call void @llvm.va_start(i8* %ap.0)
+    ret void
+}
+
+declare void @llvm.va_start(i8*) nounwind
Index: test/Transforms/FunctionImport/funcimport.ll
===================================================================
--- test/Transforms/FunctionImport/funcimport.ll
+++ test/Transforms/FunctionImport/funcimport.ll
@@ -29,7 +29,8 @@
   call void (...) @weakfunc()
   call void (...) @linkoncefunc2()
   call void (...) @referencelargelinkonce()
-  call void (...) @variadic()
+  call void (...) @variadic_no_va_start()
+  call void (...) @variadic_va_start()
   ret i32 0
 }
 
@@ -103,11 +104,19 @@
 ; INSTLIMDEF-DAG: define available_externally hidden void @funcwithpersonality.llvm.{{.*}}() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) !thinlto_src_module !0 {
 ; INSTLIM5-DAG: declare hidden void @funcwithpersonality.llvm.{{.*}}()
 
-; CHECK-DAG: declare void @variadic(...)
-declare void @variadic(...)
+; We can import variadic functions without a va_start, since the inliner
+; can handle them.
+; INSTLIMDEF-DAG: Import variadic_no_va_start
+; CHECK-DAG: define available_externally void @variadic_no_va_start(...) !thinlto_src_module !0 {
+declare void @variadic_no_va_start(...)
+
+; We shouldn't import variadic functions without a va_start, since the inliner
+; cannot handle them.
+; CHECK-DAG: declare void @variadic_va_start(...)
+declare void @variadic_va_start(...)
 
 ; INSTLIMDEF-DAG: Import globalfunc2
-; INSTLIMDEF-DAG: 13 function-import - Number of functions imported
+; INSTLIMDEF-DAG: 14 function-import - Number of functions imported
 ; INSTLIMDEF-DAG: 4 function-import - Number of global variables imported
 
 ; CHECK-DAG: !0 = !{!"{{.*}}/Inputs/funcimport.ll"}
@@ -147,7 +156,7 @@
 ; GUID-DAG: GUID {{.*}} is linkoncefunc
 
 ; DUMP:       Module [[M1:.*]] imports from 1 module
-; DUMP-NEXT:  13 functions imported from [[M2:.*]]
+; DUMP-NEXT:  14 functions imported from [[M2:.*]]
 ; DUMP-NEXT:  4 vars imported from [[M2]]
-; DUMP:       Imported 13 functions for Module [[M1]]
+; DUMP:       Imported 14 functions for Module [[M1]]
 ; DUMP-NEXT:  Imported 4 global variables for Module [[M1]]