Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -43,6 +43,10 @@
                          cl::desc("Enable unsafe double to float "
                                   "shrinking for math lib calls"));
 
+static cl::opt<bool>
+    EnableUnlockedIO("enable-io-unlocked", cl::Hidden,
+                         cl::init(false),
+                         cl::desc("Enable transformation to unlocked IO lib calls"));
 
 //===----------------------------------------------------------------------===//
 // Helper Functions
@@ -130,6 +134,9 @@
 
 static bool isLocallyOpenedFile(Value *File, CallInst *CI, IRBuilder<> &B,
                                 const TargetLibraryInfo *TLI) {
+  if (!EnableUnlockedIO)
+    return false;
+
   CallInst *FOpen = dyn_cast<CallInst>(File);
   if (!FOpen)
     return false;
Index: test/Transforms/InstCombine/no-unlocked-stdio.ll
===================================================================
--- test/Transforms/InstCombine/no-unlocked-stdio.ll
+++ test/Transforms/InstCombine/no-unlocked-stdio.ll
@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+%struct._iobuf = type { i8*, i32, i8*, i32, i32, i32, i32, i8* }
+
+@.str = private unnamed_addr constant [5 x i8] c"file\00", align 1
+@.str.1 = private unnamed_addr constant [2 x i8] c"w\00", align 1
+
+define void @external_fputc_test() {
+; CHECK-LABEL: @external_fputc_test(
+; CHECK-NEXT:    [[CALL:%.*]] = call %struct._iobuf* @fopen(i8* getelementptr inbounds ([5 x i8], [5 x i8]* @.str, i64 0, i64 0), i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.1, i64 0, i64 0))
+; CHECK-NEXT:    [[CALL1:%.*]] = call i32 @fputc(i32 99, %struct._iobuf* [[CALL]])
+; CHECK-NEXT:    ret void
+;
+  %call = call %struct._iobuf* @fopen(i8* getelementptr inbounds ([5 x i8], [5 x i8]* @.str, i64 0, i64 0), i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.1, i64 0, i64 0))
+  %call1 = call i32 @fputc(i32 99, %struct._iobuf* %call)
+  ret void
+}
+
+declare %struct._iobuf* @fopen(i8*, i8*)
+declare i32 @fputc(i32, %struct._iobuf* nocapture)
Index: test/Transforms/InstCombine/unlocked-stdio-mingw.ll
===================================================================
--- test/Transforms/InstCombine/unlocked-stdio-mingw.ll
+++ test/Transforms/InstCombine/unlocked-stdio-mingw.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -instcombine -S -mtriple=x86_64-w64-mingw32 | FileCheck %s
+; RUN: opt < %s -instcombine -S -enable-io-unlocked -mtriple=x86_64-w64-mingw32 | FileCheck %s
 
 %struct._iobuf = type { i8*, i32, i8*, i32, i32, i32, i32, i8* }
 
Index: test/Transforms/InstCombine/unlocked-stdio.ll
===================================================================
--- test/Transforms/InstCombine/unlocked-stdio.ll
+++ test/Transforms/InstCombine/unlocked-stdio.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -instcombine -S -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+; RUN: opt < %s -instcombine -enable-io-unlocked -S -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
 
 %struct._IO_FILE = type { i32, i8*, i8*, i8*, i8*, i8*, i8*, i8*, i8*, i8*, i8*, i8*, %struct._IO_marker*, %struct._IO_FILE*, i32, i32, i64, i16, i8, [1 x i8], i8*, i64, i8*, i8*, i8*, i8*, i64, i32, [20 x i8] }
 %struct._IO_marker = type { %struct._IO_marker*, %struct._IO_FILE*, i32 }