Index: lib/CodeGen/TargetPassConfig.cpp
===================================================================
--- lib/CodeGen/TargetPassConfig.cpp
+++ lib/CodeGen/TargetPassConfig.cpp
@@ -111,9 +111,16 @@
     cl::desc("Verify generated machine code"),
     cl::init(false),
     cl::ZeroOrMore);
-static cl::opt<bool> EnableMachineOutliner("enable-machine-outliner",
-    cl::Hidden,
-    cl::desc("Enable machine outliner"));
+enum RunOutliner { AlwaysOutline, NeverOutline };
+// Enable or disable the MachineOutliner.
+static cl::opt<RunOutliner> EnableMachineOutliner(
+    "enable-machine-outliner", cl::desc("Enable the machine outliner"),
+    cl::Hidden, cl::ValueOptional, cl::init(NeverOutline),
+    cl::values(clEnumValN(AlwaysOutline, "always",
+                          "Run on all functions guaranteed to be beneficial"),
+               clEnumValN(NeverOutline, "never", "Disable all outlining"),
+               // Sentinel value for unspecified option.
+               clEnumValN(AlwaysOutline, "", "")));
 // Enable or disable FastISel. Both options are needed, because
 // FastISel is enabled by default with -fast, and we wish to be
 // able to enable or disable fast-isel independently from -O0.
@@ -907,7 +914,7 @@
   addPass(&PatchableFunctionID, false);
 
   if (TM->Options.EnableMachineOutliner &&
-      EnableMachineOutliner)
+      EnableMachineOutliner == AlwaysOutline)
     addPass(createMachineOutlinerPass());
 
   // Add passes that directly emit MI after all other MI passes.
Index: test/CodeGen/AArch64/machine-outliner-flags.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/machine-outliner-flags.ll
@@ -0,0 +1,30 @@
+; RUN: llc %s -debug-pass=Structure -verify-machineinstrs \
+; RUN: -enable-machine-outliner=always -mtriple arm64---- -o /dev/null 2>&1 \
+; RUN: | FileCheck %s -check-prefix=ALWAYS
+
+; RUN: llc %s -debug-pass=Structure -verify-machineinstrs \
+; RUN: -enable-machine-outliner -mtriple arm64---- -o /dev/null 2>&1 \
+; RUN: | FileCheck %s -check-prefix=ENABLE
+
+; RUN: llc %s -debug-pass=Structure -verify-machineinstrs \
+; RUN: -enable-machine-outliner=never -mtriple arm64---- -o /dev/null 2>&1 \
+; RUN: | FileCheck %s -check-prefix=NEVER
+
+; RUN: llc %s -debug-pass=Structure -verify-machineinstrs \
+; RUN: -mtriple arm64---- -o /dev/null 2>&1 \
+; RUN: | FileCheck %s -check-prefix=NOT-ADDED
+
+; Make sure that the outliner flags all work properly. If we specify
+; -enable-machine-outliner with always or no argument, it should be added to the
+; pass pipeline. If we specify it with never, or don't pass the flag,
+; then we shouldn't add it.
+
+; ALWAYS: Machine Outliner
+; ENABLE: Machine Outliner
+; NEVER-NOT: Machine Outliner
+; NOT-ADDED-NOT: Machine Outliner
+
+define void @foo() {
+  ret void;
+}
+