Index: include/llvm/Target/TargetMachine.h
===================================================================
--- include/llvm/Target/TargetMachine.h
+++ include/llvm/Target/TargetMachine.h
@@ -210,6 +210,9 @@
   bool getO0WantsFastISel() { return O0WantsFastISel; }
   void setO0WantsFastISel(bool Enable) { O0WantsFastISel = Enable; }
   void setGlobalISel(bool Enable) { Options.EnableGlobalISel = Enable; }
+  void setMachineOutliner(bool Enable) {
+    Options.EnableMachineOutliner = Enable;
+  }
 
   bool shouldPrintMachineCode() const { return Options.PrintMachineCode; }
 
Index: include/llvm/Target/TargetOptions.h
===================================================================
--- include/llvm/Target/TargetOptions.h
+++ include/llvm/Target/TargetOptions.h
@@ -107,9 +107,9 @@
           EnableFastISel(false), EnableGlobalISel(false), UseInitArray(false),
           DisableIntegratedAS(false), RelaxELFRelocations(false),
           FunctionSections(false), DataSections(false),
-          UniqueSectionNames(true), TrapUnreachable(false),
-          EmulatedTLS(false), ExplicitEmulatedTLS(false),
-          EnableIPRA(false), EmitStackSizeSection(false) {}
+          UniqueSectionNames(true), TrapUnreachable(false), EmulatedTLS(false),
+          ExplicitEmulatedTLS(false), EnableIPRA(false),
+          EmitStackSizeSection(false), EnableMachineOutliner(false) {}
 
     /// PrintMachineCode - This flag is enabled when the -print-machineinstrs
     /// option is specified on the command line, and should enable debugging
@@ -226,6 +226,9 @@
     /// Emit section containing metadata on function stack sizes.
     unsigned EmitStackSizeSection : 1;
 
+    /// Enable outlining.
+    unsigned EnableMachineOutliner: 1;
+
     /// FloatABIType - This setting is set by -float-abi=xxx option is specfied
     /// on the command line. This setting may either be Default, Soft, or Hard.
     /// Default selects the target's default behavior. Soft selects the ABI for
Index: lib/CodeGen/TargetPassConfig.cpp
===================================================================
--- lib/CodeGen/TargetPassConfig.cpp
+++ lib/CodeGen/TargetPassConfig.cpp
@@ -114,6 +114,10 @@
 static cl::opt<bool> EnableMachineOutliner("enable-machine-outliner",
     cl::Hidden,
     cl::desc("Enable machine outliner"));
+static cl::opt<bool> DisableMachineOutliner("disable-machine-outliner",
+    cl::Hidden,
+    cl::init(false),
+    cl::desc("Disable machine outliner"));
 // 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.
@@ -905,7 +909,13 @@
   addPass(&XRayInstrumentationID, false);
   addPass(&PatchableFunctionID, false);
 
-  if (EnableMachineOutliner)
+  // If the user passes in -enable-machine-outliner we should add the outliner.
+  // If the target specifies that they want the outliner on by default, we
+  // should add it.
+  // If the user passes in -disable-machine-outliner, then that should override
+  // the previous two cases.
+  if (!DisableMachineOutliner &&
+      (EnableMachineOutliner || TM->Options.EnableMachineOutliner))
     addPass(createMachineOutlinerPass());
 
   // Add passes that directly emit MI after all other MI passes.
Index: lib/Target/AArch64/AArch64TargetMachine.cpp
===================================================================
--- lib/Target/AArch64/AArch64TargetMachine.cpp
+++ lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -268,6 +268,9 @@
   // Enable GlobalISel at or below EnableGlobalISelAt0.
   if (getOptLevel() <= EnableGlobalISelAtO)
     setGlobalISel(true);
+
+  if (getSizeOptLevel() == CodeGenSizeOpt::MinSize)
+    setMachineOutliner(true);
 }
 
 AArch64TargetMachine::~AArch64TargetMachine() = default;
Index: test/CodeGen/AArch64/machine-outliner-flags.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/machine-outliner-flags.ll
@@ -0,0 +1,15 @@
+; Ensure the MachineOutliner is enabled by default in AArch64 at Oz,
+; but not otherwise.
+; RUN: llc %s -O=z -debug-pass=Structure -mtriple arm64---- -o /dev/null 2>&1 | FileCheck %s -check-prefix=DEFAULT
+; DEFAULT: Machine Outliner
+; RUN: llc %s -O=2 -debug-pass=Structure -mtriple arm64---- -o /dev/null 2>&1 | FileCheck %s -check-prefix=NOTOZ 
+; NOTOZ-NOT: Machine Outliner
+; RUN: llc %s -O=z -disable-machine-outliner -debug-pass=Structure -mtriple arm64---- -o /dev/null 2>&1 | FileCheck %s -check-prefix=DISABLED
+; DISABLED-NOT: Machine Outliner
+; RUN: llc %s -O=z -debug-pass=Structure -mtriple=x86_64---- -o /dev/null 2>&1 | FileCheck %s -check-prefix=NOTON
+; NOTON-NOT: Machine Outliner
+; RUN: llc %s -enable-machine-outliner -disable-machine-outliner -debug-pass=Structure -mtriple=x86_64---- -o /dev/null 2>&1 | FileCheck %s -check-prefix=DISABLED2
+; DISABLED2-NOT: Machine Outliner
+define void @foo() {
+  ret void
+}