Index: include/llvm/CodeGen/Passes.h
===================================================================
--- include/llvm/CodeGen/Passes.h
+++ include/llvm/CodeGen/Passes.h
@@ -423,7 +423,7 @@
 
   /// This pass performs outlining on machine instructions directly before
   /// printing assembly.
-  ModulePass *createMachineOutlinerPass();
+  ModulePass *createMachineOutlinerPass(bool RunOnAllFunctions = true);
 
   /// This pass expands the experimental reduction intrinsics into sequences of
   /// shuffles.
Index: include/llvm/CodeGen/TargetInstrInfo.h
===================================================================
--- include/llvm/CodeGen/TargetInstrInfo.h
+++ include/llvm/CodeGen/TargetInstrInfo.h
@@ -1652,6 +1652,11 @@
                      "TargetInstrInfo::isFunctionSafeToOutlineFrom!");
   }
 
+  /// Return true if the function should be outlined from by default.
+  virtual bool shouldOutlineFromFunctionByDefault(MachineFunction &MF) const {
+    return false;
+  }
+
 private:
   unsigned CallFrameSetupOpcode, CallFrameDestroyOpcode;
   unsigned CatchRetOpcode;
Index: include/llvm/Target/TargetMachine.h
===================================================================
--- include/llvm/Target/TargetMachine.h
+++ include/llvm/Target/TargetMachine.h
@@ -205,6 +205,9 @@
   void setMachineOutliner(bool Enable) {
     Options.EnableMachineOutliner = Enable;
   }
+  void setSupportsDefaultOutlining(bool Enable) {
+    Options.SupportsDefaultOutlining = Enable;
+  }
 
   bool shouldPrintMachineCode() const { return Options.PrintMachineCode; }
 
Index: include/llvm/Target/TargetOptions.h
===================================================================
--- include/llvm/Target/TargetOptions.h
+++ include/llvm/Target/TargetOptions.h
@@ -111,7 +111,7 @@
           NoTrapAfterNoreturn(false),
           EmulatedTLS(false), ExplicitEmulatedTLS(false),
           EnableIPRA(false), EmitStackSizeSection(false),
-          EnableMachineOutliner(false) {}
+          EnableMachineOutliner(false), SupportsDefaultOutlining(false) {}
 
     /// PrintMachineCode - This flag is enabled when the -print-machineinstrs
     /// option is specified on the command line, and should enable debugging
@@ -235,6 +235,9 @@
     /// Enables the MachineOutliner pass.
     unsigned EnableMachineOutliner : 1;
 
+    /// Set if the target supports default outlining behaviour.
+    unsigned SupportsDefaultOutlining : 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/MachineOutliner.cpp
===================================================================
--- lib/CodeGen/MachineOutliner.cpp
+++ lib/CodeGen/MachineOutliner.cpp
@@ -688,6 +688,12 @@
   /// linkonceodr linkage.
   bool OutlineFromLinkOnceODRs = false;
 
+  /// Set to true if the outliner should run on all functions in the module
+  /// considered safe for outlining.
+  /// Set to true by default for compatibility with llc's -run-pass option.
+  /// Set when the pass is constructed in TargetPassConfig.
+  bool RunOnAllFunctions = true;
+
   // Collection of IR functions created by the outliner.
   std::vector<Function *> CreatedIRFunctions;
 
@@ -806,8 +812,10 @@
 char MachineOutliner::ID = 0;
 
 namespace llvm {
-ModulePass *createMachineOutlinerPass() {
-  return new MachineOutliner();
+ModulePass *createMachineOutlinerPass(bool RunOnAllFunctions) {
+  MachineOutliner *OL = new MachineOutliner();
+  OL->RunOnAllFunctions = RunOnAllFunctions;
+  return OL;
 }
 
 } // namespace llvm
@@ -1331,6 +1339,20 @@
   const TargetRegisterInfo *TRI = STI.getRegisterInfo();
   const TargetInstrInfo *TII = STI.getInstrInfo();
 
+  // If the user passed -enable-machine-outliner=always or
+  // -enable-machine-outliner, the pass will run on all functions in the module.
+  // Otherwise, if the target supports default outlining, it will run on all
+  // functions deemed by the target to be worth outlining from by default. Tell
+  // the user how the outliner is running.
+  LLVM_DEBUG(
+    dbgs() << "Machine Outliner: Running on ";
+    if (RunOnAllFunctions)
+      dbgs() << "all functions";
+    else
+      dbgs() << "target-default functions";
+    dbgs() << "\n"
+  );
+
   // If the user specifies that they want to outline from linkonceodrs, set
   // it here.
   OutlineFromLinkOnceODRs = EnableLinkOnceODROutlining;
@@ -1355,6 +1377,9 @@
     if (!MF)
       continue;
 
+    if (!RunOnAllFunctions && !TII->shouldOutlineFromFunctionByDefault(*MF))
+      continue;
+
     // We have a MachineFunction. Ask the target if it's suitable for outlining.
     // If it isn't, then move on to the next Function in the module.
     if (!TII->isFunctionSafeToOutlineFrom(*MF, OutlineFromLinkOnceODRs))
Index: lib/CodeGen/TargetPassConfig.cpp
===================================================================
--- lib/CodeGen/TargetPassConfig.cpp
+++ lib/CodeGen/TargetPassConfig.cpp
@@ -111,11 +111,11 @@
     cl::desc("Verify generated machine code"),
     cl::init(false),
     cl::ZeroOrMore);
-enum RunOutliner { AlwaysOutline, NeverOutline };
+enum RunOutliner { AlwaysOutline, NeverOutline, TargetDefault };
 // 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::Hidden, cl::ValueOptional, cl::init(TargetDefault),
     cl::values(clEnumValN(AlwaysOutline, "always",
                           "Run on all functions guaranteed to be beneficial"),
                clEnumValN(NeverOutline, "never", "Disable all outlining"),
@@ -914,8 +914,13 @@
   addPass(&PatchableFunctionID, false);
 
   if (TM->Options.EnableMachineOutliner && getOptLevel() != CodeGenOpt::None &&
-      EnableMachineOutliner == AlwaysOutline)
-    addPass(createMachineOutlinerPass());
+      EnableMachineOutliner != NeverOutline) {
+    bool RunOnAllFunctions = (EnableMachineOutliner == AlwaysOutline);
+    bool AddOutliner = RunOnAllFunctions ||
+                       TM->Options.SupportsDefaultOutlining;
+    if (AddOutliner)
+      addPass(createMachineOutlinerPass(RunOnAllFunctions));
+  }
 
   // Add passes that directly emit MI after all other MI passes.
   addPreEmitPass2();
Index: test/CodeGen/AArch64/machine-outliner-flags.ll
===================================================================
--- test/CodeGen/AArch64/machine-outliner-flags.ll
+++ test/CodeGen/AArch64/machine-outliner-flags.ll
@@ -1,9 +1,12 @@
+; REQUIRES: asserts
 ; RUN: llc %s -debug-pass=Structure -verify-machineinstrs \
-; RUN: -enable-machine-outliner=always -mtriple arm64---- -o /dev/null 2>&1 \
+; RUN: --debug-only=machine-outliner -enable-machine-outliner=always \
+; RUN: -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: --debug-only=machine-outliner -enable-machine-outliner \
+; RUN: -mtriple arm64---- -o /dev/null 2>&1 \
 ; RUN: | FileCheck %s -check-prefix=ENABLE
 
 ; RUN: llc %s -debug-pass=Structure -verify-machineinstrs \
@@ -31,7 +34,9 @@
 ;  * -enable-machine-outliner=never is passed
 
 ; ALWAYS: Machine Outliner
+; ALWAYS: Machine Outliner: Running on all functions
 ; ENABLE: Machine Outliner
+; ENABLE: Machine Outliner: Running on all functions
 ; NEVER-NOT: Machine Outliner
 ; NOT-ADDED-NOT: Machine Outliner
 ; OPTNONE-NOT: Machine Outliner