Index: include/llvm/CodeGen/TailDuplicator.h
===================================================================
--- include/llvm/CodeGen/TailDuplicator.h
+++ include/llvm/CodeGen/TailDuplicator.h
@@ -34,6 +34,7 @@
   const MachineModuleInfo *MMI;
   MachineRegisterInfo *MRI;
   bool PreRegAlloc;
+  unsigned TailDupSize;
 
   // A list of virtual registers for which to update SSA form.
   SmallVector<unsigned, 16> SSAUpdateVRs;
@@ -45,8 +46,11 @@
   DenseMap<unsigned, AvailableValsTy> SSAUpdateVals;
 
 public:
+  /// Prepare to run on a specific machine function.
+  /// @param TailDupSize - Maxmimum size of blocks to tail-duplicate.
   void initMF(MachineFunction &MF, const MachineModuleInfo *MMI,
-              const MachineBranchProbabilityInfo *MBPI);
+              const MachineBranchProbabilityInfo *MBPI,
+              unsigned TailDupSize = 0);
   bool tailDuplicateBlocks(MachineFunction &MF);
   static bool isSimpleBB(MachineBasicBlock *TailBB);
   bool shouldTailDuplicate(const MachineFunction &MF, bool IsSimple,
Index: lib/CodeGen/TailDuplicator.cpp
===================================================================
--- lib/CodeGen/TailDuplicator.cpp
+++ lib/CodeGen/TailDuplicator.cpp
@@ -57,12 +57,14 @@
 namespace llvm {
 
 void TailDuplicator::initMF(MachineFunction &MF, const MachineModuleInfo *MMIin,
-                            const MachineBranchProbabilityInfo *MBPIin) {
+                            const MachineBranchProbabilityInfo *MBPIin,
+                            unsigned TailDupSizeIn) {
   TII = MF.getSubtarget().getInstrInfo();
   TRI = MF.getSubtarget().getRegisterInfo();
   MRI = &MF.getRegInfo();
   MMI = MMIin;
   MBPI = MBPIin;
+  TailDupSize = TailDupSizeIn;
 
   assert(MBPI != nullptr && "Machine Branch Probability Info required");
 
@@ -518,12 +520,14 @@
   // duplicate only one, because one branch instruction can be eliminated to
   // compensate for the duplication.
   unsigned MaxDuplicateCount;
-  if (TailDuplicateSize.getNumOccurrences() == 0 &&
-      // FIXME: Use Function::optForSize().
-      MF.getFunction()->hasFnAttribute(Attribute::OptimizeForSize))
+  if (TailDupSize == 0 &&
+      TailDuplicateSize.getNumOccurrences() == 0 &&
+      MF.getFunction()->optForSize())
     MaxDuplicateCount = 1;
-  else
+  else if (TailDupSize == 0)
     MaxDuplicateCount = TailDuplicateSize;
+  else
+    MaxDuplicateCount = TailDupSize;
 
   // If the block to be duplicated ends in an unanalyzable fallthrough, don't
   // duplicate it.