Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -65,6 +65,11 @@
           cl::desc("Min number of instructions to consider tail merging"),
                               cl::init(3), cl::Hidden);
 
+static cl::opt<unsigned>
+    MergeMMOsThreshold("merge-mmos-threshold",
+                       cl::desc("Threshold for mergeMMOs function"),
+                       cl::init(16), cl::Hidden);
+
 namespace {
   /// BranchFolderPass - Wrap branch folder in a machine function pass.
   class BranchFolderPass : public MachineFunctionPass {
@@ -744,24 +749,40 @@
   return true;
 }
 
-static bool hasIdenticalMMOs(const MachineInstr *MI1, const MachineInstr *MI2) {
+// Add MI1's MMOs to MI2's MMOs while excluding any duplicates. The MI scheduler
+// currently doesn't handle multiple MMOs, so duplicates would likely pessimize
+// the scheduler.
+static void mergeMMOs(MachineInstr *MI1, MachineInstr *MI2) {
   auto I1 = MI1->memoperands_begin(), E1 = MI1->memoperands_end();
   auto I2 = MI2->memoperands_begin(), E2 = MI2->memoperands_end();
-  if ((E1 - I1) != (E2 - I2))
-    return false;
-  for (; I1 != E1; ++I1, ++I2) {
-    if (**I1 != **I2)
-      return false;
+  MachineFunction *MF = MI1->getParent()->getParent();
+
+  // Mostly, MI1's MMO count is 1 or zero. So we don't have to use
+  // SmallSet.
+  for (; I1 != E1; ++I1) {
+    bool IsDupMMO = false;
+    for (I2 = MI2->memoperands_begin(); I2 != E2; ++I2) {
+      if (**I1 == **I2) {
+        IsDupMMO = true;
+        break;
+      }
+    }
+    if (IsDupMMO == false) {
+      MI2->addMemOperand(*MF, *I1);
+      E2 = MI2->memoperands_end();
+    }
   }
-  return true;
 }
 
 static void
-removeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
-                               MachineBasicBlock &MBBCommon) {
-  // Remove MMOs from memory operations in the common block
-  // when they do not match the ones from the block being tail-merged.
-  // This ensures later passes conservatively compute dependencies.
+removeOrMergeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
+                                      MachineBasicBlock &MBBCommon) {
+
+  unsigned MergeThreshold = MergeMMOsThreshold;
+  // If MBB's MMOs count * MBBCommon's MMOs count is smaller than
+  // MergeThreshold,
+  // Merge MMOs from memory operations in the common block. Otherwise, we remove
+  // them.
   MachineBasicBlock *MBB = MBBIStartPos->getParent();
   // Note CommonTailLen does not necessarily matches the size of
   // the common BB nor all its instructions because of debug
@@ -791,9 +812,18 @@
            "Reached BB end within common tail length!");
     assert(MBBICommon->isIdenticalTo(&*MBBI) && "Expected matching MIIs!");
 
-    if (MBBICommon->mayLoad() || MBBICommon->mayStore())
-      if (!hasIdenticalMMOs(&*MBBI, &*MBBICommon))
+    if (MBBICommon->mayLoad() || MBBICommon->mayStore()) {
+      auto I1 = MBBI->memoperands_begin(), E1 = MBBI->memoperands_end();
+      auto I2 = MBBICommon->memoperands_begin(),
+           E2 = MBBICommon->memoperands_end();
+      unsigned I1MMOsCount = E1 - I1;
+      unsigned I2MMOsCount = E2 - I2;
+
+      if (I1MMOsCount * I2MMOsCount < MergeThreshold)
+        mergeMMOs(&*MBBI, &*MBBICommon);
+      else
         MBBICommon->clearMemRefs();
+    }
 
     ++MBBI;
     ++MBBICommon;
@@ -913,8 +943,9 @@
         continue;
       DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
                    << (i == e-1 ? "" : ", "));
-      // Remove MMOs from memory operations as needed.
-      removeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(), *MBB);
+      // Remove or merge MMOs from memory operations as needed.
+      removeOrMergeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(),
+                                            *MBB);
       // Hack the end off BB i, making it jump to BB commonTailIndex instead.
       ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
       // BB i is no longer a predecessor of SuccBB; remove it from the worklist.