Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -744,18 +744,6 @@
   return true;
 }
 
-static bool hasIdenticalMMOs(const MachineInstr *MI1, const 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;
-  }
-  return true;
-}
-
 static void
 removeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
                                MachineBasicBlock &MBBCommon) {
@@ -792,8 +780,7 @@
     assert(MBBICommon->isIdenticalTo(&*MBBI) && "Expected matching MIIs!");
 
     if (MBBICommon->mayLoad() || MBBICommon->mayStore())
-      if (!hasIdenticalMMOs(&*MBBI, &*MBBICommon))
-        MBBICommon->dropMemRefs();
+      MBBICommon->setMemRefs(MBBI->mergeMemRefsWith(*MBBI));
 
     ++MBBI;
     ++MBBICommon;
Index: lib/CodeGen/MachineInstr.cpp
===================================================================
--- lib/CodeGen/MachineInstr.cpp
+++ lib/CodeGen/MachineInstr.cpp
@@ -868,21 +868,39 @@
 
 std::pair<MachineInstr::mmo_iterator, unsigned>
 MachineInstr::mergeMemRefsWith(const MachineInstr& Other) {
-  // TODO: If we end up with too many memory operands, return the empty
-  // conservative set rather than failing asserts.
+  // If either of the incoming memrefs are empty, we must be conservative and
+  // treat this as if we've exhausted our space for memrefs and dropped them.
+  if (memoperands_empty() || Other.memoperands_empty())
+    return std::make_pair(nullptr, 0);
+
+  // If both instructions have identical memrefs, we don't need to merge them.
+  // Since many instructions have a single memref, and we tend to merge things
+  // like pairs of loads from the same location, this catches a large number of
+  // cases in practice.
+  if (NumMemRefs == Other.NumMemRefs &&
+      std::equal(memoperands_begin(), memoperands_end(),
+                 Other.memoperands_begin()))
+    return std::make_pair(MemRefs, NumMemRefs);
+  
   // TODO: consider uniquing elements within the operand lists to reduce
   // space usage and fall back to conservative information less often.
   size_t CombinedNumMemRefs = (memoperands_end() - memoperands_begin())
     + (Other.memoperands_end() - Other.memoperands_begin());
 
+  // If we don't have enough room to store this many memrefs, be conservative
+  // and drop them.  Otherwise, we'd fail asserts when trying to add them to
+  // the new instruction.
+  if (CombinedNumMemRefs != uint8_t(CombinedNumMemRefs))
+    return std::make_pair(nullptr, 0);
+
   MachineFunction *MF = getParent()->getParent();
   mmo_iterator MemBegin = MF->allocateMemRefsArray(CombinedNumMemRefs);
   mmo_iterator MemEnd = std::copy(memoperands_begin(), memoperands_end(),
                                   MemBegin);
   MemEnd = std::copy(Other.memoperands_begin(), Other.memoperands_end(),
                      MemEnd);
-  assert(MemEnd - MemBegin == CombinedNumMemRefs && "missing memrefs");
-  
+  assert(MemEnd - MemBegin == (ptrdiff_t)CombinedNumMemRefs &&
+         "missing memrefs");
   return std::make_pair(MemBegin, CombinedNumMemRefs);
 }