Index: include/llvm/Transforms/Utils/MemorySSA.h
===================================================================
--- include/llvm/Transforms/Utils/MemorySSA.h
+++ include/llvm/Transforms/Utils/MemorySSA.h
@@ -570,6 +570,15 @@
                                           MemoryAccess *Definition,
                                           MemoryAccess *InsertPt);
 
+  // \brief Splice \p What to just before \p Where.
+  //
+  // In order to be efficient, the following conditions must be met:
+  //   - \p Where  dominates \p What,
+  //   - All memory accesses in [\p Where, \p What) are no-alias with \p What.
+  //
+  // TODO: relax the MemoryDef requirement on Where.
+  void spliceMemoryAccessAbove(MemoryDef *Where, MemoryUseOrDef *What);
+
   /// \brief Remove a MemoryAccess from MemorySSA, including updating all
   /// definitions and uses.
   /// This should be called when a memory instruction that has a MemoryAccess
Index: lib/Transforms/Utils/MemorySSA.cpp
===================================================================
--- lib/Transforms/Utils/MemorySSA.cpp
+++ lib/Transforms/Utils/MemorySSA.cpp
@@ -1614,6 +1614,29 @@
   return NewAccess;
 }
 
+void MemorySSA::spliceMemoryAccessAbove(MemoryDef *Where,
+                                        MemoryUseOrDef *What) {
+  assert(What != getLiveOnEntryDef() &&
+         Where != getLiveOnEntryDef() && "Can't splice (above) LOE.");
+  assert(dominates(Where, What) && "Only upwards splices are permitted.");
+
+  if (Where == What)
+    return;
+  if (isa<MemoryDef>(What)) {
+    // TODO: possibly use removeMemoryAccess' more efficient RAUW
+    What->replaceAllUsesWith(What->getDefiningAccess());
+    What->setDefiningAccess(Where->getDefiningAccess());
+    Where->setDefiningAccess(What);
+  }
+  AccessList *Src = getWritableBlockAccesses(What->getBlock());
+  AccessList *Dest = getWritableBlockAccesses(Where->getBlock());
+  Dest->splice(AccessList::iterator(Where), *Src, What);
+
+  BlockNumberingValid.erase(What->getBlock());
+  if (What->getBlock() != Where->getBlock())
+    BlockNumberingValid.erase(Where->getBlock());
+}
+
 /// \brief Helper function to create new memory accesses
 MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I) {
   // The assume intrinsic has a control dependency which we model by claiming
Index: unittests/Transforms/Utils/MemorySSA.cpp
===================================================================
--- unittests/Transforms/Utils/MemorySSA.cpp
+++ unittests/Transforms/Utils/MemorySSA.cpp
@@ -484,3 +484,51 @@
   EXPECT_EQ(Walker->getClobberingMemoryAccess(NewLoadAccess), LoadClobber);
   EXPECT_EQ(NewLoadAccess->getDefiningAccess(), LoadClobber);
 }
+
+// Test out MemorySSA::spliceMemoryAccessAbove.
+TEST_F(MemorySSATest, SpliceAboveMemoryDef) {
+  F = Function::Create(FunctionType::get(B.getVoidTy(), {}, false),
+                       GlobalValue::ExternalLinkage, "F", &M);
+  B.SetInsertPoint(BasicBlock::Create(C, "", F));
+
+  Type *Int8 = Type::getInt8Ty(C);
+  Value *A = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "A");
+  Value *B_ = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "B");
+  Value *C = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "C");
+
+  StoreInst *StoreA0 = B.CreateStore(ConstantInt::get(Int8, 0), A);
+  StoreInst *StoreB = B.CreateStore(ConstantInt::get(Int8, 0), B_);
+  LoadInst *LoadB = B.CreateLoad(B_);
+  StoreInst *StoreA1 = B.CreateStore(ConstantInt::get(Int8, 4), A);
+  // splice this above StoreB
+  StoreInst *StoreC = B.CreateStore(ConstantInt::get(Int8, 4), C);
+  StoreInst *StoreA2 = B.CreateStore(ConstantInt::get(Int8, 4), A);
+  LoadInst *LoadC = B.CreateLoad(C);
+
+  setupAnalyses();
+  MemorySSA &MSSA = *Analyses->MSSA;
+  MemorySSAWalker &Walker = *Analyses->Walker;
+
+  StoreC->moveBefore(StoreB);
+  MSSA.spliceMemoryAccessAbove(cast<MemoryDef>(MSSA.getMemoryAccess(StoreB)),
+                               MSSA.getMemoryAccess(StoreC));
+
+  MSSA.verifyMemorySSA();
+
+  EXPECT_EQ(MSSA.getMemoryAccess(StoreB)->getDefiningAccess(),
+            MSSA.getMemoryAccess(StoreC));
+  EXPECT_EQ(MSSA.getMemoryAccess(StoreC)->getDefiningAccess(),
+            MSSA.getMemoryAccess(StoreA0));
+  EXPECT_EQ(MSSA.getMemoryAccess(StoreA2)->getDefiningAccess(),
+            MSSA.getMemoryAccess(StoreA1));
+  EXPECT_EQ(Walker.getClobberingMemoryAccess(LoadB),
+            MSSA.getMemoryAccess(StoreB));
+  EXPECT_EQ(Walker.getClobberingMemoryAccess(LoadC),
+            MSSA.getMemoryAccess(StoreC));
+
+  // exercise block numbering
+  EXPECT_TRUE(MSSA.locallyDominates(MSSA.getMemoryAccess(StoreC),
+                                    MSSA.getMemoryAccess(StoreB)));
+  EXPECT_TRUE(MSSA.locallyDominates(MSSA.getMemoryAccess(StoreA1),
+                                    MSSA.getMemoryAccess(StoreA2)));
+}