Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -124,17 +124,12 @@
         return &*Iter;
     } else {
       // Otherwise, have to walk the all access iterator.
-      auto Iter = MA->getReverseIterator();
-      ++Iter;
-      while (&*Iter != &*Defs->begin()) {
-        if (!isa<MemoryUse>(*Iter))
-          return &*Iter;
-        --Iter;
-      }
-      // At this point it must be pointing at firstdef
-      assert(&*Iter == &*Defs->begin() &&
-             "Should have hit first def walking backwards");
-      return &*Iter;
+      auto End = MSSA->getWritableBlockAccesses(MA->getBlock())->rend();
+      for (auto &U : make_range(++MA->getReverseIterator(), End))
+        if (!isa<MemoryUse>(U))
+          return cast<MemoryAccess>(&U);
+      // Note that if MA comes before Defs->begin(), we won't hit a def.
+      return nullptr;
     }
   }
   return nullptr;
Index: unittests/Analysis/MemorySSA.cpp
===================================================================
--- unittests/Analysis/MemorySSA.cpp
+++ unittests/Analysis/MemorySSA.cpp
@@ -244,6 +244,52 @@
   MSSA.verifyMemorySSA();
 }
 
+TEST_F(MemorySSATest, SinkLoad) {
+  F = Function::Create(
+      FunctionType::get(B.getVoidTy(), {B.getInt8PtrTy()}, false),
+      GlobalValue::ExternalLinkage, "F", &M);
+  BasicBlock *Entry(BasicBlock::Create(C, "", F));
+  BasicBlock *Left(BasicBlock::Create(C, "", F));
+  BasicBlock *Right(BasicBlock::Create(C, "", F));
+  BasicBlock *Merge(BasicBlock::Create(C, "", F));
+  B.SetInsertPoint(Entry);
+  B.CreateCondBr(B.getTrue(), Left, Right);
+  B.SetInsertPoint(Left, Left->begin());
+  Argument *PointerArg = &*F->arg_begin();
+  B.SetInsertPoint(Left);
+  B.CreateBr(Merge);
+  B.SetInsertPoint(Right);
+  B.CreateBr(Merge);
+
+  // Load in left block
+  B.SetInsertPoint(Left, Left->begin());
+  LoadInst *LoadInst1 = B.CreateLoad(PointerArg);
+  // Store in merge block
+  B.SetInsertPoint(Merge, Merge->begin());
+  B.CreateStore(B.getInt8(16), PointerArg);
+
+  setupAnalyses();
+  MemorySSA &MSSA = *Analyses->MSSA;
+  MemorySSAUpdater Updater(&MSSA);
+
+  // Mimic sinking of a load:
+  // - clone load
+  // - insert in "exit" block
+  // - insert in mssa
+  // - remove from original block
+
+  LoadInst *LoadInstClone = cast<LoadInst>(LoadInst1->clone());
+  Merge->getInstList().insert(Merge->begin(), LoadInstClone);
+  MemoryAccess * NewLoadAccess =
+      Updater.createMemoryAccessInBB(LoadInstClone, nullptr,
+                                     LoadInstClone->getParent(),
+                                     MemorySSA::Beginning);
+  Updater.insertUse(cast<MemoryUse>(NewLoadAccess));
+  MSSA.verifyMemorySSA();
+  Updater.removeMemoryAccess(MSSA.getMemoryAccess(LoadInst1));
+  MSSA.verifyMemorySSA();
+}
+
 TEST_F(MemorySSATest, MoveAStore) {
   // We create a diamond where there is a in the entry, a store on one side, and
   // a load at the end.  After building MemorySSA, we test updating by moving