diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -799,11 +799,12 @@
 
       LLVM_DEBUG(dbgs() << "Promoting " << *SI << " to " << *M << "\n");
 
-      assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(SI)));
-      auto *LastDef =
-          cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(SI));
-      auto *NewAccess = MSSAU->createMemoryAccessAfter(M, LastDef, LastDef);
-      MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
+      // The newly inserted memset is immediately overwritten by the original
+      // store, so we do not need to rename uses.
+      auto *StoreDef = cast<MemoryDef>(MSSA->getMemoryAccess(SI));
+      auto *NewAccess = MSSAU->createMemoryAccessBefore(
+          M, StoreDef->getDefiningAccess(), StoreDef);
+      MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/false);
 
       eraseInstruction(SI);
       NumMemSetInfer++;