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
@@ -456,8 +456,15 @@
 
       // Check to see if this stored value is of the same byte-splattable value.
       Value *StoredByte = isBytewiseValue(StoredVal, DL);
+      // If the previously stored value is undef, we can replace it with the
+      // newly stored value (which may be undef)
+      // Inversely, if the newly stored value is undef, we can replace it with
+      // the previously stored one
       if (isa<UndefValue>(ByteVal) && StoredByte)
         ByteVal = StoredByte;
+      else if (StoredByte && isa<UndefValue>(StoredByte))
+        StoredByte = ByteVal;
+
       if (ByteVal != StoredByte)
         break;