diff --git a/llvm/include/llvm/Analysis/VectorUtils.h b/llvm/include/llvm/Analysis/VectorUtils.h
--- a/llvm/include/llvm/Analysis/VectorUtils.h
+++ b/llvm/include/llvm/Analysis/VectorUtils.h
@@ -533,6 +533,12 @@
 llvm::SmallVector<int, 16>
 createSequentialMask(unsigned Start, unsigned NumInts, unsigned NumUndefs);
 
+/// Given a shuffle mask for a binary shuffle, create the equivalent shuffle
+/// mask assuming both operands are identical. This assumes that the unary
+/// shuffle will use elements from operand 0 (operand 1 will be unused).
+llvm::SmallVector<int, 16> createUnaryMask(ArrayRef<int> Mask,
+                                           unsigned NumElts);
+
 /// Concatenate a list of vectors.
 ///
 /// This function generates code that concatenate the vectors in \p Vecs into a
diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -830,6 +830,23 @@
   return Mask;
 }
 
+llvm::SmallVector<int, 16> llvm::createUnaryMask(ArrayRef<int> Mask,
+                                                 unsigned NumElts) {
+  // Avoid casts in the loop and make sure we have a reasonable number.
+  int NumEltsSigned = NumElts;
+  assert(NumEltsSigned > 0 && "Expected smaller or non-zero element count");
+
+  // If the mask chooses an element from operand 1, reduce it to choose from the
+  // corresponding element of operand 0. Undef mask elements are unchanged.
+  SmallVector<int, 16> UnaryMask;
+  for (int MaskElt : Mask) {
+    assert((MaskElt < NumEltsSigned * 2) && "Expected valid shuffle mask");
+    int UnaryElt = MaskElt >= NumEltsSigned ? MaskElt - NumEltsSigned : MaskElt;
+    UnaryMask.push_back(UnaryElt);
+  }
+  return UnaryMask;
+}
+
 /// A helper function for concatenating vectors. This function concatenates two
 /// vectors having the same element type. If the second vector has fewer
 /// elements than the first, it is padded with undefs.
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -21254,15 +21254,9 @@
   ShuffleVectorSDNode *SVN = cast<ShuffleVectorSDNode>(N);
 
   // Canonicalize shuffle v, v -> v, undef
-  if (N0 == N1) {
-    SmallVector<int, 8> NewMask;
-    for (unsigned i = 0; i != NumElts; ++i) {
-      int Idx = SVN->getMaskElt(i);
-      if (Idx >= (int)NumElts) Idx -= NumElts;
-      NewMask.push_back(Idx);
-    }
-    return DAG.getVectorShuffle(VT, SDLoc(N), N0, DAG.getUNDEF(VT), NewMask);
-  }
+  if (N0 == N1)
+    return DAG.getVectorShuffle(VT, SDLoc(N), N0, DAG.getUNDEF(VT),
+                                createUnaryMask(SVN->getMask(), NumElts));
 
   // Canonicalize shuffle undef, v -> v, undef.  Commute the shuffle mask.
   if (N0.isUndef())
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -2484,16 +2484,7 @@
   if (LHS == RHS) {
     assert(!match(RHS, m_Undef()) &&
            "Shuffle with 2 undef ops not simplified?");
-    // Remap any references to RHS to use LHS.
-    SmallVector<int, 16> Elts;
-    for (unsigned i = 0; i != VWidth; ++i) {
-      // Propagate undef elements or force mask to LHS.
-      if (Mask[i] < 0)
-        Elts.push_back(UndefMaskElem);
-      else
-        Elts.push_back(Mask[i] % LHSWidth);
-    }
-    return new ShuffleVectorInst(LHS, Elts);
+    return new ShuffleVectorInst(LHS, createUnaryMask(Mask, LHSWidth));
   }
 
   // shuffle undef, x, mask --> shuffle x, undef, mask'