Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -4137,22 +4137,12 @@
 static Value *SimplifyShuffleVectorInst(Value *Op0, Value *Op1, Constant *Mask,
                                         Type *RetTy, const Query &Q,
                                         unsigned MaxRecurse) {
-  if (isa<UndefValue>(Mask))
-    return UndefValue::get(RetTy);
-
   Type *InVecTy = Op0->getType();
   unsigned MaskNumElts = Mask->getType()->getVectorNumElements();
   unsigned InVecNumElts = InVecTy->getVectorNumElements();
 
-  auto *Op0Const = dyn_cast<Constant>(Op0);
-  auto *Op1Const = dyn_cast<Constant>(Op1);
-
-  // If all operands are constant, constant fold the shuffle.
-  if (Op0Const && Op1Const)
-    return ConstantFoldShuffleVectorInstruction(Op0Const, Op1Const, Mask);
-
-  // If only one of the operands is constant, constant fold the shuffle if the
-  // mask does not select elements from the variable operand.
+  // Canonicalization: If mask does not select elements from an input vector,
+  // replace that input vector with undef.
   bool MaskSelects0 = false, MaskSelects1 = false;
   for (unsigned i = 0; i != MaskNumElts; ++i) {
     int Idx = ShuffleVectorInst::getMaskValue(Mask, i);
@@ -4163,30 +4153,47 @@
     else
       MaskSelects1 = true;
   }
-  if (!MaskSelects0 && Op1Const)
-    return ConstantFoldShuffleVectorInstruction(UndefValue::get(InVecTy),
-                                                Op1Const, Mask);
-  if (!MaskSelects1 && Op0Const)
-    return ConstantFoldShuffleVectorInstruction(Op0Const,
-                                                UndefValue::get(InVecTy), Mask);
+  if (!MaskSelects0)
+    Op0 = UndefValue::get(InVecTy);
+  if (!MaskSelects1)
+    Op1 = UndefValue::get(InVecTy);
+
+  auto *Op0Const = dyn_cast<Constant>(Op0);
+  auto *Op1Const = dyn_cast<Constant>(Op1);
+
+  // If all operands are constant, constant fold the shuffle.
+  if (Op0Const && Op1Const)
+    return ConstantFoldShuffleVectorInstruction(Op0Const, Op1Const, Mask);
+
+  bool MaskHasAnyUndefs = false;
+  // Canonicalization: if only one input vector is contant, it shall be the
+  // second one.
+  if (Op0Const && !Op1Const) {
+    std::swap(Op0, Op1);
+    SmallVector<uint32_t, 32> NewIndices;
+    for (unsigned i = 0; i != MaskNumElts; ++i) {
+      int Idx = ShuffleVectorInst::getMaskValue(Mask, i);
+      if (Idx != -1)
+        NewIndices.push_back(Idx < (int)MaskNumElts ? Idx + MaskNumElts
+                                                    : Idx - MaskNumElts);
+      else
+        MaskHasAnyUndefs = true;
+    }
+    Mask = ConstantDataVector::get(Mask->getContext(), NewIndices);
+  }
 
   // A shuffle of a splat is always the splat itself. Legal if the shuffle's
   // value type is same as the input vectors' type.
   if (auto *OpShuf = dyn_cast<ShuffleVectorInst>(Op0))
-    if (!MaskSelects1 && RetTy == InVecTy &&
+    if (isa<UndefValue>(Op1) && RetTy == InVecTy &&
         OpShuf->getMask()->getSplatValue())
       return Op0;
-  if (auto *OpShuf = dyn_cast<ShuffleVectorInst>(Op1))
-    if (!MaskSelects0 && RetTy == InVecTy &&
-        OpShuf->getMask()->getSplatValue())
-      return Op1;
 
   // Don't fold a shuffle with undef mask elements. This may get folded in a
   // better way using demanded bits or other analysis.
   // TODO: Should we allow this?
-  for (unsigned i = 0; i != MaskNumElts; ++i)
-    if (ShuffleVectorInst::getMaskValue(Mask, i) == -1)
-      return nullptr;
+  if (MaskHasAnyUndefs)
+    return nullptr;
 
   // Check if every element of this shuffle can be mapped back to the
   // corresponding element of a single root vector. If so, we don't need this