Index: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp
@@ -4085,8 +4085,9 @@
 static Value *SimplifyShuffleVectorInst(Value *Op0, Value *Op1, Constant *Mask,
                                         Type *RetTy, const Query &Q,
                                         unsigned MaxRecurse) {
+  Type *InVecTy = Op0->getType();
   unsigned MaskNumElts = Mask->getType()->getVectorNumElements();
-  unsigned InVecNumElts = Op0->getType()->getVectorNumElements();
+  unsigned InVecNumElts = InVecTy->getVectorNumElements();
 
   auto *Op0Const = dyn_cast<Constant>(Op0);
   auto *Op1Const = dyn_cast<Constant>(Op1);
@@ -4108,11 +4109,22 @@
       MaskSelects1 = true;
   }
   if (!MaskSelects0 && Op1Const)
-    return ConstantFoldShuffleVectorInstruction(UndefValue::get(Op0->getType()),
+    return ConstantFoldShuffleVectorInstruction(UndefValue::get(InVecTy),
                                                 Op1Const, Mask);
   if (!MaskSelects1 && Op0Const)
-    return ConstantFoldShuffleVectorInstruction(
-        Op0Const, UndefValue::get(Op0->getType()), Mask);
+    return ConstantFoldShuffleVectorInstruction(Op0Const,
+                                                UndefValue::get(InVecTy), Mask);
+
+  // 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 &&
+        OpShuf->getMask()->getSplatValue())
+      return Op0;
+  if (auto *OpShuf = dyn_cast<ShuffleVectorInst>(Op1))
+    if (!MaskSelects0 && RetTy == InVecTy &&
+        OpShuf->getMask()->getSplatValue())
+      return Op1;
 
   return nullptr;
 }
Index: llvm/trunk/test/Transforms/InstSimplify/shufflevector.ll
===================================================================
--- llvm/trunk/test/Transforms/InstSimplify/shufflevector.ll
+++ llvm/trunk/test/Transforms/InstSimplify/shufflevector.ll
@@ -29,14 +29,87 @@
 define <4 x i32> @splat_operand(<4 x i32> %x) {
 ; CHECK-LABEL: @splat_operand(
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> [[SPLAT]], <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
-; CHECK-NEXT:    ret <4 x i32> [[SHUF]]
+; CHECK-NEXT:    ret <4 x i32> [[SPLAT]]
 ;
   %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
   %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
   ret <4 x i32> %shuf
 }
 
+define <4 x i32> @splat_operand1(<4 x i32> %x, <4 x i32> %y) {
+; CHECK-LABEL: @splat_operand1(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> [[Y:%.*]], <4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <4 x i32> [[SPLAT]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> %y, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
+  ret <4 x i32> %shuf
+}
+
+define <4 x i32> @splat_operand2(<4 x i32> %x, <4 x i32> %y) {
+; CHECK-LABEL: @splat_operand2(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <4 x i32> [[SPLAT]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> %splat, <4 x i32> %y, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
+  ret <4 x i32> %shuf
+}
+
+define <4 x i32> @splat_operand3(<4 x i32> %x) {
+; CHECK-LABEL: @splat_operand3(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <4 x i32> [[SPLAT]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> zeroinitializer, <4 x i32> %splat, <4 x i32> <i32 7, i32 6, i32 5, i32 5>
+  ret <4 x i32> %shuf
+}
+
+define <8 x i32> @splat_operand_negative(<4 x i32> %x) {
+; CHECK-LABEL: @splat_operand_negative(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> [[SPLAT]], <4 x i32> undef, <8 x i32> <i32 0, i32 3, i32 2, i32 1, i32 undef, i32 undef, i32 undef, i32 undef>
+; CHECK-NEXT:    ret <8 x i32> [[SHUF]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <8 x i32> <i32 0, i32 3, i32 2, i32 1, i32 undef, i32 undef, i32 undef, i32 undef>
+  ret <8 x i32> %shuf
+}
+
+define <4 x i32> @splat_operand_negative2(<4 x i32> %x, <4 x i32> %y) {
+; CHECK-LABEL: @splat_operand_negative2(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> [[SPLAT]], <4 x i32> [[Y:%.*]], <4 x i32> <i32 0, i32 3, i32 4, i32 1>
+; CHECK-NEXT:    ret <4 x i32> [[SHUF]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> %splat, <4 x i32> %y, <4 x i32> <i32 0, i32 3, i32 4, i32 1>
+  ret <4 x i32> %shuf
+}
+
+define <4 x i32> @splat_operand_negative3(<4 x i32> %x, <4 x i32> %y) {
+; CHECK-LABEL: @splat_operand_negative3(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> [[Y:%.*]], <4 x i32> [[SPLAT]], <4 x i32> <i32 0, i32 3, i32 4, i32 1>
+; CHECK-NEXT:    ret <4 x i32> [[SHUF]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> %y, <4 x i32> %splat, <4 x i32> <i32 0, i32 3, i32 4, i32 1>
+  ret <4 x i32> %shuf
+}
+
+define <4 x i32> @splat_operand_negative4(<4 x i32> %x) {
+; CHECK-LABEL: @splat_operand_negative4(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> <i32 2, i32 undef, i32 2, i32 undef>
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> [[SPLAT]], <4 x i32> undef, <4 x i32> <i32 0, i32 2, i32 undef, i32 undef>
+; CHECK-NEXT:    ret <4 x i32> [[SHUF]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> <i32 2, i32 undef, i32 2, i32 undef>
+  %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32> <i32 0, i32 2, i32 undef, i32 undef>
+  ret <4 x i32> %shuf
+}
+
 define <4 x i32> @undef_mask(<4 x i32> %x) {
 ; CHECK-LABEL: @undef_mask(
 ; CHECK-NEXT:    ret <4 x i32> undef