Index: llvm/trunk/lib/IR/Constants.cpp
===================================================================
--- llvm/trunk/lib/IR/Constants.cpp
+++ llvm/trunk/lib/IR/Constants.cpp
@@ -2261,6 +2261,9 @@
              isExact ? PossiblyExactOperator::IsExact : 0);
 }
 
+// FIXME: Add a parameter to specify the operand number for non-commutative ops.
+// For example, the operand 1 identity constant for any shift is the null value
+// because shift-by-0 always returns operand 0.
 Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty) {
   switch (Opcode) {
   default:
@@ -2277,6 +2280,8 @@
 
   case Instruction::And:
     return Constant::getAllOnesValue(Ty);
+
+  // FIXME: FAdd / FMul?
   }
 }
 
Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1182,6 +1182,50 @@
   return {};
 }
 
+static Instruction *foldSelectShuffleWith1Binop(ShuffleVectorInst &Shuf) {
+  assert(Shuf.isSelect() && "Must have select-equivalent shuffle");
+
+  // Are we shuffling together some value and that same value after it has been
+  // modified by a binop with a constant?
+  Value *Op0 = Shuf.getOperand(0), *Op1 = Shuf.getOperand(1);
+  Constant *C;
+  bool Op0IsBinop;
+  if (match(Op0, m_BinOp(m_Specific(Op1), m_Constant(C))))
+    Op0IsBinop = true;
+  else if (match(Op1, m_BinOp(m_Specific(Op0), m_Constant(C))))
+    Op0IsBinop = false;
+  else
+    return nullptr;
+
+  auto *BO = cast<BinaryOperator>(Op0IsBinop ? Op0 : Op1);
+  Value *X = Op0IsBinop ? Op1 : Op0;
+  // TODO: Allow div/rem by accounting for potential UB due to undef elements.
+  if (BO->isIntDivRem())
+    return nullptr;
+
+  // The identity constant for a binop leaves a variable operand unchanged. For
+  // a vector, this is a splat of something like 0, -1, or 1.
+  // If there's no identity constant for this binop, we're done.
+  BinaryOperator::BinaryOps BOpcode = BO->getOpcode();
+  Constant *IdC = ConstantExpr::getBinOpIdentity(BOpcode, Shuf.getType());
+  if (!IdC)
+    return nullptr;
+
+  // Shuffle identity constants into the lanes that return the original value.
+  // Example: shuf (mul X, {-1,-2,-3,-4}), X, {0,5,6,3} --> mul X, {-1,1,1,-4}
+  // Example: shuf X, (add X, {-1,-2,-3,-4}), {0,1,6,7} --> add X, {0,0,-3,-4}
+  // The existing binop constant vector remains in the same operand position.
+  Constant *Mask = Shuf.getMask();
+  Constant *NewC = Op0IsBinop ? ConstantExpr::getShuffleVector(C, IdC, Mask) :
+                                ConstantExpr::getShuffleVector(IdC, C, Mask);
+
+  // shuf (bop X, C), X, M --> bop X, C'
+  // shuf X, (bop X, C), M --> bop X, C'
+  Instruction *NewBO = BinaryOperator::Create(BOpcode, X, NewC);
+  NewBO->copyIRFlags(BO);
+  return NewBO;
+}
+
 /// Try to fold shuffles that are the equivalent of a vector select.
 static Instruction *foldSelectShuffle(ShuffleVectorInst &Shuf,
                                       InstCombiner::BuilderTy &Builder,
@@ -1189,6 +1233,9 @@
   if (!Shuf.isSelect())
     return nullptr;
 
+  if (Instruction *I = foldSelectShuffleWith1Binop(Shuf))
+    return I;
+
   BinaryOperator *B0, *B1;
   if (!match(Shuf.getOperand(0), m_BinOp(B0)) ||
       !match(Shuf.getOperand(1), m_BinOp(B1)))
Index: llvm/trunk/test/Transforms/InstCombine/shuffle_select.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/shuffle_select.ll
+++ llvm/trunk/test/Transforms/InstCombine/shuffle_select.ll
@@ -6,8 +6,7 @@
 
 define <4 x i32> @add(<4 x i32> %v) {
 ; CHECK-LABEL: @add(
-; CHECK-NEXT:    [[B:%.*]] = add <4 x i32> [[V:%.*]], <i32 11, i32 undef, i32 13, i32 undef>
-; CHECK-NEXT:    [[S:%.*]] = shufflevector <4 x i32> [[B]], <4 x i32> [[V]], <4 x i32> <i32 0, i32 5, i32 2, i32 7>
+; CHECK-NEXT:    [[S:%.*]] = add <4 x i32> [[V:%.*]], <i32 11, i32 0, i32 13, i32 0>
 ; CHECK-NEXT:    ret <4 x i32> [[S]]
 ;
   %b = add <4 x i32> %v, <i32 11, i32 12, i32 13, i32 14>
@@ -34,8 +33,7 @@
 
 define <4 x i32> @mul(<4 x i32> %v) {
 ; CHECK-LABEL: @mul(
-; CHECK-NEXT:    [[B:%.*]] = mul nuw nsw <4 x i32> [[V:%.*]], <i32 undef, i32 12, i32 undef, i32 14>
-; CHECK-NEXT:    [[S:%.*]] = shufflevector <4 x i32> [[V]], <4 x i32> [[B]], <4 x i32> <i32 undef, i32 5, i32 2, i32 7>
+; CHECK-NEXT:    [[S:%.*]] = mul nuw nsw <4 x i32> [[V:%.*]], <i32 undef, i32 12, i32 1, i32 14>
 ; CHECK-NEXT:    ret <4 x i32> [[S]]
 ;
   %b = mul nsw nuw <4 x i32> %v, <i32 11, i32 12, i32 13, i32 14>
@@ -80,8 +78,7 @@
 
 define <3 x i42> @and(<3 x i42> %v) {
 ; CHECK-LABEL: @and(
-; CHECK-NEXT:    [[B:%.*]] = and <3 x i42> [[V:%.*]], <i42 undef, i42 12, i42 undef>
-; CHECK-NEXT:    [[S:%.*]] = shufflevector <3 x i42> [[V]], <3 x i42> [[B]], <3 x i32> <i32 0, i32 4, i32 undef>
+; CHECK-NEXT:    [[S:%.*]] = and <3 x i42> [[V:%.*]], <i42 -1, i42 12, i42 undef>
 ; CHECK-NEXT:    ret <3 x i42> [[S]]
 ;
   %b = and <3 x i42> %v, <i42 11, i42 12, i42 13>
@@ -96,7 +93,7 @@
 define <4 x i32> @or(<4 x i32> %v) {
 ; CHECK-LABEL: @or(
 ; CHECK-NEXT:    [[B:%.*]] = or <4 x i32> [[V:%.*]], <i32 11, i32 12, i32 13, i32 14>
-; CHECK-NEXT:    [[S:%.*]] = shufflevector <4 x i32> [[B]], <4 x i32> [[V]], <4 x i32> <i32 4, i32 5, i32 2, i32 3>
+; CHECK-NEXT:    [[S:%.*]] = or <4 x i32> [[V]], <i32 0, i32 0, i32 13, i32 14>
 ; CHECK-NEXT:    call void @use_v4i32(<4 x i32> [[B]])
 ; CHECK-NEXT:    ret <4 x i32> [[S]]
 ;
@@ -108,8 +105,7 @@
 
 define <4 x i32> @xor(<4 x i32> %v) {
 ; CHECK-LABEL: @xor(
-; CHECK-NEXT:    [[B:%.*]] = xor <4 x i32> [[V:%.*]], <i32 undef, i32 12, i32 undef, i32 undef>
-; CHECK-NEXT:    [[S:%.*]] = shufflevector <4 x i32> [[V]], <4 x i32> [[B]], <4 x i32> <i32 0, i32 5, i32 2, i32 3>
+; CHECK-NEXT:    [[S:%.*]] = xor <4 x i32> [[V:%.*]], <i32 0, i32 12, i32 0, i32 0>
 ; CHECK-NEXT:    ret <4 x i32> [[S]]
 ;
   %b = xor <4 x i32> %v, <i32 11, i32 12, i32 13, i32 14>