Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -463,6 +463,31 @@
   return BinaryOperator::Create(LogicOp->getOpcode(), NarrowOp0, NarrowC);
 }
 
+static Instruction *shrinkShuffle(TruncInst &Trunc,
+                                  InstCombiner::BuilderTy &Builder) {
+  auto *Shuf = dyn_cast<ShuffleVectorInst>(Trunc.getOperand(0));
+  if (!Shuf || !Shuf->hasOneUse())
+    return nullptr;
+
+  Type *DestTy = Trunc.getType();
+  if (auto *C = dyn_cast<Constant>(Shuf->getOperand(1))) {
+    // trunc (shuffle X, C, Mask) --> shuffle (trunc X), C', Mask
+    Constant *NarrowC = ConstantExpr::getTrunc(C, DestTy);
+    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), DestTy);
+    return new ShuffleVectorInst(NarrowOp, NarrowC, Shuf->getMask());
+  }
+  // TODO: Canonicalize constant to operand 1, so we don't need to match this
+  // pattern?
+  if (auto *C = dyn_cast<Constant>(Shuf->getOperand(0))) {
+    // trunc (shuffle C, X, Mask) --> shuffle C', (trunc X), Mask
+    Constant *NarrowC = ConstantExpr::getTrunc(C, DestTy);
+    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(1), DestTy);
+    return new ShuffleVectorInst(NarrowC, NarrowOp, Shuf->getMask());
+  }
+
+  return nullptr;
+}
+
 Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
@@ -554,6 +579,9 @@
   if (Instruction *I = shrinkBitwiseLogic(CI))
     return I;
 
+  if (Instruction *I = shrinkShuffle(CI, *Builder))
+    return I;
+
   if (Src->hasOneUse() && isa<IntegerType>(SrcTy) &&
       shouldChangeType(SrcTy, DestTy)) {
     // Transform "trunc (shl X, cst)" -> "shl (trunc X), cst" so long as the
Index: test/Transforms/InstCombine/trunc.ll
===================================================================
--- test/Transforms/InstCombine/trunc.ll
+++ test/Transforms/InstCombine/trunc.ll
@@ -468,8 +468,8 @@
 
 define <4 x i8> @shuf1(<4 x i32> %x) {
 ; CHECK-LABEL: @shuf1(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> %x, <4 x i32> <i32 undef, i32 3634, i32 90, i32 undef>, <4 x i32> <i32 1, i32 5, i32 6, i32 2>
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc <4 x i32> [[SHUF]] to <4 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i32> %x to <4 x i8>
+; CHECK-NEXT:    [[TRUNC:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> <i8 undef, i8 50, i8 90, i8 undef>, <4 x i32> <i32 1, i32 5, i32 6, i32 2>
 ; CHECK-NEXT:    ret <4 x i8> [[TRUNC]]
 ;
   %shuf = shufflevector <4 x i32> %x, <4 x i32> <i32 35, i32 3634, i32 90, i32 -1>, <4 x i32> <i32 1, i32 5, i32 6, i32 2>
@@ -482,8 +482,8 @@
 
 define <4 x i8> @shuf2(<4 x i32> %x) {
 ; CHECK-LABEL: @shuf2(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> <i32 -3500, i32 undef, i32 undef, i32 -1>, <4 x i32> %x, <4 x i32> <i32 3, i32 6, i32 6, i32 0>
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc <4 x i32> [[SHUF]] to <4 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i32> %x to <4 x i8>
+; CHECK-NEXT:    [[TRUNC:%.*]] = shufflevector <4 x i8> <i8 84, i8 undef, i8 undef, i8 -1>, <4 x i8> [[TMP1]], <4 x i32> <i32 3, i32 6, i32 6, i32 0>
 ; CHECK-NEXT:    ret <4 x i8> [[TRUNC]]
 ;
   %shuf = shufflevector <4 x i32> <i32 -3500, i32 3634, i32 90, i32 -1>, <4 x i32> %x, <4 x i32> <i32 3, i32 6, i32 6, i32 0>