Prefer shuffles to selects. Backends love shuffles!

llvm-svn: 164763

1 files changed, 19 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 291e80019e8..70483ceb062 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -903,7 +903,7 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
     return &SI;
   }
 
-  if (VectorType* VecTy = dyn_cast<VectorType>(SI.getType())) {
+  if (VectorType *VecTy = dyn_cast<VectorType>(SI.getType())) {
     unsigned VWidth = VecTy->getNumElements();
     APInt UndefElts(VWidth, 0);
     APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
@@ -912,6 +912,24 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
         return ReplaceInstUsesWith(SI, V);
       return &SI;
     }
+
+    if (ConstantVector *CV = dyn_cast<ConstantVector>(CondVal)) {
+      // Form a shufflevector instruction.
+      SmallVector<Constant *, 8> Mask(VWidth);
+      Type *Int32Ty = Type::getInt32Ty(CV->getContext());
+      for (unsigned i = 0; i != VWidth; ++i) {
+        Constant *Elem = cast<Constant>(CV->getOperand(i));
+        if (ConstantInt *E = dyn_cast<ConstantInt>(Elem))
+          Mask[i] = ConstantInt::get(Int32Ty, i + (E->isZero() ? VWidth : 0));
+        else if (isa<UndefValue>(Elem))
+          Mask[i] = UndefValue::get(Int32Ty);
+        else
+          return 0;
+      }
+      Constant *MaskVal = ConstantVector::get(Mask);
+      Value *V = Builder->CreateShuffleVector(TrueVal, FalseVal, MaskVal);
+      return ReplaceInstUsesWith(SI, V);
+    }
   }
 
   return 0;