[DAGCombine] Fix a bug in a BUILD_VECTOR combine

When trying to convert a BUILD_VECTOR into a shuffle, we try to split a single source vector that is twice as wide as the destination vector. 
We can not do this when we also need the zero vector to create a blend.
This fixes PR22774.

Differential Revision: http://reviews.llvm.org/D8040

llvm-svn: 231219

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index c7a9df543a0..4a074a03d9a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11358,7 +11358,9 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
       } else if (VecInT.getSizeInBits() == VT.getSizeInBits() * 2) {
         // If the input vector is too large, try to split it.
         // We don't support having two input vectors that are too large.
-        if (VecIn2.getNode())
+        // If the zero vector was used, we can not split the vector,
+        // since we'd need 3 inputs.
+        if (UsesZeroVector || VecIn2.getNode())
           return SDValue();
 
         if (!TLI.isExtractSubvectorCheap(VT, VT.getVectorNumElements()))
@@ -11370,7 +11372,6 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
           DAG.getConstant(VT.getVectorNumElements(), TLI.getVectorIdxTy()));
         VecIn1 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, VT, VecIn1,
           DAG.getConstant(0, TLI.getVectorIdxTy()));
-        UsesZeroVector = false;
       } else
         return SDValue();
     }