[DAGCombiner] Fold extract_subvector of undef to undef. Fold away inserting undef subvectors.

llvm-svn: 293152

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 75f4a470212..17608b1f0a5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13880,6 +13880,10 @@ SDValue DAGCombiner::visitEXTRACT_SUBVECTOR(SDNode* N) {
   EVT NVT = N->getValueType(0);
   SDValue V = N->getOperand(0);
 
+  // Extract from UNDEF is UNDEF.
+  if (V.isUndef())
+    return DAG.getUNDEF(NVT);
+
   if (V->getOpcode() == ISD::CONCAT_VECTORS) {
     // Combine:
     //    (extract_subvec (concat V1, V2, ...), i)
@@ -14522,6 +14526,10 @@ SDValue DAGCombiner::visitINSERT_SUBVECTOR(SDNode *N) {
   SDValue N1 = N->getOperand(1);
   SDValue N2 = N->getOperand(2);
 
+  // If inserting an UNDEF, just return the original vector.
+  if (N1.isUndef())
+    return N0;
+
   // Combine INSERT_SUBVECTORs where we are inserting to the same index.
   // INSERT_SUBVECTOR( INSERT_SUBVECTOR( Vec, SubOld, Idx ), SubNew, Idx )
   // --> INSERT_SUBVECTOR( Vec, SubNew, Idx )