[TargetLowering] Add UNDEF folding to SimplifyDemandedVectorElts

If all the demanded elements of the SimplifyDemandedVectorElts are known to be UNDEF, we can simplify to an ISD::UNDEF node.

Zero constant folding will be handled in a future patch - its a little trickier as we often have bitcasted zero values.

Differential Revision: https://reviews.llvm.org/D55511

llvm-svn: 348784

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index b45453b52e1..1f5ea31c815 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1783,8 +1783,13 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     break;
   }
   }
-
   assert((KnownUndef & KnownZero) == 0 && "Elements flagged as undef AND zero");
+
+  // Constant fold all undef cases.
+  // TODO: Handle zero cases as well.
+  if (DemandedElts.isSubsetOf(KnownUndef))
+    return TLO.CombineTo(Op, TLO.DAG.getUNDEF(VT));
+
   return false;
 }