DAGCombiner: Fold away vector extract of insert with the same index

This only really matters when the index is non-constant since the
constant case already gets taken care of by other combines.

llvm-svn: 274569

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index a8a76311225..6e6dfd855f8 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -12397,6 +12397,14 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
       return DAG.getNode(ISD::TRUNCATE, SDLoc(N), NVT, BCSrc);
   }
 
+  // extract_vector_elt (insert_vector_elt vec, val, idx), idx) -> val
+  //
+  // This only really matters if the index is non-constant since other combines
+  // on the constant elements already work.
+  if (InVec.getOpcode() == ISD::INSERT_VECTOR_ELT &&
+      EltNo == InVec.getOperand(2))
+    return InVec.getOperand(1);
+
   // Transform: (EXTRACT_VECTOR_ELT( VECTOR_SHUFFLE )) -> EXTRACT_VECTOR_ELT.
   // We only perform this optimization before the op legalization phase because
   // we may introduce new vector instructions which are not backed by TD