[DAGCombiner] after simplifying demanded elements of vector operand of extract, revisit the extract; 2nd try

This is a retry of rL349051 (reverted at rL349056). I changed the check for dead-ness from
number of uses to an opcode test for DELETED_NODE based on existing similar code.

Differential Revision: https://reviews.llvm.org/D55655

llvm-svn: 349058

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7768d12c75c..ceb7e482603 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -15698,8 +15698,13 @@ SDValue DAGCombiner::visitEXTRACT_VECTOR_ELT(SDNode *N) {
       if (CstElt->getAPIntValue().ult(VT.getVectorNumElements()))
         DemandedElts.setBit(CstElt->getZExtValue());
     }
-    if (SimplifyDemandedVectorElts(InVec, DemandedElts, true))
+    if (SimplifyDemandedVectorElts(InVec, DemandedElts, true)) {
+      // We simplified the vector operand of this extract element. If this
+      // extract is not dead, visit it again so it is folded properly.
+      if (N->getOpcode() != ISD::DELETED_NODE)
+        AddToWorklist(N);
       return SDValue(N, 0);
+    }
   }
 
   bool BCNumEltsChanged = false;