[SelectionDAG] Fold insert_subvector(undef, extract_subvector(v, c), c) -> v in getNode

This is already done in DAGCombiner::visitINSERT_SUBVECTOR, but this helps a number of shuffles across different vector widths recognise when they come from the same source.

llvm-svn: 363542

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 224265dfee5..fea8d7ad894 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5539,6 +5539,12 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       // Trivial insertion.
       if (VT.getSimpleVT() == N2.getSimpleValueType())
         return N2;
+
+      // If this is an insert of an extracted vector into an undef vector, we
+      // can just use the input to the extract.
+      if (N1.isUndef() && N2.getOpcode() == ISD::EXTRACT_SUBVECTOR &&
+          N2.getOperand(1) == N3 && N2.getOperand(0).getValueType() == VT)
+        return N2.getOperand(0);
     }
     break;
   }