[SDAG] fold insert_vector_elt with undef index

Similar to:
rG4c47617627fb

This makes the DAG behavior consistent with IR's insertelement.

https://bugs.llvm.org/show_bug.cgi?id=42689

I've tried to maintain test intent for AArch64 and WebAssembly
by replacing undef index operands with something else.

2 files changed, 9 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index f00e332de9c..45947939482 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -16574,10 +16574,6 @@ SDValue DAGCombiner::visitINSERT_VECTOR_ELT(SDNode *N) {
   SDValue EltNo = N->getOperand(2);
   SDLoc DL(N);
 
-  // If the inserted element is an UNDEF, just use the input vector.
-  if (InVal.isUndef())
-    return InVec;
-
   EVT VT = InVec.getValueType();
   unsigned NumElts = VT.getVectorNumElements();
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 8ea7ce94da1..4ffc55c62f2 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5506,6 +5506,15 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     // INSERT_VECTOR_ELT into out-of-bounds element is an UNDEF
     if (N3C && N3C->getZExtValue() >= N1.getValueType().getVectorNumElements())
       return getUNDEF(VT);
+
+    // Undefined index can be assumed out-of-bounds, so that's UNDEF too.
+    if (N3.isUndef())
+      return getUNDEF(VT);
+
+    // If the inserted element is an UNDEF, just use the input vector.
+    if (N2.isUndef())
+      return N1;
+
     break;
   }
   case ISD::INSERT_SUBVECTOR: {