1 files changed, 31 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 75fb6559b9b..b8af674a1e2 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -19537,6 +19537,37 @@ SDValue DAGCombiner::SimplifyVBinOp(SDNode *N) {
     }
   }
 
+  // Make sure all but the first op are undef.
+  auto ConcatWithUndef = [](SDValue Concat) {
+    return Concat.getOpcode() == ISD::CONCAT_VECTORS &&
+           std::all_of(std::next(Concat->op_begin()), Concat->op_end(),
+                       [](const SDValue &Op) {
+                         return Op.isUndef();
+                       });
+  };
+
+  // The following pattern is likely to emerge with vector reduction ops. Moving
+  // the binary operation ahead of the concat may allow using a narrower vector
+  // instruction that has better performance than the wide version of the op:
+  // VBinOp (concat X, undef), (concat Y, undef) --> concat (VBinOp X, Y), VecC
+  if (ConcatWithUndef(LHS) && ConcatWithUndef(RHS) &&
+      (LHS.hasOneUse() || RHS.hasOneUse())) {
+    SDValue X = LHS.getOperand(0);
+    SDValue Y = RHS.getOperand(0);
+    EVT NarrowVT = X.getValueType();
+    if (NarrowVT == Y.getValueType() &&
+        TLI.isOperationLegalOrCustomOrPromote(Opcode, NarrowVT)) {
+      // (binop undef, undef) may not return undef, so compute that result.
+      SDLoc DL(N);
+      SDValue VecC =
+          DAG.getNode(Opcode, DL, NarrowVT, DAG.getUNDEF(NarrowVT),
+                      DAG.getUNDEF(NarrowVT));
+      SmallVector<SDValue, 4> Ops(LHS.getNumOperands(), VecC);
+      Ops[0] = DAG.getNode(Opcode, DL, NarrowVT, X, Y);
+      return DAG.getNode(ISD::CONCAT_VECTORS, DL, VT, Ops);
+    }
+  }
+
   if (SDValue V = scalarizeBinOpOfSplats(N, DAG))
     return V;