[DAGCombine] (addcarry 0, 0, X) -> (ext/trunc X)

Summary:
While this makes some case better and some case worse - so it's unclear if it is a worthy combine just by itself - this is a useful canonicalisation.

As per discussion in D32756 .

Reviewers: jyknight, nemanjai, mkuper, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32916

llvm-svn: 303441

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 0ccee175abf..db2a428688d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2138,6 +2138,17 @@ SDValue DAGCombiner::visitADDCARRY(SDNode *N) {
   if (isNullConstant(CarryIn))
     return DAG.getNode(ISD::UADDO, DL, N->getVTList(), N0, N1);
 
+  // fold (addcarry 0, 0, X) -> (and (ext/trunc X), 1) and no carry.
+  if (isNullConstant(N0) && isNullConstant(N1)) {
+    EVT VT = N0.getValueType();
+    EVT CarryVT = CarryIn.getValueType();
+    SDValue CarryExt = DAG.getBoolExtOrTrunc(CarryIn, DL, VT, CarryVT);
+    AddToWorklist(CarryExt.getNode());
+    return CombineTo(N, DAG.getNode(ISD::AND, DL, VT, CarryExt,
+                                    DAG.getConstant(1, DL, VT)),
+                     DAG.getConstant(0, DL, CarryVT));
+  }
+
   if (SDValue Combined = visitADDCARRYLike(N0, N1, CarryIn, N))
     return Combined;