[DAGCombine][ARM][X86] (sub Carry, X) -> (addcarry (sub 0, X), 0, Carry) fold

Summary:
`DAGCombiner::visitADDLikeCommutative()` already has a sibling fold:
`(add X, Carry) -> (addcarry X, 0, Carry)`

This fold, as suggested by @efriedma, helps recover from //some//
of the regressions of D62266

Reviewers: efriedma, deadalnix

Subscribers: javed.absar, kristof.beyls, llvm-commits, efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62392

llvm-svn: 372259

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index a7e50685548..edb5ecb4d3d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3384,6 +3384,18 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
     }
   }
 
+  if (TLI.isOperationLegalOrCustom(ISD::ADDCARRY, VT)) {
+    // (sub Carry, X)  ->  (addcarry (sub 0, X), 0, Carry)
+    if (SDValue Carry = getAsCarry(TLI, N0)) {
+      SDValue X = N1;
+      SDValue Zero = DAG.getConstant(0, DL, VT);
+      SDValue NegX = DAG.getNode(ISD::SUB, DL, VT, Zero, X);
+      return DAG.getNode(ISD::ADDCARRY, DL,
+                         DAG.getVTList(VT, Carry.getValueType()), NegX, Zero,
+                         Carry);
+    }
+  }
+
   return SDValue();
 }