[DAGCombiner] Don't combine (addcarry (uaddo X, Y), 0, Carry) -> (addcarry X, Y, Carry) if the Carry comes from the uaddo.

Summary:
The uaddo won't be removed and the addcarry will still be
dependent on the uaddo. So we'll just increase the use count
of X and Y and potentially require a COPY.

Reviewers: spatel, RKSimon, deadalnix

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64190

llvm-svn: 365149

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index b0e017571f6..c00f89267fb 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2966,8 +2966,11 @@ SDValue DAGCombiner::visitADDCARRYLike(SDValue N0, SDValue N1, SDValue CarryIn,
                                        SDNode *N) {
   // Iff the flag result is dead:
   // (addcarry (add|uaddo X, Y), 0, Carry) -> (addcarry X, Y, Carry)
+  // Don't do this if the Carry comes from the uaddo. It won't remove the uaddo
+  // or the dependency between the instructions.
   if ((N0.getOpcode() == ISD::ADD ||
-       (N0.getOpcode() == ISD::UADDO && N0.getResNo() == 0)) &&
+       (N0.getOpcode() == ISD::UADDO && N0.getResNo() == 0 &&
+        N0.getValue(1) != CarryIn)) &&
       isNullConstant(N1) && !N->hasAnyUseOfValue(1))
     return DAG.getNode(ISD::ADDCARRY, SDLoc(N), N->getVTList(),
                        N0.getOperand(0), N0.getOperand(1), CarryIn);