[DAGCombine] Do not try to deduplicate commutative operations if both operand are the same.

Summary: It is creating useless work as the commuted nodes is the same as the node we are working on in that case.

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

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33840

llvm-svn: 310832

1 files changed, 3 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 88dc0f4bde6..bd11c5aa1b3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1614,15 +1614,15 @@ SDValue DAGCombiner::combine(SDNode *N) {
     }
   }
 
-  // If N is a commutative binary node, try commuting it to enable more
-  // sdisel CSE.
+  // If N is a commutative binary node, try eliminate it if the commuted
+  // version is already present in the DAG.
   if (!RV.getNode() && TLI.isCommutativeBinOp(N->getOpcode()) &&
       N->getNumValues() == 1) {
     SDValue N0 = N->getOperand(0);
     SDValue N1 = N->getOperand(1);
 
     // Constant operands are canonicalized to RHS.
-    if (isa<ConstantSDNode>(N0) || !isa<ConstantSDNode>(N1)) {
+    if (N0 != N1 && (isa<ConstantSDNode>(N0) || !isa<ConstantSDNode>(N1))) {
       SDValue Ops[] = {N1, N0};
       SDNode *CSENode = DAG.getNodeIfExists(N->getOpcode(), N->getVTList(), Ops,
                                             N->getFlags());