[DAGCombiner] Teach DAG combiner that A-(B-C) can be folded to A+(C-B)

This can be useful since addition is commutable, and subtraction is not.

This matches a transform that is also done by InstCombine.

llvm-svn: 338181

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7a99687757f..963081c2e95 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2671,6 +2671,12 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
     return DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0),
                        N0.getOperand(1).getOperand(0));
 
+  // fold (A-(B-C)) -> A+(C-B)
+  if (N1.getOpcode() == ISD::SUB && N1.hasOneUse())
+    return DAG.getNode(ISD::ADD, DL, VT, N0,
+                       DAG.getNode(ISD::SUB, DL, VT, N1.getOperand(1),
+                                   N1.getOperand(0)));
+
   // fold (X - (-Y * Z)) -> (X + (Y * Z))
   if (N1.getOpcode() == ISD::MUL && N1.hasOneUse()) {
     if (N1.getOperand(0).getOpcode() == ISD::SUB &&