[DAGCombine] ((A-c1)+c2) -> (A+(c2-c1)) constant-fold

Summary:
This was the root cause of the endless combine loop in D62257

https://rise4fun.com/Alive/d3W

Reviewers: RKSimon, spatel, craig.topper, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62662

llvm-svn: 362131

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 49cd7596f5a..9a06077587d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2161,6 +2161,15 @@ SDValue DAGCombiner::visitADDLike(SDNode *N) {
     return N0;
 
   if (isConstantOrConstantVector(N1, /* NoOpaque */ true)) {
+    // fold ((A-c1)+c2) -> (A+(c2-c1))
+    if (N0.getOpcode() == ISD::SUB &&
+        isConstantOrConstantVector(N0.getOperand(1), /* NoOpaque */ true)) {
+      SDValue Sub = DAG.FoldConstantArithmetic(ISD::SUB, DL, VT, N1.getNode(),
+                                               N0.getOperand(1).getNode());
+      assert(Sub && "Constant folding failed");
+      return DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(0), Sub);
+    }
+
     // fold ((c1-A)+c2) -> (c1+c2)-A
     if (N0.getOpcode() == ISD::SUB &&
         isConstantOrConstantVector(N0.getOperand(0), /* NoOpaque */ true)) {