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

Summary: https://rise4fun.com/Alive/B0A

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

Reviewed By: RKSimon

Subscribers: javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62691

llvm-svn: 362135

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 8bbd5cd2012..b6164ac4ded 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2895,6 +2895,16 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
     return DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), NewC);
   }
 
+  // fold (c1-A)-c2 -> (c1-c2)-A
+  if (N0.getOpcode() == ISD::SUB &&
+      isConstantOrConstantVector(N1, /* NoOpaques */ true) &&
+      isConstantOrConstantVector(N0.getOperand(0), /* NoOpaques */ true)) {
+    SDValue NewC = DAG.FoldConstantArithmetic(
+        ISD::SUB, DL, VT, N0.getOperand(0).getNode(), N1.getNode());
+    assert(NewC && "Constant folding failed");
+    return DAG.getNode(ISD::SUB, DL, VT, NewC, N0.getOperand(1));
+  }
+
   // fold ((A+(B+or-C))-B) -> A+or-C
   if (N0.getOpcode() == ISD::ADD &&
       (N0.getOperand(1).getOpcode() == ISD::SUB ||