[DAGCombiner][X86][SystemZ] Canonicalize SSUBO with immediate RHS to SADDO by negating the immediate.

This lines up with what we do for regular subtract and it matches up better with X86 assumptions in isel patterns that add with immediate is more canonical than sub with immediate.

Differential Revision: https://reviews.llvm.org/D60020

llvm-svn: 358027

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 5d7890ee08f..c61b24f8d21 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3037,6 +3037,14 @@ SDValue DAGCombiner::visitSUBO(SDNode *N) {
     return CombineTo(N, DAG.getConstant(0, DL, VT),
                      DAG.getConstant(0, DL, CarryVT));
 
+  ConstantSDNode *N1C = getAsNonOpaqueConstant(N1);
+
+  // fold (subox, c) -> (addo x, -c)
+  if (IsSigned && N1C && !N1C->getAPIntValue().isMinSignedValue()) {
+    return DAG.getNode(ISD::SADDO, DL, N->getVTList(), N0,
+                       DAG.getConstant(-N1C->getAPIntValue(), DL, VT));
+  }
+
   // fold (subo x, 0) -> x + no borrow
   if (isNullOrNullSplat(N1))
     return CombineTo(N, N0, DAG.getConstant(0, DL, CarryVT));