[DAGCombiner] Add add/sub saturation undef handling

Match ConstantFolding.cpp:
(add_sat x, undef) -> -1
(sub_sat x, undef) -> 0

2 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 9a40774cff8..25f2fc66b73 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2194,6 +2194,10 @@ SDValue DAGCombiner::visitADDSAT(SDNode *N) {
       return N1;
   }
 
+  // fold (add_sat x, undef) -> -1
+  if (N0.isUndef() || N1.isUndef())
+    return DAG.getAllOnesConstant(DL, VT);
+
   if (DAG.isConstantIntBuildVectorOrConstantInt(N0)) {
     // canonicalize constant to RHS
     if (!DAG.isConstantIntBuildVectorOrConstantInt(N1))
@@ -2792,6 +2796,10 @@ SDValue DAGCombiner::visitSUBSAT(SDNode *N) {
       return N0;
   }
 
+  // fold (sub_sat x, undef) -> 0
+  if (N0.isUndef() || N1.isUndef())
+    return DAG.getConstant(0, DL, VT);
+
   // TODO Constant Folding
 
   // fold (sub_sat x, 0) -> x
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index a0ee80c0dcd..dc5b5e3123f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5180,6 +5180,8 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       case ISD::SDIV:
       case ISD::UREM:
       case ISD::SREM:
+      case ISD::SSUBSAT:
+      case ISD::USUBSAT:
         return getConstant(0, DL, VT);    // fold op(undef, arg2) -> 0
       }
     }
@@ -5203,8 +5205,12 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       return getUNDEF(VT);       // fold op(arg1, undef) -> undef
     case ISD::MUL:
     case ISD::AND:
+    case ISD::SSUBSAT:
+    case ISD::USUBSAT:
       return getConstant(0, DL, VT);  // fold op(arg1, undef) -> 0
     case ISD::OR:
+    case ISD::SADDSAT:
+    case ISD::UADDSAT:
       return getAllOnesConstant(DL, VT);
     }
   }