[DAGCombiner] Do various combine on usubo.

Summary: This essentially does the same transform as for SUBC.

Reviewers: jyknight, nemanjai, mkuper, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30437

llvm-svn: 297404

1 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 046c9e9520c..626dfaea957 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -236,6 +236,7 @@ namespace {
     SDValue visitSUB(SDNode *N);
     SDValue visitADDC(SDNode *N);
     SDValue visitSUBC(SDNode *N);
+    SDValue visitUSUBO(SDNode *N);
     SDValue visitADDE(SDNode *N);
     SDValue visitSUBE(SDNode *N);
     SDValue visitMUL(SDNode *N);
@@ -1400,6 +1401,7 @@ SDValue DAGCombiner::visit(SDNode *N) {
   case ISD::SUB:                return visitSUB(N);
   case ISD::ADDC:               return visitADDC(N);
   case ISD::SUBC:               return visitSUBC(N);
+  case ISD::USUBO:              return visitUSUBO(N);
   case ISD::ADDE:               return visitADDE(N);
   case ISD::SUBE:               return visitSUBE(N);
   case ISD::MUL:                return visitMUL(N);
@@ -2133,6 +2135,38 @@ SDValue DAGCombiner::visitSUBC(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitUSUBO(SDNode *N) {
+  SDValue N0 = N->getOperand(0);
+  SDValue N1 = N->getOperand(1);
+  EVT VT = N0.getValueType();
+  if (VT.isVector())
+    return SDValue();
+
+  EVT CarryVT = N->getValueType(1);
+  SDLoc DL(N);
+
+  // If the flag result is dead, turn this into an SUB.
+  if (!N->hasAnyUseOfValue(1))
+    return CombineTo(N, DAG.getNode(ISD::SUB, DL, VT, N0, N1),
+                     DAG.getUNDEF(CarryVT));
+
+  // fold (usubo x, x) -> 0 + no borrow
+  if (N0 == N1)
+    return CombineTo(N, DAG.getConstant(0, DL, VT),
+                     DAG.getConstant(0, DL, CarryVT));
+
+  // fold (usubo x, 0) -> x + no borrow
+  if (isNullConstant(N1))
+    return CombineTo(N, N0, DAG.getConstant(0, DL, CarryVT));
+
+  // Canonicalize (usubo -1, x) -> ~x, i.e. (xor x, -1) + no borrow
+  if (isAllOnesConstant(N0))
+    return CombineTo(N, DAG.getNode(ISD::XOR, DL, VT, N1, N0),
+                     DAG.getConstant(0, DL, CarryVT));
+
+  return SDValue();
+}
+
 SDValue DAGCombiner::visitSUBE(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);