142     Op op_ref = GetRef<Op>(op);  in VisitExpr_()  local
143     CHECK(rev_map.count(op_ref))  in VisitExpr_()
145     return std::make_shared<ADFunction>([this, op_ref](const Type& orig_type,  in VisitExpr_()
153       auto orig = CallNode::make(op_ref, call_args, attrs, type_args);  in VisitExpr_()
156       backprop_actions.push_back([this, args, orig, ret, op_ref](LetList* ll) {  in VisitExpr_()
157         tvm::Array<Expr> rev = rev_map[op_ref](orig, ret->reverse);  in VisitExpr_()
405     Op op_ref = GetRef<Op>(op_node);  in VisitCheckpoint()  local
406     CHECK(op_ref->name == "annotation.checkpoint") << "expected checkpoint annotation";  in VisitCheckpoint()
433       Op op_ref = GetRef<Op>(op_node);  in VisitExpr_()  local
435       if (op_ref->name == "annotation.checkpoint") {  in VisitExpr_()
439       CHECK(rev_map.count(op_ref))  in VisitExpr_()
459             tvm::Array<Expr> rev = rev_map[op_ref](orig, GetGrad(call->checked_type(), ret, ll));  in VisitExpr_()
508       Op op_ref = GetRef<Op>(op);  in MissingGrad()  local
509       if (op_ref->name != "annotation.checkpoint" && !rev_map.count(op_ref)) {  in MissingGrad()
510         op_names.insert(op_ref->name);  in MissingGrad()