diff --git a/imperative/src/impl/ops/indexing.cpp b/imperative/src/impl/ops/indexing.cpp
new file mode 100644
index 00000000..7d012cf1
--- /dev/null
+++ b/imperative/src/impl/ops/indexing.cpp
@@ -0,0 +1,76 @@
+/**
+ * \file imperative/src/impl/ops/indexing.cpp
+ * MegEngine is Licensed under the Apache License, Version 2.0 (the "License")
+ *
+ * Copyright (c) 2014-2021 Megvii Inc. All rights reserved.
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT ARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ */
+
+#include "megbrain/imperative/ops/autogen.h"
+
+#include "../op_trait.h"
+
+#include "megbrain/opr/indexing.h"
+
+namespace mgb {
+namespace imperative {
+
+namespace {
+namespace indexing_one_hot {
+
+std::tuple<SmallVector<LogicalTensorDesc>, bool> infer_output_attrs_fallible(
+        const OpDef& def,
+        const SmallVector<LogicalTensorDesc>& input_descs) {
+    auto& op = def.cast_final_safe<IndexingOneHot>();
+
+    mgb_assert(input_descs.size() == 2, "IndexingOneHot expects two inputs");
+
+    auto comp_node = input_descs[0].comp_node;
+    TensorLayout src = input_descs[0].layout,
+            index = input_descs[1].layout;
+
+    mgb_assert(index.dtype == dtype::Int32(), "index dtype must be int32");
+
+    if (!src.ndim) {
+        return {{{{{}, src.dtype}, comp_node}}, false};
+    }
+
+    mgb_assert(src.ndim >= 2, "src ndim must be at least 2");
+    mgb_assert(src.is_contiguous(), "src should be contiguous");
+    mgb_assert(op.axis >= 0 && op.axis < src.ndim, "axis %d not exists in src", op.axis);
+
+    TensorLayout dst = src;
+    dst.shape[op.axis] = 1;
+    dst.init_contiguous_stride();
+
+    if (!index.ndim) {
+        return {{{dst, comp_node}}, false};
+    }
+
+    mgb_assert(index.is_contiguous(), "index should be all contiguous");
+    mgb_assert(index.eq_shape(src.remove_axis(op.axis)), "index shape doesn't match src");
+
+    return {{{dst, comp_node}}, true};
+}
+
+auto apply_on_var_node(const OpDef& def, const VarNodeArray& inputs) {
+    auto&& op = static_cast<const IndexingOneHot&>(def);
+    mgb_assert(inputs.size() == 2);
+    OperatorNodeConfig config{op.make_name()};
+    return opr::IndexingOneHot::make(inputs[0], inputs[1], op.param(), config);
+}
+
+OP_TRAIT_REG(IndexingOneHot, IndexingOneHot)
+    .infer_output_attrs_fallible(infer_output_attrs_fallible)
+    .apply_on_var_node(apply_on_var_node)
+    .fallback();
+
+}  // namespace indexing_one_hot
+}  // anonymous namespace
+}  // namespace imperative
+}  // namespace mgb
+
+// vim: syntax=cpp.doxygen foldmethod=marker foldmarker=f{{{,f}}}
diff --git a/imperative/src/impl/ops/specializations.cpp b/imperative/src/impl/ops/specializations.cpp
index af07254d..3d596d3e 100644
--- a/imperative/src/impl/ops/specializations.cpp
+++ b/imperative/src/impl/ops/specializations.cpp
@@ -303,20 +303,6 @@ OP_TRAIT_REG(GroupLocal, GroupLocal)
 }  // namespace
 
 namespace {
-namespace indexing_one_hot {
-auto apply_on_var_node(const OpDef& def, const VarNodeArray& inputs) {
-    auto&& op = static_cast<const IndexingOneHot&>(def);
-    mgb_assert(inputs.size() == 2);
-    OperatorNodeConfig config{op.make_name()};
-    return opr::IndexingOneHot::make(inputs[0], inputs[1], op.param(), config);
-}
-OP_TRAIT_REG(IndexingOneHot, IndexingOneHot)
-        .apply_on_var_node(apply_on_var_node)
-        .fallback();
-}  // namespace indexing_one_hot
-}  // namespace
-
-namespace {
 namespace indexing_set_one_hot {
 auto apply_on_var_node(const OpDef& def, const VarNodeArray& inputs) {
     auto&& op = static_cast<const IndexingSetOneHot&>(def);