[RELAY][[PASS] ForwardRewrite pass.

include/tvm/relay/expr.h

@@ -415,6 +415,32 @@ class TupleGetItemNode : public ExprNode {
 
 RELAY_DEFINE_NODE_REF(TupleGetItem, TupleGetItemNode, Expr);
 
+/*!
+ * \brief Base class of the temporary expression.
+ *
+ * TempExprs are pass specific expression that can be
+ * useful to define intermediate result in the
+ * rewriting pass such as layout or type transformation.
+ *
+ * Subclass TempExprNode allows us to pattern match on
+ * specific kind TempExpr and use them for expression rewriting.
+ *
+ * TempExpr should only be used within a pass,
+ */
+class TempExprNode : public ExprNode {
+ public:
+  /*!
+   * \brief Convert the expression to a normal(non-temp) Expr.
+   * \return The corresponding normal(non-temp) expression.
+   */
+  virtual Expr Realize() const = 0;
+
+  static constexpr const char* _type_key = "relay.TempExpr";
+  TVM_DECLARE_BASE_NODE_INFO(TempExprNode, ExprNode);
+};
+
+RELAY_DEFINE_NODE_REF(TempExpr, TempExprNode, Expr);
+
 // implementataions
 template<typename TTypeNode>
 inline const TTypeNode* ExprNode::type_as() const {

include/tvm/relay/op.h

@@ -276,6 +276,16 @@ class GenericOpMap {
    */
   template <typename ValueType>
   inline ValueType get(const Op& op, ValueType def_value) const;
+  /*!
+   * \brief get the corresponding value element at op with default value.
+   * \param expr The key to the map
+   * \param def_value The default value when the key does not exist
+   *         or if expr is not an Op.
+   * \return the const reference to the content value.
+   * \tparam ValueType The content value type.
+   */
+  template <typename ValueType>
+  inline ValueType get(const Expr& expr, ValueType def_value) const;
 
  private:
   friend class OpRegistry;
@@ -313,6 +323,14 @@ class OpMap {
    * \return the const reference to the content value.
    */
   inline ValueType get(const Op& op, ValueType def_value) const;
+  /*!
+   * \brief get the corresponding value element at op with default value.
+   * \param expr The key to the map
+   * \param def_value The default value when the key does not exist
+   *         or if expr is not an Op.
+   * \return the const reference to the content value.
+   */
+  inline ValueType get(const Expr& expr, ValueType def_value) const;
 
  private:
   friend class Op;
@@ -496,6 +514,21 @@ inline ValueType GenericOpMap::get(const Op& op, ValueType value) const {
   }
 }
 
+template <typename ValueType>
+inline ValueType GenericOpMap::get(const Expr& expr, ValueType value) const {
+  CHECK(expr.defined());
+  if (const OpNode* op = expr.as<OpNode>()) {
+    const uint32_t idx = op->index_;
+    if (idx < data_.size() && data_[idx].second != 0) {
+      return data_[idx].first;
+    } else {
+      return value;
+    }
+  } else {
+    return value;
+  }
+}
+
 template <typename ValueType>
 inline int OpMap<ValueType>::count(const Op& op) const {
   return map_.count(op);
@@ -505,12 +538,19 @@ template <typename ValueType>
 inline ValueType OpMap<ValueType>::operator[](const Op& op) const {
   return map_[op];
 }
+
 template <typename ValueType>
 inline ValueType OpMap<ValueType>::get(const Op& op,
                                        ValueType def_value) const {
   return map_.get<ValueType>(op, def_value);
 }
 
+template <typename ValueType>
+inline ValueType OpMap<ValueType>::get(const Expr& expr,
+                                       ValueType def_value) const {
+  return map_.get<ValueType>(expr, def_value);
+}
+
 /*!
  * \brief Check that an expression is a "primtive operator".
  *

include/tvm/relay/op_attr_types.h

@@ -85,6 +85,25 @@ using FTVMSchedule = runtime::TypedPackedFunc<
   Schedule(const Attrs& attrs,
            const Array<Tensor>& outs,
            const Target& target)>;
+
+/*!
+ * \brief Forward rewriting rule for a specific op.
+ *
+ * \param ref_call The reference old call type to be rewritten.
+ *                 We can make use of the op and type information.
+ * \param new_args The new arguments (some of them could be TempExpr).
+ * \param ctx  Optional context information about ref_call.
+ * \return The rewriten result call, can also return nullptr,
+ *         which indicate the rewriter should use the default fallback
+ *         rule that realizes all its input and compose the call.
+ *
+ * \note When we register the function, we can register
+ *       a different signature with ctx to be a specific node type.
+ */
+using FForwardRewrite = runtime::TypedPackedFunc<
+  Expr(const Call& ref_call,
+       const Array<Expr>& new_args,
+       const NodeRef& ctx)>;
 }  // namespace relay
 }  // namespace tvm
 #endif  // TVM_RELAY_OP_ATTR_TYPES_H_

include/tvm/relay/pass.h

@@ -158,6 +158,17 @@ Expr FoldConstant(const Expr& expr);
  */
 Expr FuseOps(const Expr& expr, int fuse_opt_level);
 
+/*!
+ * \brief Apply rewrite rules to rewrite the expr in post DFS order.
+ * \param expr The expression.
+ * \param rewrite_map_attr_name The Op's attr name which corresponds to the rewrite
+ *                              rule function.
+ * \param fcontext Additional callback to provide context argument for each call node.
+ * \return The rewritten expression.
+ */
+Expr ForwardRewrite(const Expr& expr,
+                    const std::string& rewrite_map_attr_name,
+                    std::function<NodeRef(const Call&)> fcontext = nullptr);
 
 /*! \brief A hashing structure in the style of std::hash. */
 struct StructuralHash {

include/tvm/runtime/packed_func.h

@@ -73,6 +73,8 @@ class PackedFunc {
   using FType = std::function<void (TVMArgs args, TVMRetValue* rv)>;
   /*! \brief default constructor */
   PackedFunc() {}
+  /*! \brief constructor from null */
+  PackedFunc(std::nullptr_t null) {}  // NOLINT(*)
   /*!
    * \brief constructing a packed function from a std::function.
    * \param body the internal container of packed function.