[TIR] Introduce tir::PrimFunc (apache#5070)

This PR introduces tir::PrimFunc which will be used as the TIR function
container in the unified IR.

Also streamlined the function attributes a bit further.
- All common attributes are under tvm::attr
- TIR specific attributes are under tvm::tir::attr and comes with a tir prefix
- Use stl_style for attributes for now

include/tvm/ir/function.h

@@ -32,6 +32,36 @@
 
 namespace tvm {
 
+/*!
+ * \brief Possible Calling conventions.
+ *
+ *  NOTE: The calling convention also implies
+ *  the way we implement the function during lowering.
+ */
+enum class CallingConv : int {
+  /*!
+   * \brief Default calling convetion.
+   *
+   * - Uses the native calling convention of the target.
+   * - Implementation: specified by the native target.
+   */
+  kDefault = 0,
+  /*!
+   * \brief Device kernel launch
+   *
+   * - Call by PackedFunc calling convention.
+   * - Implementation: defined by device runtime(e.g. runtime/cuda)
+   */
+  kDeviceKernelLaunch = 2,
+  /*!
+   * \brief PackedFunc that exposes a CPackedFunc signature.
+   *
+   * - Calling by PackedFunc calling convention.
+   * - Implementation: Expose a function with the CPackedFunc signature.
+   */
+  kCPackedFunc = 3,
+};
+
 /*!
  * \brief Base node of all functions.
  *
@@ -115,5 +145,74 @@ class BaseFunc : public RelayExpr {
   TVM_DEFINE_OBJECT_REF_METHODS(BaseFunc, RelayExpr, BaseFuncNode);
 };
 
+/*!
+ * \brief Create a new function that copies func, but overrides
+ *        the attribute value key with the value.
+ *
+ * \param func The input function.
+ * \param attr_key The attribute key.
+ * \param attr_value The value attribute value.
+ *
+ * \tparam TFunc The corresponding function type.
+ *
+ * \returns The new function with updated attributes.
+ *
+ * \note This function performs copy on write optimization for func.
+ *       If we move a uniquely referenced func into WithAttr,
+ *       then no additional copy will be performed.
+ *
+ *       This is also why we make it as a function instead of a member function
+ *       and why we pass by value in the first argument.
+ *
+ * \code
+ *
+ *  // Recommended way to trigger copy on write
+ *  func = WithAttr(std::move(func), "key1", value1);
+ *  func = WithAttr(std::move(func), "key2", value2);
+ *
+ * \endcode
+ */
+template<typename TFunc,
+         typename = typename std::enable_if<
+           std::is_base_of<BaseFunc, TFunc>::value>::type>
+inline TFunc WithAttr(TFunc func,
+                      const std::string& attr_key,
+                      ObjectRef attr_value) {
+  using TNode = typename TFunc::ContainerType;
+  static_assert(TNode::_type_final, "Can only operate on the leaf nodes");
+  TNode* node = func.CopyOnWrite();
+  if (node->attrs.defined()) {
+    node->attrs.CopyOnWrite()->dict.Set(attr_key, attr_value);
+  } else {
+    Map<std::string, ObjectRef> dict = {{attr_key, attr_value}};
+    node->attrs = DictAttrs(dict);
+  }
+  return func;
+}
+
+/*!
+ * \brief Generic attribute names that can be attached to any function.
+ *
+ * \sa tvm::tir::attr, tvm::relay::attr
+ */
+namespace attr {
+/*!
+ * \brief Indicates the special calling convention.
+ *
+ * Type: Integer
+ *
+ * \sa tvm::CallingConv
+ */
+constexpr const char* kCallingConv = "calling_conv";
+
+/*!
+ * \brief Compilation target of the function.
+ *
+ * Type: Target
+ *
+ * \sa tvm::Target
+ */
+constexpr const char* kTarget = "target";
+}  // namespace attr
 }  // namespace tvm
 #endif  // TVM_IR_FUNCTION_H_

include/tvm/ir/type.h

@@ -276,6 +276,13 @@ class TupleType : public Type {
   TVM_DEFINE_OBJECT_REF_METHODS(TupleType, Type, TupleTypeNode);
 };
 
+/*!
+ * \return a type that represents void.
+ */
+inline Type VoidType() {
+  return TupleType::Empty();
+}
+
 /*!
  * \brief Potential Constraints in a function.
  * \sa TypeConstraint

include/tvm/relay/function.h

@@ -114,33 +114,6 @@ class Function : public BaseFunc {
   TVM_DEFINE_OBJECT_REF_COW_METHOD(FunctionNode);
 };
 
-/*!
- * \brief Create a new function that copies func, but overrides
- *        the attribute value key with the value.
- *
- * \param func The input function.
- * \param attr_key The attribute key.
- * \param attr_value The value attribute value.
- *
- * \returns The new function with updated attributes.
- *
- * \note This function performs copy on write optimization for func.
- *       If we move a uniquely referenced func into WithAttr,
- *       then no additional copy will be performed.
- *
- *       This is also why we make it as a function instead of a member function
- *       and why we pass by value in the first argument.
- *
- * \code
- *
- *  // Recommended way to trigger copy on write
- *  func = WithAttr(std::move(func), "key1", value1);
- *  func = WithAttr(std::move(func), "key2", value2);
- *
- * \endcode
- */
-TVM_DLL Function WithAttr(Function func, const std::string& attr_key, ObjectRef attr_value);
-
 /*!
  * \brief namespace of the attributes that can be attached to a relay::Function.
  */

include/tvm/tir/function.h

@@ -0,0 +1,177 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*!
+ * \file tvm/tir/function.h
+ * \brief TIR Function.
+ */
+#ifndef TVM_TIR_FUNCTION_H_
+#define TVM_TIR_FUNCTION_H_
+
+#include <tvm/ir/function.h>
+#include <tvm/tir/expr.h>
+#include <tvm/tir/buffer.h>
+#include <tvm/tir/stmt.h>
+#include <string>
+
+
+namespace tvm {
+namespace tir {
+
+/*!
+ * \brief Primitive functions that contains TIR statements.
+ *
+ * The PrimFunc provides low-level code representation does not
+ * automatically manage
+ *
+ * \sa PrimFunc
+ */
+class PrimFuncNode : public BaseFuncNode {
+ public:
+  /*! \brief Function parameters */
+  Array<tir::Var> params;
+  /*! \brief The body of the function */
+  tir::Stmt body;
+  /*! \brief The return type of the function. */
+  Type ret_type;
+  /*!
+   * \brief Maps some parameters to specific Buffer data structures.
+   *
+   *  buffer_map provides a way to express data structure's field and shape
+   *  constraints. The provided information is used in the program analysis
+   *  and the code generation.
+   *
+   *  - It defines the vars in the Buffer (m, n) in the cases below when
+   *    they appears in the buffer_map for the first time.
+   *  - When a var appears multiple times, they translate into runtime
+   *    assertion to check the field constraint.
+   *
+   *  \code
+   *
+   *   # The corresponding fields of f are as follows
+   *   #
+   *   # - f.params = [a, b]
+   *   # - f.buffer_map = {a: A, b: B}
+   *   # - A = decl_buffer(shape=[m, n])
+   *   # - B = decl_buffer(shape=[m, n])
+   *
+   *   def f(a, b):
+   *       m, n = var(), var()
+   *       A = bind_buffer(a, shape=[m, n])
+   *       B = bind_buffer(b, shape=[m, n])
+   *       # body
+   *
+   *  \endcode
+   *
+   *  buffer_map is a sugar to express:
+   *  - Parameter unpacking: e.g. I can load a.shape[0] to get value of m
+   *  - Constraint checking: a.shape[0] must equal b.shape[0] because they
+   *    both corresponds to m.
+
+   *  While we could have express parameter unpacking and constraint using
+   *  normal statements, making buffer_map as first class citizen of PrimFunc
+   *  will make program analysis much easier.
+   *
+   * \note This field can be nullptr
+   */
+  Map<tir::Var, Buffer> buffer_map;
+
+  void VisitAttrs(tvm::AttrVisitor* v) {
+    v->Visit("params", &params);
+    v->Visit("body", &body);
+    v->Visit("ret_type", &ret_type);
+    v->Visit("buffer_map", &buffer_map);
+    v->Visit("attrs", &attrs);
+    v->Visit("span", &span);
+    v->Visit("_checked_type_", &checked_type_);
+  }
+
+  /*!
+   * \brief Return the derived function annotation of this function.
+   *
+   * \return The function type annotation.
+   * \note The function type annotation of PrimExpr is
+   *       directly derived from the Vars without the need of type inference.
+   */
+  TVM_DLL FuncType func_type_annotation() const;
+
+  static constexpr const char* _type_key = "tir.PrimFunc";
+  TVM_DECLARE_FINAL_OBJECT_INFO(PrimFuncNode, BaseFuncNode);
+};
+
+/*!
+ * \brief Managed reference to PrimFuncNode.
+ * \sa PrimFuncNode
+ */
+class PrimFunc : public BaseFunc {
+ public:
+  /*!
+   * \brief Constructor
+   * \param params The parameters of the function.
+   * \param body The body of the function.
+   * \param ret_type The return type of the function.
+   * \param buffer_map The buffer map for parameter buffer unpacking.
+   * \param attrs Additional function attributes.
+   */
+  TVM_DLL PrimFunc(Array<tir::Var> params,
+                   Stmt body,
+                   Type ret_type = VoidType(),
+                   Map<tir::Var, Buffer> buffer_map = NullValue<Map<tir::Var, Buffer>>(),
+                   DictAttrs attrs = NullValue<DictAttrs>());
+
+  TVM_DEFINE_OBJECT_REF_METHODS(PrimFunc, BaseFunc, PrimFuncNode);
+  TVM_DEFINE_OBJECT_REF_COW_METHOD(PrimFuncNode);
+};
+
+/*!
+ * \brief PrimFunc specific attribute names.
+ *
+ * \sa tvm::attr
+ */
+namespace attr {
+/*!
+ * \brief List of thread IterVar that a DeviceLaunch function corresponds to.
+ *
+ * Type: Array<tir::IterVar>
+ *
+ * We call a device kernel launch function f using the following convention:
+ *
+ * Call(f,
+ *      [arg1, arg2, ..., arg_n,
+ *       work_size_1, work_size_2, ... work_size_m])
+ *
+ * Here n = len(arg), m = len(work_size) = len(device_thread_axis).
+ *
+ * The list of device_thread_axis indicates how can be bind the
+ * work_size arguments to the corresponding threads.
+ *
+ * \sa tvm::CallingConv::kDeviceKernelLaunch
+ */
+constexpr const char* kDeviceThreadAxis = "tir.device_thread_axis";
+
+/*!
+ * \brief Whether to set noalias rule on the function arguments.
+ *
+ * Type: Integer
+ */
+constexpr const char* kNoAlias = "tir.noalias";
+}  // namespace attr
+}  // namespace tir
+}  // namespace tvm
+#endif  // TVM_TIR_FUNCTION_H_

include/tvm/tir/op.h

@@ -28,6 +28,7 @@
 #ifndef TVM_TIR_OP_H_
 #define TVM_TIR_OP_H_
 
+#include <tvm/ir/type.h>
 #include <tvm/tir/expr.h>
 #include <tvm/tir/stmt.h>
 
@@ -37,13 +38,24 @@
 
 
 namespace tvm {
+
 // Most common operators can be overloaded by argument type(PrimExpr).
 // So we put them under the root namespace.
 // It is also necessary to overload operators for PrimExpr.
 //
 // We put more developer oriented APIs -- make_const and is_const under tir
 // as they are more specific to the tir namespace.
 
+/*!
+ * \brief Get the type of the expression under the unified type system.
+ *
+ * This function could return a more refined type than
+ * the runtime type provided by expr->dtype
+ *
+ * \sa tvm/ir/type.h for discussion about the relation between Type and runtime::DataType.
+ */
+TVM_DLL Type GetType(const PrimExpr& expr);
+
 /*!
  * Query the maximum possible value of dtype.
  * \param dtype The data type.

python/tvm/ir/__init__.py

@@ -21,7 +21,8 @@
 from .type import TypeConstraint, FuncType, IncompleteType, RelayRefType
 from .tensor_type import TensorType
 from .type_relation import TypeCall, TypeRelation
-from .expr import BaseExpr, PrimExpr, RelayExpr, GlobalVar, BaseFunc, Range
+from .expr import BaseExpr, PrimExpr, RelayExpr, GlobalVar, Range
+from .function import BaseFunc
 from .adt import Constructor, TypeData
 from .module import IRModule
 from .attrs import Attrs, DictAttrs, make_node

python/tvm/ir/expr.py

@@ -51,15 +51,6 @@ def checked_type(self):
         return ret
 
 
-class BaseFunc(RelayExpr):
-    """Base class of all functions."""
-    @property
-    def attrs(self):
-        """Return the attrs member of the function.
-        """
-        return _ffi_api.BaseFunc_Attrs(self)
-
-
 @tvm._ffi.register_object("relay.GlobalVar")
 class GlobalVar(RelayExpr):
     """A global variable in the IR.