[GitHub] [tvm-rfcs] electriclilies commented on a change in pull request #45: [RELAY] Add virtual device as a first class field to Relay expressions

[GitBox <gi...@apache.org>]

electriclilies commented on a change in pull request #45:
URL: https://github.com/apache/tvm-rfcs/pull/45#discussion_r760674611



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File path: rfcs/1111-first-class-virtual-device.md
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+- Feature Name: Add virtual device information as a first class field on Relay expressions.
+- Start Date: 2021-11-20
+- RFC PR: [apache/tvm-rfcs#1111](https://github.com/apache/tvm-rfcs/pull/1111)
+- GitHub Issue: [apache/tvm#1111](https://github.com/apache/tvm/issues/1111)
+
+# Summary
+[summary]: #summary
+
+I propose adding a new field to Relay expressions, virtual_device_. This field will contain virtual device information [currently called SEScope].
+
+# Motivation
+[motivation]: #motivation
+
+Currently, the virtual device information (called SEScope today, but we will rename it soon) is stored in Function attributes and in on_device Relay ops. This op is a wrapper op that contains the virtual device information for an expression.
+
+Here's an example of how the virtual device information is stored in the program today (example from test_pass_plan_devices.py):
+
+(note that SEScope is just the virtual device, but we have not renamed it yet).
+
+```
+"""
+#[version = "0.0.5"]
+def @main(%a: Tensor[(5, 7), float32], %b: Tensor[(5, 7), float32],
+          %c: Tensor[(5, 7), float32], %d: Tensor[(5, 7), float32],
+           param_se_scopes=[meta[SEScope][0], meta[SEScope][0], meta[SEScope][1], meta[SEScope][1]],
+           result_se_scope=meta[SEScope][1]) {
+  %0 = add(%a, %b);
+  %1 = on_device(%0, se_scope=meta[SEScope][0], is_fixed=True);
+  %2 = device_copy(%1, src_se_scope=meta[SEScope][0], dst_se_scope=meta[SEScope][1]);
+  %3 = add(%c, %d);
+  subtract(%2, %3)
+}
+"""
+```
+
+Using this method to store the virtual device information has proven to be very fragile.
+
+Normal visitors that don't care about virtual devices need to peek inside on_device ops.
+
+Additionally, we need DeviceAware visitors to be able to know the virtual device of sub-expressions. Notice in the example above that on_device doesn't wrap every expression. Let's say we want to know the virtual device of %3 while visiting it. We can't look it up directly since the information is not stored on the node. So how do we get the information? Well, instead of a normal visitor, we need to use a DeviceAware visitor, which keeps track of the current virtual device when it visits sub-expressions. We can then get the virtual device from the DeviceAware visitor itself.
+
+Making virtual devices first class eliminates the need for this complexity, and will allow us to implement more features in device and memory planning in the future.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Users can introduce new information about the virtual device through the on_device op. This behavior is the same as it was before.
+
+However, let's say you want to write a pass that uses the virtual devices after device planning. Now, you'll be able to use the virtual device directly in your pass, just like you can with the checked_type_ field.
+
+For example, in this visitor, we can just look at the virtual device directly. 
+
+```
+  Expr VisitExpr_(const LetNode* let_node) final {
+    Expr expr = GetRef<Expr>(let_node);
+    // Iterate through chained lets, provided they all agree on their device type.
+    SEScope scope = expr->virtual_device_;
+    ...
+ ```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The AST change will occur in include/tvm/ir/expr.h:
+
+```
+class RelayExprNode : public BaseExprNode {
+ public:
+  /*!
+   * \brief Stores the result of type inference(type checking).
+   *
+   * \note This can be undefined before type inference.
+   *       This value is discarded during serialization.
+   */
+  mutable Type checked_type_ = Type(nullptr);
+  /*!
+   * \return The checked_type
+   */
+  inline const Type& checked_type() const;
+  /*!
+   * \brief Check if the inferred(checked) type of the Expr
+   *  is backed by a TTypeNode and return it.
+   *
+   * \note This function will thrown an error if the node type
+   *       of this Expr is not TTypeNode.
+   *
+   * \return The corresponding TTypeNode pointer.
+   * \tparam The specific TypeNode we look for.
+   */
+  template <typename TTypeNode>
+  inline const TTypeNode* type_as() const;
+
+  /*!
+   * \brief The virtual device (SEScope) for this node (the result of device planning).
+   *
+   * \note Unfortunately, the type of virtual_device_ needs to be ObjectRef to avoid a circular import.
+   *       We can forward-declare the SEScope type for the getter function, but not for the field
+   *       itself.
+   */
+  mutable ObjectRef virtual_device_;
+
+  /*!
+   * \return The virtual device (currently called SEScope, this will be changing soon.)
+   */
+  SEScope virtual_device() const;
+
+  static constexpr const char* _type_key = "RelayExpr";
+  static constexpr const uint32_t _type_child_slots = 22;
+  TVM_DECLARE_BASE_OBJECT_INFO(RelayExprNode, BaseExprNode);
+};
+```
+
+Additionally, I will add virtual_device_ to the WithFields methods.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+One challenge with making virtual devices first class is that passes in TVM do not propagate all fields when they visit expressions. You can see this today with spans-- most visitors do not preserve spans. When we introduce the virtual device field, we will need to ensure that it is propagated correctly throughout the Relay program. To do this, I introduced WithFields, a COW constructor that copies extra fields, including spans. I will extend WithFields to also copy virtual devices. Then, we can use WithFields to ensure that the virtual device field is correctly propagated. 

Review comment:
       Yes, will do




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