[TVMScript] Add object path tracing to StructuralEqual

Motivation: when two IR objects fail a structural equality check, currently there is no easy way to
find out which part of the IR caused the mismatch. In this PR, we modify the `StructuralEqual`
infrastructure to also optionally return a pair of `ObjectPath` objects that point to the mismatch.
(See apache#11977). In the upcoming PRs, we will pass these paths to the
TIR printer, so that it could highlight the mismatch location nicely.

Tracking issue: apache#11912

include/tvm/node/reflection.h

@@ -404,5 +404,11 @@ inline bool ReflectionVTable::GetReprBytes(const Object* self, std::string* repr
   }
 }
 
+/*!
+ * \brief Given an object and an address of its attribute, return the key of the attribute.
+ * \return nullptr if no attribute with the given address exists.
+ */
+Optional<String> GetAttrKeyByAddress(const Object* object, const void* attr_address);
+
 }  // namespace tvm
 #endif  // TVM_NODE_REFLECTION_H_

include/tvm/node/structural_equal.h

@@ -24,6 +24,7 @@
 #define TVM_NODE_STRUCTURAL_EQUAL_H_
 
 #include <tvm/node/functor.h>
+#include <tvm/node/object_path.h>
 #include <tvm/runtime/container/array.h>
 #include <tvm/runtime/data_type.h>
 
@@ -56,6 +57,31 @@ class BaseValueEqual {
   }
 };
 
+/*!
+ * \brief Pair of `ObjectPath`s, one for each object being tested for structural equality.
+ */
+struct ObjectPathPair {
+  ObjectPath lhs_path;
+  ObjectPath rhs_path;
+};
+
+// Could be replaced with std::optional<ObjectPathPair>
+class OptionalObjectPathPair {
+ public:
+  OptionalObjectPathPair() = default;
+
+  OptionalObjectPathPair(const ObjectPathPair& p)  // NOLINT(runtime/explicit)
+      : lhs_path(p.lhs_path), rhs_path(p.rhs_path) {}
+
+  bool defined() const { return lhs_path.defined(); }
+
+  ObjectPathPair value() const { return {lhs_path.value(), rhs_path.value()}; }
+
+ private:
+  Optional<ObjectPath> lhs_path;
+  Optional<ObjectPath> rhs_path;
+};
+
 /*!
  * \brief Content-aware structural equality comparator for objects.
  *
@@ -99,7 +125,10 @@ class StructuralEqual : public BaseValueEqual {
  * equality checking. Instead, it can store the necessary equality conditions
  * and check later via an internally managed stack.
  */
-class SEqualReducer : public BaseValueEqual {
+class SEqualReducer {
+ private:
+  struct PathTracingData;
+
  public:
   /*! \brief Internal handler that defines custom behaviors.. */
   class Handler {
@@ -110,12 +139,24 @@ class SEqualReducer : public BaseValueEqual {
      * \param lhs The left operand.
      * \param rhs The right operand.
      * \param map_free_vars Whether do we allow remap variables if possible.
+     * \param current_paths Optional paths to `lhs` and `rhs` objects, for error traceability.
      *
      * \return false if there is an immediate failure, true otherwise.
      * \note This function may save the equality condition of (lhs == rhs) in an internal
      *       stack and try to resolve later.
      */
-    virtual bool SEqualReduce(const ObjectRef& lhs, const ObjectRef& rhs, bool map_free_vars) = 0;
+    virtual bool SEqualReduce(const ObjectRef& lhs, const ObjectRef& rhs, bool map_free_vars,
+                              const OptionalObjectPathPair& current_paths) = 0;
+
+    /*!
+     * \brief Mark the comparison as failed, but don't fail immediately.
+     *
+     * This is useful for producing better error messages when comparing containers.
+     * For example, if two array sizes mismatch, it's better to mark the comparison as failed
+     * but compare array elements anyway, so that we could find the true first mismatch.
+     */
+    virtual void DeferFail(const ObjectPathPair& mismatch_paths) = 0;
+
     /*!
      * \brief Lookup the graph node equal map for vars that are already mapped.
      *
@@ -129,28 +170,72 @@ class SEqualReducer : public BaseValueEqual {
      * \brief Mark current comparison as graph node equal comparison.
      */
     virtual void MarkGraphNode() = 0;
-  };
 
-  using BaseValueEqual::operator();
+   protected:
+    using PathTracingData = SEqualReducer::PathTracingData;
+  };
 
   /*! \brief default constructor */
   SEqualReducer() = default;
   /*!
    * \brief Constructor with a specific handler.
    * \param handler The equal handler for objects.
+   * \param tracing_data Optional pointer to the path tracing data.
    * \param map_free_vars Whether or not to map free variables.
    */
-  explicit SEqualReducer(Handler* handler, bool map_free_vars)
-      : handler_(handler), map_free_vars_(map_free_vars) {}
+  explicit SEqualReducer(Handler* handler, const PathTracingData* tracing_data, bool map_free_vars)
+      : handler_(handler), tracing_data_(tracing_data), map_free_vars_(map_free_vars) {}
+
+  /*!
+   * \brief Reduce condition to comparison of two attribute values.
+   * \param lhs The left operand.
+   * \param rhs The right operand.
+   * \return the immediate check result.
+   */
+  bool operator()(const double& lhs, const double& rhs) const;
+  bool operator()(const int64_t& lhs, const int64_t& rhs) const;
+  bool operator()(const uint64_t& lhs, const uint64_t& rhs) const;
+  bool operator()(const int& lhs, const int& rhs) const;
+  bool operator()(const bool& lhs, const bool& rhs) const;
+  bool operator()(const std::string& lhs, const std::string& rhs) const;
+  bool operator()(const DataType& lhs, const DataType& rhs) const;
+
+  template <typename ENum, typename = typename std::enable_if<std::is_enum<ENum>::value>::type>
+  bool operator()(const ENum& lhs, const ENum& rhs) const {
+    using Underlying = typename std::underlying_type<ENum>::type;
+    static_assert(std::is_same<Underlying, int>::value,
+                  "Enum must have `int` as the underlying type");
+    return EnumAttrsEqual(static_cast<int>(lhs), static_cast<int>(rhs), &lhs, &rhs);
+  }
+
   /*!
    * \brief Reduce condition to comparison of two objects.
    * \param lhs The left operand.
    * \param rhs The right operand.
    * \return the immediate check result.
    */
-  bool operator()(const ObjectRef& lhs, const ObjectRef& rhs) const {
-    return handler_->SEqualReduce(lhs, rhs, map_free_vars_);
+  bool operator()(const ObjectRef& lhs, const ObjectRef& rhs) const;
+
+  /*!
+   * \brief Reduce condition to comparison of two objects.
+   *
+   * Like `operator()`, but with an additional `paths` parameter that specifies explicit object
+   * paths for `lhs` and `rhs`. This is useful for implementing SEqualReduce() methods for container
+   * objects like Array and Map, or other custom objects that store nested objects that are not
+   * simply attributes.
+   *
+   * Can only be called when `IsPathTracingEnabled()` is `true`.
+   *
+   * \param lhs The left operand.
+   * \param rhs The right operand.
+   * \param paths Object paths for `lhs` and `rhs`.
+   * \return the immediate check result.
+   */
+  bool operator()(const ObjectRef& lhs, const ObjectRef& rhs, const ObjectPathPair& paths) const {
+    ICHECK(IsPathTracingEnabled()) << "Path tracing must be enabled when calling this function";
+    return ObjectAttrsEqual(lhs, rhs, map_free_vars_, &paths);
   }
+
   /*!
    * \brief Reduce condition to comparison of two definitions,
    *        where free vars can be mapped.
@@ -162,9 +247,8 @@ class SEqualReducer : public BaseValueEqual {
    * \param rhs The right operand.
    * \return the immediate check result.
    */
-  bool DefEqual(const ObjectRef& lhs, const ObjectRef& rhs) {
-    return handler_->SEqualReduce(lhs, rhs, true);
-  }
+  bool DefEqual(const ObjectRef& lhs, const ObjectRef& rhs);
+
   /*!
    * \brief Reduce condition to comparison of two arrays.
    * \param lhs The left operand.
@@ -173,13 +257,20 @@ class SEqualReducer : public BaseValueEqual {
    */
   template <typename T>
   bool operator()(const Array<T>& lhs, const Array<T>& rhs) const {
-    // quick specialization for Array to reduce amount of recursion
-    // depth as array comparison is pretty common.
-    if (lhs.size() != rhs.size()) return false;
-    for (size_t i = 0; i < lhs.size(); ++i) {
-      if (!(operator()(lhs[i], rhs[i]))) return false;
+    if (tracing_data_ == nullptr) {
+      // quick specialization for Array to reduce amount of recursion
+      // depth as array comparison is pretty common.
+      if (lhs.size() != rhs.size()) return false;
+      for (size_t i = 0; i < lhs.size(); ++i) {
+        if (!(operator()(lhs[i], rhs[i]))) return false;
+      }
+      return true;
     }
-    return true;
+
+    // If tracing is enabled, fall back to the regular path
+    const ObjectRef& lhs_obj = lhs;
+    const ObjectRef& rhs_obj = rhs;
+    return (*this)(lhs_obj, rhs_obj);
   }
   /*!
    * \brief Implementation for equality rule of var type objects(e.g. TypeVar, tir::Var).
@@ -198,9 +289,41 @@ class SEqualReducer : public BaseValueEqual {
   /*! \return Get the internal handler. */
   Handler* operator->() const { return handler_; }
 
+  /*! \brief Check if this reducer is tracing paths to the first mismatch. */
+  bool IsPathTracingEnabled() const { return tracing_data_ != nullptr; }
+
+  /*!
+   * \brief Get the paths of the currently compared objects.
+   *
+   * Can only be called when `IsPathTracingEnabled()` is true.
+   */
+  const ObjectPathPair& GetCurrentObjectPaths() const;
+
+  /*!
+   * \brief Specify the object paths of a detected mismatch.
+   *
+   * Can only be called when `IsPathTracingEnabled()` is true.
+   */
+  void RecordMismatchPaths(const ObjectPathPair& paths) const;
+
  private:
+  bool EnumAttrsEqual(int lhs, int rhs, const void* lhs_address, const void* rhs_address) const;
+
+  bool ObjectAttrsEqual(const ObjectRef& lhs, const ObjectRef& rhs, bool map_free_vars,
+                        const ObjectPathPair* paths) const;
+
+  static void GetPathsFromAttrAddressesAndStoreMismatch(const void* lhs_address,
+                                                        const void* rhs_address,
+                                                        const PathTracingData* tracing_data);
+
+  template <typename T>
+  static bool CompareAttributeValues(const T& lhs, const T& rhs,
+                                     const PathTracingData* tracing_data);
+
   /*! \brief Internal class pointer. */
   Handler* handler_;
+  /*! \brief Pointer to the current path tracing context, or nullptr if path tracing is disabled. */
+  const PathTracingData* tracing_data_;
   /*! \brief Whether or not to map free vars. */
   bool map_free_vars_;
 };

python/tvm/ir/base.py

@@ -191,8 +191,8 @@ def structural_equal(lhs, rhs, map_free_vars=False):
         The left operand.
 
     map_free_vars : bool
-        Whether or not shall we map free vars that does
-        not bound to any definitions as equal to each other.
+        Whether free variables (i.e. variables without a definition site) should be mapped
+        as equal to each other.
 
     Return
     ------
@@ -209,6 +209,37 @@ def structural_equal(lhs, rhs, map_free_vars=False):
     return bool(tvm.runtime._ffi_node_api.StructuralEqual(lhs, rhs, False, map_free_vars))
 
 
+def get_first_structural_mismatch(lhs, rhs, map_free_vars=False):
+    """Like structural_equal(), but returns the ObjectPaths of the first detected mismatch.
+
+    Parameters
+    ----------
+    lhs : Object
+        The left operand.
+
+    rhs : Object
+        The left operand.
+
+    map_free_vars : bool
+        Whether free variables (i.e. variables without a definition site) should be mapped
+        as equal to each other.
+
+    Returns
+    -------
+    mismatch: Optional[Tuple[ObjectPath, ObjectPath]]
+        `None` if `lhs` and `rhs` are structurally equal.
+        Otherwise, a tuple of two ObjectPath objects that point to the first detected mismtach.
+    """
+    lhs = tvm.runtime.convert(lhs)
+    rhs = tvm.runtime.convert(rhs)
+    mismatch = tvm.runtime._ffi_node_api.GetFirstStructuralMismatch(lhs, rhs, map_free_vars)
+    if len(mismatch) == 0:
+        return None
+    else:
+        assert len(mismatch) == 2
+        return tuple(mismatch)
+
+
 def assert_structural_equal(lhs, rhs, map_free_vars=False):
     """Assert lhs and rhs are structurally equal to each other.
 

python/tvm/runtime/__init__.py

@@ -19,6 +19,7 @@
 # class exposures
 from .packed_func import PackedFunc
 from .object import Object
+from .object_path import ObjectPath
 from .object_generic import ObjectGeneric, ObjectTypes
 from .ndarray import NDArray, DataType, DataTypeCode, Device
 from .module import Module, num_threads

src/node/reflection.cc

@@ -281,4 +281,48 @@ TVM_REGISTER_GLOBAL("node.NodeGetAttr").set_body(NodeGetAttr);
 TVM_REGISTER_GLOBAL("node.NodeListAttrNames").set_body(NodeListAttrNames);
 
 TVM_REGISTER_GLOBAL("node.MakeNode").set_body(MakeNode);
+
+namespace {
+// Attribute visitor class for finding the attribute key by its address
+class GetAttrKeyByAddressVisitor : public AttrVisitor {
+ public:
+  explicit GetAttrKeyByAddressVisitor(const void* attr_address)
+      : attr_address_(attr_address), key_(nullptr) {}
+
+  void Visit(const char* key, double* value) final { DoVisit(key, value); }
+  void Visit(const char* key, int64_t* value) final { DoVisit(key, value); }
+  void Visit(const char* key, uint64_t* value) final { DoVisit(key, value); }
+  void Visit(const char* key, int* value) final { DoVisit(key, value); }
+  void Visit(const char* key, bool* value) final { DoVisit(key, value); }
+  void Visit(const char* key, std::string* value) final { DoVisit(key, value); }
+  void Visit(const char* key, void** value) final { DoVisit(key, value); }
+  void Visit(const char* key, DataType* value) final { DoVisit(key, value); }
+  void Visit(const char* key, runtime::NDArray* value) final { DoVisit(key, value); }
+  void Visit(const char* key, runtime::ObjectRef* value) final { DoVisit(key, value); }
+
+  const char* GetKey() const { return key_; }
+
+ private:
+  const void* attr_address_;
+  const char* key_;
+
+  void DoVisit(const char* key, const void* candidate) {
+    if (attr_address_ == candidate) {
+      key_ = key;
+    }
+  }
+};
+}  // anonymous namespace
+
+Optional<String> GetAttrKeyByAddress(const Object* object, const void* attr_address) {
+  GetAttrKeyByAddressVisitor visitor(attr_address);
+  ReflectionVTable::Global()->VisitAttrs(const_cast<Object*>(object), &visitor);
+  const char* key = visitor.GetKey();
+  if (key == nullptr) {
+    return NullOpt;
+  } else {
+    return String(key);
+  }
+}
+
 }  // namespace tvm