[Relay] Added Merge Composite pass (apache#4771)

* [Relay] Added MergeComposite pass

This pass allows for patterns to be wrapped
in a function marked with 'Composite' and a
composite function name. This is intended to be
used with the external codegen for the cases where
an external operator maps to multiple Relay
operators. In that case, the mapping can be expressed
as a pattern and assigned a name.

For more information on this pass and its motivation,
see the RFC:
https://discuss.tvm.ai/t/rfc-external-codegen-defining-composite-relay-operators/5470

Change-Id: Icb1b803a9f0ac57c529143200228f3bb5793afc0

* [Relay] Merge composite tests

Added tests for the merge_composite pass.

Change-Id: I1728b4a05b0c1c36140a40f1afe028fde62185dd

* Merge composite additional test

Change-Id: I9bc7d6053c575e9468ac5abc31214c6ad8507e46

* Support priority order in merge_composite

The order in which the patterns are matched
was currently random as an unordered_map was
used to store the pattern table. This uses
arrays instead so that a distinct priority
order of matching can be defined. Additional
tests have also been added to verify this
behaviour.

Change-Id: Ief347df4262639138d5d9d7c8cee7ef233af7b56

* Improved merge composite docs

Change-Id: Ie3a72045ecc3f13ad3c302fbdf192b7296a306a8

* Removed unused variable

Change-Id: I7814d5fde368ffaf1b3d6d806060c774c7720364

* Remove unnecessary op check

Change-Id: I38e78d2acd5b86cb8e837be72ff9d72cd10bcf33

* Improve styling on composite function creation

Change-Id: I37add1c3134e0b5d5085fe1eb9daf8e06890fa8c

* Comment reword

Change-Id: Ie05872dcbbe0c3e1190b0597083b9a64e6b66c66

* Stylistic changes to avoid std::move

Change-Id: I43a93995bbf10530399900c992aa99dd4ae4575f

* Relax a check in ExtractPattern

Change-Id: I0faef77a66c55f83f09e6e47c561ffaea63dedfa

* Remove new line

Change-Id: Ifdd02c12087a7e1a0a9b54825669bc0de8f13c3d

* Removed MatchPattern from MergeComposite

This is not necessary now that ExtractPattern
can fulfill the same purpose.

Change-Id: I14dc020afa8e50f2df4c0a2efb88a011987f8196

* Removed a new line

Change-Id: I8b50f0c9069aa1bcaccbe68eb421031f01a64842

* Improved docs for merge composite

Change-Id: Ib1959a35c856e7ea5639de2e4ef314a54f44caf5

* Fixed free vars in test

Change-Id: I2b7f273db275964ec0e9820560663f0808adee79

* Handle case where root arg might not be a call

Change-Id: I4eeea3ce723d3ba337d110dcc690377daebe8626

* Removed blank line

Change-Id: I07f5392c0e95cfe3cfa5c333703cc6f82d6034fb

* Change to CHECK_EQ

Change-Id: I5c5d62d3cd57f72508b30b926f72091ae6f0d1cc

* Revised a conditional

Change-Id: I23a7897ca15a7cd076db5039dc653a4b8c27e803

* Improved doc styling

Change-Id: I377f0a1c1ac70f3b8d7584b0c49bddc8c6c134ef

* Fail extraction if vars conflict

Change-Id: I78e36d805e8ed6b55e61d490212a967c857554a4

* Added further merge composite tests

Change-Id: Ib1d800409fca4c1834c7fe0cab5a26ab99a26820

Co-authored-by: lhutton1 <35535092+lhutton1@users.noreply.github.com>

include/tvm/relay/expr.h

@@ -561,6 +561,8 @@ constexpr const char* kParams = "__params__";
 constexpr const char* kExternalSymbol = "ExternalSymbol";
 /*! \brief Mark if the function should be avoided being optimized. */
 constexpr const char* kSkipOptimization = "SkipOptimization";
+/*! \brief Treat the function as a composite operator. */
+constexpr const char* kComposite = "Composite";
 }  // namespace attr
 
 }  // namespace relay

python/tvm/relay/transform.py

@@ -513,6 +513,31 @@ def Legalize(legalize_map_attr_name="FTVMLegalize"):
     return _transform.Legalize(legalize_map_attr_name)
 
 
+def MergeComposite(pattern_table):
+    """Merge multiple operators into a single composite relay function.
+
+    Parameters
+    ----------
+    pattern_table : list(tuple)
+        A list of (pattern_name, pattern) tuples.
+        The order of the patterns in the list will determine the order
+        of priority in which they are matched.
+
+    Returns
+    -------
+    ret : tvm.relay.Pass
+        The registered pass that merges operators into a single composite
+        relay function.
+    """
+    pattern_names = []
+    patterns = []
+    for pattern_name, pattern in pattern_table:
+        pattern_names.append(pattern_name)
+        patterns.append(pattern)
+
+    return _transform.MergeComposite(pattern_names, patterns)
+
+
 def RewriteAnnotatedOps(fallback_device):
     """Rewrite the annotated program where annotation operators, e.g.
     `on_deivce`, mark which device an expression should be scheduled to.

src/relay/pass/merge_composite.cc

@@ -0,0 +1,218 @@
+/*
+ * 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 src/relay/pass/merge_composite.cc
+ * \brief Merges expressions matching patterns into functions marked
+ * as 'composite'. This is primarily intended to be used alongside the
+ * external codegen infrastructure to support the case where multiple
+ * Relay operators map to a single external operator.
+ */
+
+#include <tvm/te/operation.h>
+#include <tvm/relay/analysis.h>
+#include <tvm/relay/expr_functor.h>
+#include <tvm/relay/op_attr_types.h>
+#include <tvm/relay/transform.h>
+
+namespace tvm {
+namespace relay {
+namespace merge_composite {
+
+class MergeCompositeWrapper : public ExprMutator {
+ public:
+  explicit MergeCompositeWrapper(const std::string& pattern_name, const Expr& pattern)
+    : pattern_name_(pattern_name), pattern_(pattern) {}
+
+  Expr ExtractPattern(const Var& pattern, const Expr& root,
+          Map<std::string, Array<Expr>>* var_map) {
+    if (var_map->find(pattern->name_hint()) == var_map->end()) {
+      // if we haven't encountered this var yet, make a new free var and associate
+      // it with the value at 'root'
+      auto free_var = VarNode::make(pattern->name_hint(), Type());
+      var_map->Set(pattern->name_hint(), Array<Expr>({free_var, root}));
+      return std::move(free_var);
+    } else {
+      // if we have encountered this var already, return the free var that was created
+      auto vars = (*var_map)[pattern->name_hint()];
+      auto free_var = vars[0];
+      auto graph_expr = vars[1];
+      // make sure to first check they both map to the same node in the graph
+      if (graph_expr != root) {
+        return Expr();
+      }
+      return (*var_map)[pattern->name_hint()][0];
+    }
+  }
+
+  Expr ExtractPattern(const Constant& pattern, const Expr& root,
+          Map<std::string, Array<Expr>>* var_map) {
+    return root;
+  }
+
+  /*!
+   * \brief Try and extract a given pattern from a graph as a subgraph.
+   * \param pattern The pattern to extract.
+   * \param root The graph to extract from.
+   * \param var_map A map between free vars in the subgraph and nodes in the graph.
+   * \return The extracted subgraph.
+   *
+   * \note How does this work?
+   *
+   * A pattern consists of Relay expression containing only operator call nodes, constants
+   * and free variables. The free variables indicate where the pattern can 'attach' in your
+   * graph. This function takes the final call node of the pattern and the call node currently
+   * being traversed in the Relay graph. It traverses through the pattern in lockstep with call node
+   * from the graph (referred to as the 'root' node here) to check they're identical. If at any point
+   * they differ, an empty expression is returned to signify the extract failed. If a free var is
+   * reached in the pattern, the corresponding value in the root is associated with the name of the
+   * free var (via the var_map) so that when we construct the composite function, the inputs match
+   * up correctly with the rest of the graph. The return value of this function when successful is
+   * a new Relay expression ready to be wrapped into a composite function.
+   */
+  Expr ExtractPattern(const Call& pattern, const Call& root,
+          Map<std::string, Array<Expr>>* var_map) {
+    // check to make sure both calls are to operators (not functions)
+    if (!pattern->op->IsInstance<OpNode>() || !root->op->IsInstance<OpNode>())
+      return Expr();
+    if (pattern->op.as<OpNode>()->name != root->op.as<OpNode>()->name)
+      return Expr();
+
+    unsigned int i = 0;
+    Array<Expr> new_args;
+    for (const auto& arg : pattern->args) {
+      Expr new_arg;
+      if (arg->IsInstance<CallNode>()) {
+        // fail if the root argument is not also a call node
+        if (!root->args[i]->IsInstance<CallNode>()) {
+          return Expr();
+        }
+        // if it's a call node, recursively call this function
+        new_arg = ExtractPattern(Downcast<Call>(arg),
+                                 Downcast<Call>(root->args[i]),
+                                 var_map);
+      } else if (arg->IsInstance<VarNode>()) {
+        // if there's a var in the pattern, it must be a free var
+        // so call the function to update the var_map
+        new_arg = ExtractPattern(Downcast<Var>(arg),
+                                 root->args[i],
+                                 var_map);
+      } else if (arg->IsInstance<ConstantNode>()) {
+        // if there's a constant, simply get the corresponding
+        // value of the constant from the root
+        new_arg = ExtractPattern(Downcast<Constant>(arg),
+                                 root->args[i],
+                                 var_map);
+      }
+      if (!new_arg.defined()) {
+        return Expr();
+      }
+      new_args.push_back(new_arg);
+      i++;
+    }
+    return CallNode::make(root->op, new_args, root->attrs);
+  }
+
+  Expr VisitExpr_(const CallNode* cn) {
+    Call call = GetRef<Call>(cn);
+    if (call->op->IsInstance<FunctionNode>()) {
+      Function func = Downcast<Function>(call->op);
+      CHECK(func.defined());
+      const auto name_node = FunctionGetAttr(func, attr::kComposite).as<tir::StringImmNode>();
+      // don't step into existing composite functions
+      if (name_node && name_node->value != "") {
+        tvm::Array<tvm::relay::Expr> new_args;
+        for (const auto& arg : call->args) {
+          auto new_e = this->Mutate(arg);
+          new_args.push_back(new_e);
+        }
+        return CallNode::make(call->op, new_args, call->attrs);
+      }
+    }
+
+    Expr expr = ExprMutator::VisitExpr_(cn);
+    call = Downcast<Call>(expr);
+    if (!call->op->IsInstance<OpNode>())
+      return std::move(call);
+
+    // only call patterns are supported
+    Call pattern = Downcast<Call>(pattern_);
+    CHECK(pattern.defined());
+    Map<std::string, Array<Expr>> args_map;
+    auto extract = ExtractPattern(pattern, call, &args_map);
+    if (extract.defined()) {
+      auto free_vars = FreeVars(extract);
+      // make the composite function
+      auto f = FunctionNode::make(free_vars, extract, call->checked_type_, {}, Attrs());
+      f = FunctionSetAttr(f, attr::kComposite, tir::StringImmNode::make(pattern_name_));
+      f = FunctionSetAttr(f, attr::kPrimitive, tvm::Integer(1));
+      // find the expressions associated with the free vars using the args_map
+      // this tells us which expressions should be given as inputs to the composite function
+      Array<Expr> args;
+      for (const auto& free_var : free_vars) {
+        args.push_back(args_map[free_var->name_hint()][1]);
+      }
+      auto new_call = CallNode::make(f, args);
+      return std::move(new_call);
+    }
+    return std::move(call);
+  }
+
+ private:
+  /*! \brief The name of the pattern to match */
+  std::string pattern_name_;
+  /*! \brief The pattern to match */
+  Expr pattern_;
+};
+
+Expr MergeComposite(const Expr& expr,
+    const Array<tir::StringImm>& pattern_names, const Array<Expr>& patterns) {
+  CHECK_EQ(pattern_names.size(), patterns.size());
+  Expr merged_expr = expr;
+  // merge the patterns one-by-one in order
+  for (size_t i = 0; i < patterns.size(); i++) {
+    std::string pattern_name = pattern_names[i]->value;
+    Expr pattern = patterns[i];
+    merged_expr = MergeCompositeWrapper(pattern_name, pattern).Mutate(merged_expr);
+  }
+  return merged_expr;
+}
+
+}  // namespace merge_composite
+
+namespace transform {
+
+Pass MergeComposite(const tvm::Array<tir::StringImm>& pattern_names,
+    const tvm::Array<Expr>& patterns) {
+  runtime::TypedPackedFunc<Function(Function, IRModule, PassContext)> pass_func =
+      [=](Function f, IRModule m, PassContext pc) {
+        return Downcast<Function>(
+            relay::merge_composite::MergeComposite(f, pattern_names, patterns));
+      };
+  auto func_pass = CreateFunctionPass(pass_func, 0, "MergeComposite", {});
+  return func_pass;
+}
+
+TVM_REGISTER_GLOBAL("relay._transform.MergeComposite")
+.set_body_typed(MergeComposite);
+
+}  // namespace transform
+
+}  // namespace relay
+}  // namespace tvm