[GitHub] [tvm] jwfromm commented on a diff in pull request #14262: [Unity][Transform] Simple Dead Code Elimination

["jwfromm (via GitHub)" <gi...@apache.org>]

jwfromm commented on code in PR #14262:
URL: https://github.com/apache/tvm/pull/14262#discussion_r1133143457


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src/relax/transform/dead_code_elimination.cc:
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@@ -0,0 +1,104 @@
+/*
+ * 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/relax/transform/dead_code_elimination.cc
+ * \brief Dead code elimination pass.
+ * Currently it removes:
+ *   1. Unused local VarBindings in a DataflowBlock.
+ */
+
+#include <tvm/relax/expr.h>
+#include <tvm/relax/expr_functor.h>
+#include <tvm/relax/transform.h>
+
+namespace tvm {
+namespace relax {
+
+class DeadCodeEliminator : public ExprMutator {
+ private:
+  Expr VisitExpr_(const VarNode* op) final {
+    used_vars_.insert(GetRef<Var>(op));
+    return GetRef<Expr>(op);
+  }
+
+  Expr VisitExpr_(const DataflowVarNode* op) final {
+    used_vars_.insert(GetRef<Var>(op));
+    return GetRef<Expr>(op);
+  }
+
+  void VisitBinding_(const VarBindingNode* binding) { this->VisitExpr(binding->value); }
+
+  void VisitBinding_(const MatchCastNode* binding) {
+    this->VisitExpr(binding->value);
+    this->VisitAndCheckStructInfoFieldUnchanged(binding->struct_info);
+  }
+
+  BindingBlock VisitBindingBlock_(const DataflowBlockNode* block) final {
+    // reverse scan the data flow block to find the used vars
+    used_vars_.clear();
+    std::vector<Binding> new_bindings;
+    for (auto rit = block->bindings.rbegin(); rit != block->bindings.rend(); rit++) {

Review Comment:
   I think this bit could use more explanation since it's making a lot of assumptions about the structure and ordering of visiting bindings that aren't obvious to the reader. For example, since we've emptied `used_vars`, the check below is assuming the first visited nodes won't be `DataFlowVarNode`, but what are they instead?



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