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diff --git a/src/ast/local-graph.h b/src/ast/local-graph.h
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+/*
+ * Copyright 2017 WebAssembly Community Group participants
+ *
+ * Licensed 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.
+ */
+
+#ifndef wasm_ast_local_graph_h
+#define wasm_ast_local_graph_h
+
+namespace wasm {
+
+//
+// Finds the connections between get_locals and set_locals, creating
+// a graph of those ties. This is useful for "ssa-style" optimization,
+// in which you want to know exactly which sets are relevant for a
+// a get, so it is as if each get has just one set, logically speaking
+// (see the SSA pass for actually creating new local indexes based
+// on this).
+//
+// TODO: the algorithm here is pretty simple, but also pretty slow,
+//       we should optimize it. e.g. we rely on set_interaction
+//       here, and worse we only use it to compute the size...
+struct LocalGraph : public PostWalker<LocalGraph> {
+  // main API
+
+  // the constructor computes getSetses, the sets affecting each get
+  LocalGraph(Function* func, Module* module);
+
+  // the set_locals relevant for an index or a get.
+  typedef std::set<SetLocal*> Sets;
+
+  // externally useful information
+  std::map<GetLocal*, Sets> getSetses; // the sets affecting each get. a nullptr set means the initial
+                                       // value (0 for a var, the received value for a param)
+  std::map<Expression*, Expression**> locations; // where each get and set is (for easy replacing)
+
+  // optional computation: compute the influence graphs between sets and gets
+  // (useful for algorithms that propagate changes)
+
+  std::unordered_map<GetLocal*, std::unordered_set<SetLocal*>> getInfluences; // for each get, the sets whose values are influenced by that get
+  std::unordered_map<SetLocal*, std::unordered_set<GetLocal*>> setInfluences; // for each set, the gets whose values are influenced by that set
+
+  void computeInfluences();
+
+private:
+  // we map local index => the set_locals for that index.
+  // a nullptr set means there is a virtual set, from a param
+  // initial value or the zero init initial value.
+  typedef std::vector<Sets> Mapping;
+
+  // internal state
+  Index numLocals;
+  Mapping currMapping;
+  std::vector<Mapping> mappingStack; // used in ifs, loops
+  std::map<Name, std::vector<Mapping>> breakMappings; // break target => infos that reach it
+  std::vector<std::vector<GetLocal*>> loopGetStack; //  stack of loops, all the gets in each, so we can update them for back branches
+
+public:
+  void doWalkFunction(Function* func);
+
+  // control flow
+
+  void visitBlock(Block* curr);
+
+  void finishIf();
+
+  static void afterIfCondition(LocalGraph* self, Expression** currp);
+  static void afterIfTrue(LocalGraph* self, Expression** currp);
+  static void afterIfFalse(LocalGraph* self, Expression** currp);
+  static void beforeLoop(LocalGraph* self, Expression** currp);
+  void visitLoop(Loop* curr);
+  void visitBreak(Break* curr);
+  void visitSwitch(Switch* curr);
+  void visitReturn(Return *curr);
+  void visitUnreachable(Unreachable *curr);
+
+  // local usage
+
+  void visitGetLocal(GetLocal* curr);
+  void visitSetLocal(SetLocal* curr);
+
+  // traversal
+
+  static void scan(LocalGraph* self, Expression** currp);
+
+  // helpers
+
+  void setUnreachable(Mapping& mapping);
+
+  bool isUnreachable(Mapping& mapping);
+
+  // merges a bunch of infos into one.
+  // if we need phis, writes them into the provided vector. the caller should
+  // ensure those are placed in the right location
+  Mapping& merge(std::vector<Mapping>& mappings);
+};
+
+} // namespace wasm
+
+#endif // wasm_ast_local_graph_h
+