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diff --git a/src/ast/hashed.h b/src/ast/hashed.h
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+++ b/src/ast/hashed.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_hashed_h
+
+#include "support/hash.h"
+#include "wasm.h"
+#include "ast_utils.h"
+
+namespace wasm {
+
+// An expression with a cached hash value
+struct HashedExpression {
+  Expression* expr;
+  size_t hash;
+
+  HashedExpression(Expression* expr) : expr(expr) {
+    if (expr) {
+      hash = ExpressionAnalyzer::hash(expr);
+    }
+  }
+
+  HashedExpression(const HashedExpression& other) : expr(other.expr), hash(other.hash) {}
+};
+
+struct ExpressionHasher {
+  size_t operator()(const HashedExpression value) const {
+    return value.hash;
+  }
+};
+
+struct ExpressionComparer {
+  bool operator()(const HashedExpression a, const HashedExpression b) const {
+    if (a.hash != b.hash) return false;
+    return ExpressionAnalyzer::equal(a.expr, b.expr);
+  }
+};
+
+template<typename T>
+class HashedExpressionMap : public std::unordered_map<HashedExpression, T, ExpressionHasher, ExpressionComparer> {
+};
+
+} // namespace wasm
+
+#endif // _wasm_ast_hashed_h
+