diff --git a/libjulia/optimiser/CommonSubexpressionEliminator.cpp b/libjulia/optimiser/CommonSubexpressionEliminator.cpp
new file mode 100644
index 000000000..229bd35eb
--- /dev/null
+++ b/libjulia/optimiser/CommonSubexpressionEliminator.cpp
@@ -0,0 +1,48 @@
+/*(
+ This file is part of solidity.
+
+ solidity is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ solidity is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with solidity. If not, see .
+*/
+/**
+ * Optimisation stage that replaces expressions known to be the current value of a variable
+ * in scope by a reference to that variable.
+ */
+
+#include
+
+#include
+#include
+
+#include
+
+using namespace std;
+using namespace dev;
+using namespace dev::julia;
+
+void CommonSubexpressionEliminator::visit(Expression& _e)
+{
+ // Single exception for substitution: We do not substitute one variable for another.
+ if (_e.type() != typeid(Identifier))
+ // TODO this search rather inefficient.
+ for (auto const& var: m_value)
+ {
+ solAssert(var.second, "");
+ if (SyntacticalEqualityChecker::equal(_e, *var.second))
+ {
+ _e = Identifier{locationOf(_e), var.first};
+ break;
+ }
+ }
+ DataFlowAnalyzer::visit(_e);
+}
diff --git a/libjulia/optimiser/CommonSubexpressionEliminator.h b/libjulia/optimiser/CommonSubexpressionEliminator.h
new file mode 100644
index 000000000..a8ca3abba
--- /dev/null
+++ b/libjulia/optimiser/CommonSubexpressionEliminator.h
@@ -0,0 +1,45 @@
+/*
+ This file is part of solidity.
+
+ solidity is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ solidity is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with solidity. If not, see .
+*/
+/**
+ * Optimisation stage that replaces expressions known to be the current value of a variable
+ * in scope by a reference to that variable.
+ */
+
+#pragma once
+
+#include
+
+namespace dev
+{
+namespace julia
+{
+
+/**
+ * Optimisation stage that replaces expressions known to be the current value of a variable
+ * in scope by a reference to that variable.
+ *
+ * Prerequisite: Disambiguator
+ */
+class CommonSubexpressionEliminator: public DataFlowAnalyzer
+{
+protected:
+ using ASTModifier::visit;
+ virtual void visit(Expression& _e) override;
+};
+
+}
+}
diff --git a/test/libjulia/CommonSubexpression.cpp b/test/libjulia/CommonSubexpression.cpp
new file mode 100644
index 000000000..8a575c48b
--- /dev/null
+++ b/test/libjulia/CommonSubexpression.cpp
@@ -0,0 +1,102 @@
+/*
+ This file is part of solidity.
+
+ solidity is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ solidity is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with solidity. If not, see .
+*/
+/**
+ * Unit tests for the common subexpression eliminator optimizer stage.
+ */
+
+#include
+
+#include
+
+#include
+
+#include
+
+#include
+#include
+
+using namespace std;
+using namespace dev;
+using namespace dev::julia;
+using namespace dev::julia::test;
+using namespace dev::solidity;
+
+
+#define CHECK(_original, _expectation)\
+do\
+{\
+ assembly::AsmPrinter p;\
+ Block b = disambiguate(_original, false);\
+ (CommonSubexpressionEliminator{})(b);\
+ string result = p(b);\
+ BOOST_CHECK_EQUAL(result, format(_expectation, false));\
+}\
+while(false)
+
+BOOST_AUTO_TEST_SUITE(IuliaCSE)
+
+BOOST_AUTO_TEST_CASE(smoke_test)
+{
+ CHECK("{ }", "{ }");
+}
+
+BOOST_AUTO_TEST_CASE(trivial)
+{
+ CHECK(
+ "{ let a := mul(1, codesize()) let b := mul(1, codesize()) }",
+ "{ let a := mul(1, codesize()) let b := a }"
+ );
+}
+
+BOOST_AUTO_TEST_CASE(non_movable_instr)
+{
+ CHECK(
+ "{ let a := mload(1) let b := mload(1) }",
+ "{ let a := mload(1) let b := mload(1) }"
+ );
+}
+
+BOOST_AUTO_TEST_CASE(non_movable_instr2)
+{
+ CHECK(
+ "{ let a := gas() let b := gas() }",
+ "{ let a := gas() let b := gas() }"
+ );
+}
+
+BOOST_AUTO_TEST_CASE(branches_if)
+{
+ CHECK(
+ "{ let b := 1 if b { b := 1 } let c := 1 }",
+ "{ let b := 1 if b { b := b } let c := 1 }"
+ );
+}
+
+BOOST_AUTO_TEST_CASE(branches_for)
+{
+ CHECK(
+ "{ let a := 1 let b := codesize()"
+ "for { } lt(1, codesize()) { mstore(1, codesize()) a := add(a, codesize()) }"
+ "{ mstore(1, codesize()) } mstore(1, codesize()) }",
+
+ "{ let a := 1 let b := codesize()"
+ "for { } lt(1, b) { mstore(1, b) a := add(a, b) }"
+ "{ mstore(1, b) } mstore(1, b) }"
+ );
+}
+
+BOOST_AUTO_TEST_SUITE_END()