Also apply simplification rules that require multiple identical sub-expressions.

libjulia/optimiser/ExpressionSimplifier.cpp

@@ -40,6 +40,7 @@ void ExpressionSimplifier::visit(Expression& _expression)
 	ASTModifier::visit(_expression);
 	while (auto match = SimplificationRules::findFirstMatch(_expression))
 	{
+		// Do not apply the rule if it removes non-constant parts of the expression.
 		// TODO: The check could actually be less strict than "movable".
 		// We only require "Does not cause side-effects".
 		if (std::get<2>(*match) && !MovableChecker(_expression).movable())

libjulia/optimiser/SimplificationRules.cpp

@@ -22,6 +22,8 @@
 
 #include <libjulia/optimiser/Utilities.h>
 #include <libjulia/optimiser/ASTCopier.h>
+#include <libjulia/optimiser/Semantics.h>
+#include <libjulia/optimiser/SyntacticalEquality.h>
 
 #include <libsolidity/inlineasm/AsmData.h>
 
@@ -121,11 +123,19 @@ bool Pattern::matches(Expression const& _expr) const
 	{
 		assertThrow(m_arguments.empty(), OptimizerException, "");
 	}
-	// We do not support matching multiple expressions that require the same value.
+	// We support matching multiple expressions that require the same value
+	// based on identical ASTs, which have to be movable.
 	if (m_matchGroup)
 	{
 		if (m_matchGroups->count(m_matchGroup))
-			return false;
+		{
+			Expression const* firstMatch = (*m_matchGroups)[m_matchGroup];
+			assertThrow(firstMatch, OptimizerException, "Match set but to null.");
+			return
+				SyntacticalEqualityChecker::equal(*firstMatch, _expr) &&
+				MovableChecker(_expr).movable();
+		}
+		else
 			(*m_matchGroups)[m_matchGroup] = &_expr;
 	}
 	return true;

libjulia/optimiser/SyntacticalEquality.cpp

@@ -0,0 +1,75 @@
+/*(
+	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 <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Component that can compare ASTs for equality on a syntactic basis.
+ */
+
+#include <libjulia/optimiser/SyntacticalEquality.h>
+
+#include <libsolidity/inlineasm/AsmData.h>
+#include <libsolidity/interface/Exceptions.h>
+
+#include <libdevcore/CommonData.h>
+
+using namespace std;
+using namespace dev;
+using namespace dev::julia;
+
+bool SyntacticalEqualityChecker::equal(Expression const& _e1, Expression const& _e2)
+{
+	if (_e1.type() != _e2.type())
+		return false;
+
+	// TODO This should be replaced by some kind of AST walker as soon as it gets
+	// more complex.
+	if (_e1.type() == typeid(FunctionalInstruction))
+	{
+		auto const& e1 = boost::get<FunctionalInstruction>(_e1);
+		auto const& e2 = boost::get<FunctionalInstruction>(_e2);
+		return
+			e1.instruction == e2.instruction &&
+			equalVector(e1.arguments, e2.arguments);
+	}
+	else if (_e1.type() == typeid(FunctionCall))
+	{
+		auto const& e1 = boost::get<FunctionCall>(_e1);
+		auto const& e2 = boost::get<FunctionCall>(_e2);
+		return
+			equal(e1.functionName, e2.functionName) &&
+			equalVector(e1.arguments, e2.arguments);
+	}
+	else if (_e1.type() == typeid(Identifier))
+		return boost::get<Identifier>(_e1).name == boost::get<Identifier>(_e2).name;
+	else if (_e1.type() == typeid(Literal))
+	{
+		auto const& e1 = boost::get<Literal>(_e1);
+		auto const& e2 = boost::get<Literal>(_e2);
+		return e1.kind == e2.kind && e1.value == e2.value && e1.type == e2.type;
+	}
+	else
+	{
+		solAssert(false, "Invlid expression");
+	}
+	return false;
+}
+
+bool SyntacticalEqualityChecker::equalVector(vector<Expression> const& _e1, vector<Expression> const& _e2)
+{
+	return _e1.size() == _e2.size() &&
+		std::equal(begin(_e1), end(_e1), begin(_e2), SyntacticalEqualityChecker::equal);
+
+}

libjulia/optimiser/SyntacticalEquality.h

@@ -0,0 +1,50 @@
+/*
+	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 <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Component that can compare ASTs for equality on a syntactic basis.
+ */
+
+#pragma once
+
+#include <libjulia/ASTDataForward.h>
+
+#include <vector>
+
+namespace dev
+{
+namespace julia
+{
+
+/**
+ * Component that can compare ASTs for equality on a syntactic basis.
+ * Ignores source locations but requires exact matches otherwise.
+ *
+ * TODO: Only implemented for Expressions for now.
+ * A future version might also recognize renamed variables and thus could be used to
+ * remove duplicate functions.
+ */
+class SyntacticalEqualityChecker
+{
+public:
+	static bool equal(Expression const& _e1, Expression const& _e2);
+
+protected:
+	static bool equalVector(std::vector<Expression> const& _e1, std::vector<Expression> const& _e2);
+};
+
+}
+}

test/libjulia/Simplifier.cpp

@@ -87,6 +87,30 @@ BOOST_AUTO_TEST_CASE(constant_propagation)
 	);
 }
 
+BOOST_AUTO_TEST_CASE(identity_rules_simple)
+{
+	CHECK(
+		"{ let a := mload(0) let b := sub(a, a) }",
+		"{ let a := mload(0) let b := 0 }"
+	);
+}
+
+BOOST_AUTO_TEST_CASE(identity_rules_complex)
+{
+	CHECK(
+		"{ let a := sub(calldataload(0), calldataload(0)) }",
+		"{ let a := 0 }"
+	);
+}
+
+BOOST_AUTO_TEST_CASE(identity_rules_negative)
+{
+	CHECK(
+		"{ let a := sub(calldataload(1), calldataload(0)) }",
+		"{ let a := sub(calldataload(1), calldataload(0)) }"
+	);
+}
+
 BOOST_AUTO_TEST_CASE(including_function_calls)
 {
 	CHECK(