Redundant assign eliminator.

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diff --git a/libyul/optimiser/RedundantAssignEliminator.h b/libyul/optimiser/RedundantAssignEliminator.h
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+/*
+	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/>.
+*/
+/**
+ * Optimiser component that removes assignments to variables that are not used
+ * until they go out of scope or are re-assigned.
+ */
+
+#pragma once
+
+#include <libyul/ASTDataForward.h>
+
+#include <libyul/optimiser/ASTWalker.h>
+
+#include <map>
+
+namespace dev
+{
+namespace yul
+{
+
+/**
+ * Optimiser component that removes assignments to variables that are not used
+ * until they go out of scope or are re-assigned. This component
+ * respects the control-flow and takes it into account for removal.
+ *
+ * Example:
+ *
+ * {
+ *   let a
+ *   a := 1
+ *   a := 2
+ *   b := 2
+ *   if calldataload(0)
+ *   {
+ *     b := mload(a)
+ *   }
+ *   a := b
+ * }
+ *
+ * In the example, "a := 1" can be removed because the value from this assignment
+ * is not used in any control-flow branch (it is replaced right away).
+ * The assignment "a := 2" is also overwritten by "a := b" at the end,
+ * but there is a control-flow path (through the condition body) which uses
+ * the value from "a := 2" and thus, this assignment cannot be removed.
+ *
+ * Detailed rules:
+ *
+ * The AST is traversed twice: in an information gathering step and in the
+ * actual removal step. During information gathering, we maintain a
+ * mapping from assignment statements to the three states
+ * "unused", "undecided" and "used".
+ * When an assignment is visited, it is added to the mapping in the "undecided" state
+ * (see remark about for loops below) and every other assignment to the same variable
+ * that is still in the "undecided" state is changed to "unused".
+ * When a variable is referenced, the state of any assignment to that variable still
+ * in the "undecided" state is changed to "used".
+ * At points where control flow splits, a copy
+ * of the mapping is handed over to each branch. At points where control flow
+ * joins, the two mappings coming from the two branches are combined in the following way:
+ * Statements that are only in one mapping or have the same state are used unchanged.
+ * Conflicting values are resolved in the following way:
+ * "unused", "undecided" -> "undecided"
+ * "unused", "used" -> "used"
+ * "undecided, "used" -> "used".
+ *
+ * For for-loops, the condition, body and post-part are visited twice, taking
+ * the joining control-flow at the condition into account.
+ * In other words, we create three control flow paths: Zero runs of the loop,
+ * one run and two runs and then combine them at the end.
+ * Running at most twice is enough because there are only three different states.
+ *
+ * For switch statements that have a "default"-case, there is no control-flow
+ * part that skips the switch.
+ *
+ * When a variable goes out of scope, all statements still in the "undecided"
+ * state are changed to "unused", unless the variable is the return
+ * parameter of a function - there, the state changes to "used".
+ *
+ * In the second traversal, all assignments that are in the "unused" state are removed.
+ *
+ *
+ * This step is usually run right after the SSA transform to complete
+ * the generation of the pseudo-SSA.
+ *
+ * Prerequisite: Disambiguator.
+ */
+class RedundantAssignEliminator: public ASTWalker
+{
+public:
+	RedundantAssignEliminator(RedundantAssignEliminator const&) = default;
+	RedundantAssignEliminator& operator=(RedundantAssignEliminator const&) = default;
+	RedundantAssignEliminator(RedundantAssignEliminator&&) = default;
+	RedundantAssignEliminator& operator=(RedundantAssignEliminator&&) = default;
+
+	void operator()(Identifier const& _identifier) override;
+	void operator()(VariableDeclaration const& _variableDeclaration) override;
+	void operator()(Assignment const& _assignment) override;
+	void operator()(If const& _if) override;
+	void operator()(Switch const& _switch) override;
+	void operator()(FunctionDefinition const&) override;
+	void operator()(ForLoop const&) override;
+	void operator()(Block const& _block) override;
+
+	static void run(Block& _ast);
+
+private:
+	RedundantAssignEliminator() {}
+
+	class State
+	{
+	public:
+		enum Value { Unused, Undecided, Used };
+		State(Value _value = Undecided): m_value(_value) {}
+		bool operator==(State _other) const { return m_value == _other.m_value; }
+		bool operator!=(State _other) const { return !operator==(_other); }
+		void join(State _other)
+		{
+			// Using "max" works here because of the order of the values in the enum.
+			m_value = Value(std::max(int(_other.m_value), int(m_value)));
+		}
+	private:
+		Value m_value = Undecided;
+	};
+
+	/**
+	 * Takes care about storing the list of declared variables and
+	 * sets them to "unused" when it is destroyed.
+	 */
+	class BlockScope
+	{
+	public:
+		explicit BlockScope(RedundantAssignEliminator& _rae): m_rae(_rae)
+		{
+			swap(m_rae.m_declaredVariables, m_outerDeclaredVariables);
+		}
+		~BlockScope()
+		{
+			for (auto const& var: m_rae.m_declaredVariables)
+				m_rae.changeUndecidedTo(var, State::Unused);
+			swap(m_rae.m_declaredVariables, m_outerDeclaredVariables);
+		}
+
+	private:
+		RedundantAssignEliminator& m_rae;
+		std::set<std::string> m_outerDeclaredVariables;
+	};
+
+	/// Joins the assignment mapping with @a _other according to the rules laid out
+	/// above.
+	/// Will destroy @a _other.
+	void join(RedundantAssignEliminator& _other);
+	void changeUndecidedTo(std::string const& _variable, State _newState);
+
+	std::set<std::string> m_declaredVariables;
+	std::map<std::string, std::map<Assignment const*, State>> m_assignments;
+};
+
+class AssignmentRemover: public ASTModifier
+{
+public:
+	explicit AssignmentRemover(std::set<Assignment const*> const& _toRemove):
+		m_toRemove(_toRemove)
+	{}
+	void operator()(Block& _block) override;
+
+private:
+	std::set<Assignment const*> const& m_toRemove;
+};
+
+}
+}