Add SSAReverser to the yul optimiser.

libyul/optimiser/SSAReverser.cpp

@@ -0,0 +1,71 @@
+/*
+	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/>.
+*/
+#include <libyul/optimiser/SSAReverser.h>
+#include <libyul/AsmData.h>
+#include <libdevcore/CommonData.h>
+
+using namespace std;
+using namespace dev;
+using namespace yul;
+
+void SSAReverser::operator()(Block& _block)
+{
+	walkVector(_block.statements);
+	iterateReplacingWindow<2>(
+		_block.statements,
+		[&](Statement& _stmt1, Statement& _stmt2) -> boost::optional<vector<Statement>>
+		{
+			// Replaces
+			//   let a_1 := E
+			//   a := a_1
+			// with
+			//   a := E
+			//   let a_1 := a
+
+			auto* varDecl = boost::get<VariableDeclaration>(&_stmt1);
+			auto* assignment = boost::get<Assignment>(&_stmt2);
+
+			if (!varDecl || !assignment)
+				return {};
+
+			auto* identifier = boost::get<Identifier>(assignment->value.get());
+
+			if (
+				varDecl->variables.size() == 1 &&
+				varDecl->value &&
+				assignment->variableNames.size() == 1 &&
+				identifier &&
+				identifier->name == varDecl->variables.front().name
+			)
+			{
+				vector<Statement> result;
+				result.emplace_back(Assignment{
+					std::move(assignment->location),
+					assignment->variableNames,
+					std::move(varDecl->value)
+				});
+				result.emplace_back(VariableDeclaration{
+							std::move(varDecl->location),
+							std::move(varDecl->variables),
+							std::make_unique<Expression>(std::move(assignment->variableNames.front()))
+				});
+				return { std::move(result) };
+			}
+			return {};
+		}
+	);
+}