Add SSAReverser to the yul optimiser.

4 files changed, 137 insertions, 3 deletions

diff --git a/libyul/CMakeLists.txt b/libyul/CMakeLists.txt
index 44af2fc3..259f43f8 100644
--- a/libyul/CMakeLists.txt
+++ b/libyul/CMakeLists.txt
@@ -80,6 +80,8 @@ add_library(yul
 	optimiser/RedundantAssignEliminator.h
 	optimiser/Rematerialiser.cpp
 	optimiser/Rematerialiser.h
+	optimiser/SSAReverser.cpp
+	optimiser/SSAReverser.h
 	optimiser/SSATransform.cpp
 	optimiser/SSATransform.h
 	optimiser/SSAValueTracker.cpp
diff --git a/libyul/optimiser/SSAReverser.cpp b/libyul/optimiser/SSAReverser.cpp
new file mode 100644
index 00000000..313a677a
--- /dev/null
+++ b/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 {};
+		}
+	);
+}
diff --git a/libyul/optimiser/SSAReverser.h b/libyul/optimiser/SSAReverser.h
new file mode 100644
index 00000000..a4a11074
--- /dev/null
+++ b/libyul/optimiser/SSAReverser.h
@@ -0,0 +1,54 @@
+/*
+	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/>.
+*/
+#pragma once
+
+#include <libyul/optimiser/ASTWalker.h>
+
+namespace yul
+{
+
+/**
+ * Reverses the SSA transformation.
+ *
+ * In particular, the SSA transform will rewrite
+ *
+ * 		a := E
+ *
+ * to
+ *
+ * 		let a_1 := E
+ * 		a := a_1
+ *
+ * 	To undo this transformation, the SSAReverser changes this back to
+ *
+ * 		a := E
+ * 		let a_1 := a
+ *
+ * 	After that the CSE can replace references of a_1 by references to a,
+ * 	after which the unused pruner can remove the declaration of a_1.
+ *
+ * 	Prerequisites: None
+ *
+ */
+class SSAReverser: public ASTModifier
+{
+public:
+	using ASTModifier::operator();
+	void operator()(Block& _block) override;
+};
+
+}
diff --git a/libyul/optimiser/Suite.cpp b/libyul/optimiser/Suite.cpp
index 38c0bf49..63931554 100644
--- a/libyul/optimiser/Suite.cpp
+++ b/libyul/optimiser/Suite.cpp
@@ -35,6 +35,7 @@
 #include <libyul/optimiser/UnusedPruner.h>
 #include <libyul/optimiser/ExpressionSimplifier.h>
 #include <libyul/optimiser/CommonSubexpressionEliminator.h>
+#include <libyul/optimiser/SSAReverser.h>
 #include <libyul/optimiser/SSATransform.h>
 #include <libyul/optimiser/StructuralSimplifier.h>
 #include <libyul/optimiser/RedundantAssignEliminator.h>
@@ -88,9 +89,10 @@ void OptimiserSuite::run(
 		UnusedPruner::runUntilStabilised(_dialect, ast, reservedIdentifiers);
 		CommonSubexpressionEliminator{_dialect}(ast);
 		UnusedPruner::runUntilStabilised(_dialect, ast, reservedIdentifiers);
-		SSATransform::run(ast, dispenser);
-		RedundantAssignEliminator::run(_dialect, ast);
-		RedundantAssignEliminator::run(_dialect, ast);
+
+		SSAReverser{}(ast);
+		CommonSubexpressionEliminator{_dialect}(ast);
+		UnusedPruner::runUntilStabilised(_dialect, ast, reservedIdentifiers);
 
 		ExpressionJoiner::run(ast);
 		ExpressionJoiner::run(ast);
@@ -127,6 +129,11 @@ void OptimiserSuite::run(
 	UnusedPruner::runUntilStabilised(_dialect, ast);
 	ExpressionJoiner::run(ast);
 	UnusedPruner::runUntilStabilised(_dialect, ast);
+
+	SSAReverser{}(ast);
+	CommonSubexpressionEliminator{_dialect}(ast);
+	UnusedPruner::runUntilStabilised(_dialect, ast, reservedIdentifiers);
+
 	ExpressionJoiner::run(ast);
 	Rematerialiser::run(_dialect, ast);
 	UnusedPruner::runUntilStabilised(_dialect, ast);