Add SSAReverser to the yul optimiser.

16 files changed, 470 insertions, 82 deletions

diff --git a/libdevcore/CommonData.h b/libdevcore/CommonData.h
index 1d668f26..98331936 100644
--- a/libdevcore/CommonData.h
+++ b/libdevcore/CommonData.h
@@ -263,6 +263,59 @@ void iterateReplacing(std::vector<T>& _vector, const F& _f)
 		_vector = std::move(modifiedVector);
 }
 
+
+namespace detail
+{
+template <typename T, typename F, std::size_t... I>
+void iterateReplacingWindow(std::vector<T>& _vector, F const& _f, std::index_sequence<I...>)
+{
+	// Concept: _f must be Callable, must accept sizeof...(I) parameters of type T&, must return optional<vector<T>>
+	bool useModified = false;
+	std::vector<T> modifiedVector;
+	size_t i = 0;
+	for (; i + sizeof...(I) <= _vector.size(); ++i)
+	{
+		if (boost::optional<std::vector<T>> r = _f(_vector[i + I]...))
+		{
+			if (!useModified)
+			{
+				std::move(_vector.begin(), _vector.begin() + i, back_inserter(modifiedVector));
+				useModified = true;
+			}
+			modifiedVector += std::move(*r);
+			i += sizeof...(I) - 1;
+		}
+		else if (useModified)
+			modifiedVector.emplace_back(std::move(_vector[i]));
+	}
+	if (useModified)
+	{
+		for (; i < _vector.size(); ++i)
+			modifiedVector.emplace_back(std::move(_vector[i]));
+		_vector = std::move(modifiedVector);
+	}
+}
+
+}
+
+/// Function that iterates over the vector @param _vector,
+/// calling the function @param _f on sequences of @tparam N of its
+/// elements. If @param _f returns a vector, these elements are replaced by
+/// the returned vector and the iteration continues with the next @tparam N elements.
+/// If the function does not return a vector, the iteration continues with an overlapping
+/// sequence of @tparam N elements that starts with the second element of the previous
+/// iteration.
+/// During the iteration, the original vector is only valid
+/// on the current element and after that. The actual replacement takes
+/// place at the end, but already visited elements might be invalidated.
+/// If nothing is replaced, no copy is performed.
+template <std::size_t N, typename T, typename F>
+void iterateReplacingWindow(std::vector<T>& _vector, F const& _f)
+{
+	// Concept: _f must be Callable, must accept N parameters of type T&, must return optional<vector<T>>
+	detail::iterateReplacingWindow(_vector, _f, std::make_index_sequence<N>{});
+}
+
 /// @returns true iff @a _str passess the hex address checksum test.
 /// @param _strict if false, hex strings with only uppercase or only lowercase letters
 /// are considered valid.
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);
diff --git a/test/libyul/YulOptimizerTest.cpp b/test/libyul/YulOptimizerTest.cpp
index 59cde4fe..914e0d6a 100644
--- a/test/libyul/YulOptimizerTest.cpp
+++ b/test/libyul/YulOptimizerTest.cpp
@@ -38,6 +38,7 @@
 #include <libyul/optimiser/ExpressionSimplifier.h>
 #include <libyul/optimiser/UnusedPruner.h>
 #include <libyul/optimiser/ExpressionJoiner.h>
+#include <libyul/optimiser/SSAReverser.h>
 #include <libyul/optimiser/SSATransform.h>
 #include <libyul/optimiser/RedundantAssignEliminator.h>
 #include <libyul/optimiser/StructuralSimplifier.h>
@@ -223,6 +224,23 @@ bool YulOptimizerTest::run(ostream& _stream, string const& _linePrefix, bool con
 		disambiguate();
 		EquivalentFunctionCombiner::run(*m_ast);
 	}
+	else if (m_optimizerStep == "ssaReverser")
+	{
+		disambiguate();
+		SSAReverser{}(*m_ast);
+	}
+	else if (m_optimizerStep == "ssaAndBack")
+	{
+		disambiguate();
+		// apply SSA
+		NameDispenser nameDispenser{*m_dialect, *m_ast};
+		SSATransform::run(*m_ast, nameDispenser);
+		RedundantAssignEliminator::run(*m_dialect, *m_ast);
+		// reverse SSA
+		SSAReverser{}(*m_ast);
+		CommonSubexpressionEliminator{*m_dialect}(*m_ast);
+		UnusedPruner::runUntilStabilised(*m_dialect, *m_ast);
+	}
 	else if (m_optimizerStep == "fullSuite")
 		OptimiserSuite::run(*m_dialect, *m_ast, *m_analysisInfo);
 	else
diff --git a/test/libyul/yulOptimizerTests/fullSuite/abi2.yul b/test/libyul/yulOptimizerTests/fullSuite/abi2.yul
index 887399b6..41a52c67 100644
--- a/test/libyul/yulOptimizerTests/fullSuite/abi2.yul
+++ b/test/libyul/yulOptimizerTests/fullSuite/abi2.yul
@@ -1073,12 +1073,12 @@
 // fullSuite
 // {
 //     let _2 := mload(1)
-//     let _172 := mload(0)
-//     if slt(sub(_2, _172), 64)
+//     let _153 := mload(0)
+//     if slt(sub(_2, _153), 64)
 //     {
 //         revert(0, 0)
 //     }
-//     sstore(0, and(calldataload(_172), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF))
+//     sstore(0, and(calldataload(_153), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF))
 //     let x0, x1, x2, x3, x4 := abi_decode_tuple_t_addresst_uint256t_bytes_calldata_ptrt_enum$_Operation_$1949(mload(7), mload(8))
 //     sstore(x1, x0)
 //     sstore(x3, x2)
@@ -1093,40 +1093,40 @@
 //         value0_57 := and(calldataload(add(headStart_55, value4)), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
 //         value1_58 := calldataload(add(headStart_55, 32))
 //         let offset_62 := calldataload(add(headStart_55, 64))
-//         let _220 := 0xffffffffffffffff
-//         if gt(offset_62, _220)
+//         let _201 := 0xffffffffffffffff
+//         if gt(offset_62, _201)
 //         {
 //             revert(value4, value4)
 //         }
-//         let _222 := add(headStart_55, offset_62)
-//         if iszero(slt(add(_222, 0x1f), dataEnd_56))
+//         let _203 := add(headStart_55, offset_62)
+//         if iszero(slt(add(_203, 0x1f), dataEnd_56))
 //         {
 //             revert(value4, value4)
 //         }
-//         let abi_decode_length_15_116 := calldataload(_222)
-//         if gt(abi_decode_length_15_116, _220)
+//         let abi_decode_length_15_246 := calldataload(_203)
+//         if gt(abi_decode_length_15_246, _201)
 //         {
 //             revert(value4, value4)
 //         }
-//         if gt(add(add(_222, abi_decode_length_15_116), 32), dataEnd_56)
+//         if gt(add(add(_203, abi_decode_length_15_246), 32), dataEnd_56)
 //         {
 //             revert(value4, value4)
 //         }
-//         value2_59 := add(_222, 32)
-//         value3 := abi_decode_length_15_116
-//         let _225 := calldataload(add(headStart_55, 96))
-//         if iszero(lt(_225, 3))
+//         value2_59 := add(_203, 32)
+//         value3 := abi_decode_length_15_246
+//         let _206 := calldataload(add(headStart_55, 96))
+//         if iszero(lt(_206, 3))
 //         {
 //             revert(value4, value4)
 //         }
-//         value4 := _225
+//         value4 := _206
 //     }
 //     function abi_encode_tuple_t_bytes32_t_address_t_uint256_t_bytes32_t_enum$_Operation_$1949_t_uint256_t_uint256_t_uint256_t_address_t_address_t_uint256__to_t_bytes32_t_address_t_uint256_t_bytes32_t_uint8_t_uint256_t_uint256_t_uint256_t_address_t_address_t_uint256_(headStart_252, value10_253, value9_254, value8_255, value7_256, value6_257, value5_258, value4_259, value3_260, value2_261, value1_262, value0_263) -> tail_264
 //     {
 //         tail_264 := add(headStart_252, 352)
 //         mstore(headStart_252, value0_263)
-//         let _439 := 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
-//         mstore(add(headStart_252, 32), and(value1_262, _439))
+//         let _413 := 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
+//         mstore(add(headStart_252, 32), and(value1_262, _413))
 //         mstore(add(headStart_252, 64), value2_261)
 //         mstore(add(headStart_252, 96), value3_260)
 //         if iszero(lt(value4_259, 3))
@@ -1137,8 +1137,8 @@
 //         mstore(add(headStart_252, 160), value5_258)
 //         mstore(add(headStart_252, 192), value6_257)
 //         mstore(add(headStart_252, 224), value7_256)
-//         mstore(add(headStart_252, 256), and(value8_255, _439))
-//         mstore(add(headStart_252, 288), and(value9_254, _439))
+//         mstore(add(headStart_252, 256), and(value8_255, _413))
+//         mstore(add(headStart_252, 288), and(value9_254, _413))
 //         mstore(add(headStart_252, 320), value10_253)
 //     }
 // }
diff --git a/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul b/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul
index a8cac6c6..05a35673 100644
--- a/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul
+++ b/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul
@@ -460,12 +460,12 @@
 // {
 //     let _1 := 0x20
 //     let _2 := 0
-//     let _268 := mload(_2)
+//     let _218 := mload(_2)
 //     let abi_encode_pos := _1
-//     let abi_encode_length_68 := mload(_268)
+//     let abi_encode_length_68 := mload(_218)
 //     mstore(_1, abi_encode_length_68)
 //     abi_encode_pos := 64
-//     let abi_encode_srcPtr := add(_268, _1)
+//     let abi_encode_srcPtr := add(_218, _1)
 //     let abi_encode_i_69 := _2
 //     for {
 //     }
@@ -474,66 +474,65 @@
 //         abi_encode_i_69 := add(abi_encode_i_69, 1)
 //     }
 //     {
-//         let _668 := mload(abi_encode_srcPtr)
-//         let abi_encode_pos_71_760 := abi_encode_pos
-//         let abi_encode_srcPtr_73_762 := _668
-//         let abi_encode_i_74_763 := _2
+//         let _580 := mload(abi_encode_srcPtr)
+//         let abi_encode_pos_71_672 := abi_encode_pos
+//         let abi_encode_srcPtr_73_674 := _580
+//         let abi_encode_i_74_675 := _2
 //         for {
 //         }
-//         lt(abi_encode_i_74_763, 0x3)
+//         lt(abi_encode_i_74_675, 0x3)
 //         {
-//             abi_encode_i_74_763 := add(abi_encode_i_74_763, 1)
+//             abi_encode_i_74_675 := add(abi_encode_i_74_675, 1)
 //         }
 //         {
-//             mstore(abi_encode_pos_71_760, and(mload(abi_encode_srcPtr_73_762), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF))
-//             abi_encode_srcPtr_73_762 := add(abi_encode_srcPtr_73_762, _1)
-//             abi_encode_pos_71_760 := add(abi_encode_pos_71_760, _1)
+//             mstore(abi_encode_pos_71_672, and(mload(abi_encode_srcPtr_73_674), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF))
+//             abi_encode_srcPtr_73_674 := add(abi_encode_srcPtr_73_674, _1)
+//             abi_encode_pos_71_672 := add(abi_encode_pos_71_672, _1)
 //         }
 //         abi_encode_srcPtr := add(abi_encode_srcPtr, _1)
 //         abi_encode_pos := add(abi_encode_pos, 0x60)
 //     }
-//     let _270 := mload(64)
-//     let _271 := mload(_1)
-//     if slt(sub(_270, _271), 128)
+//     let _220 := mload(64)
+//     let _221 := mload(_1)
+//     if slt(sub(_220, _221), 128)
 //     {
 //         revert(_2, _2)
 //     }
-//     let abi_decode_offset_64 := calldataload(add(_271, 64))
+//     let abi_decode_offset_64 := calldataload(add(_221, 64))
 //     let abi_decode__74 := 0xffffffffffffffff
 //     if gt(abi_decode_offset_64, abi_decode__74)
 //     {
 //         revert(_2, _2)
 //     }
-//     let abi_decode_value2_367 := abi_decode_t_array$_t_uint256_$dyn_memory_ptr(add(_271, abi_decode_offset_64), _270)
-//     let abi_decode_offset_65 := calldataload(add(_271, 96))
+//     let abi_decode_value2_317 := abi_decode_t_array$_t_uint256_$dyn_memory_ptr(add(_221, abi_decode_offset_64), _220)
+//     let abi_decode_offset_65 := calldataload(add(_221, 96))
 //     if gt(abi_decode_offset_65, abi_decode__74)
 //     {
 //         revert(_2, _2)
 //     }
-//     let abi_decode_value3_368 := abi_decode_t_array$_t_array$_t_uint256_$2_memory_$dyn_memory_ptr(add(_271, abi_decode_offset_65), _270)
-//     sstore(calldataload(_271), calldataload(add(_271, _1)))
-//     sstore(abi_decode_value2_367, abi_decode_value3_368)
+//     let abi_decode_value3_318 := abi_decode_t_array$_t_array$_t_uint256_$2_memory_$dyn_memory_ptr(add(_221, abi_decode_offset_65), _220)
+//     sstore(calldataload(_221), calldataload(add(_221, _1)))
+//     sstore(abi_decode_value2_317, abi_decode_value3_318)
 //     sstore(_2, abi_encode_pos)
 //     function abi_decode_t_array$_t_array$_t_uint256_$2_memory_$dyn_memory_ptr(offset_3, end_4) -> array_5
 //     {
 //         if iszero(slt(add(offset_3, 0x1f), end_4))
 //         {
-//             revert(0, 0)
+//             revert(array_5, array_5)
 //         }
 //         let length_6 := calldataload(offset_3)
-//         let array_5_115 := allocateMemory(array_allocation_size_t_array$_t_address_$dyn_memory(length_6))
-//         array_5 := array_5_115
-//         let dst_7 := array_5_115
-//         mstore(array_5_115, length_6)
+//         array_5 := allocateMemory(array_allocation_size_t_array$_t_address_$dyn_memory(length_6))
+//         let dst_7 := array_5
+//         mstore(array_5, length_6)
 //         let _16 := 0x20
-//         dst_7 := add(array_5_115, _16)
+//         dst_7 := add(array_5, _16)
 //         let src_8 := add(offset_3, _16)
 //         if gt(add(add(offset_3, mul(length_6, 0x40)), _16), end_4)
 //         {
 //             revert(0, 0)
 //         }
-//         let i_9_346 := 0
-//         let i_9 := i_9_346
+//         let i_9_296 := 0
+//         let i_9 := i_9_296
 //         for {
 //         }
 //         lt(i_9, length_6)
@@ -543,17 +542,17 @@
 //         {
 //             if iszero(slt(add(src_8, 0x1f), end_4))
 //             {
-//                 revert(i_9_346, i_9_346)
+//                 revert(i_9_296, i_9_296)
 //             }
-//             let abi_decode_array_13_124 := allocateMemory(array_allocation_size_t_array$_t_uint256_$2_memory(0x2))
-//             let abi_decode_dst_15 := abi_decode_array_13_124
+//             let abi_decode_array_13_300 := allocateMemory(array_allocation_size_t_array$_t_uint256_$2_memory(0x2))
+//             let abi_decode_dst_15 := abi_decode_array_13_300
 //             let abi_decode_src_16 := src_8
-//             let abi_decode__289 := add(src_8, 0x40)
-//             if gt(abi_decode__289, end_4)
+//             let abi_decode__239 := add(src_8, 0x40)
+//             if gt(abi_decode__239, end_4)
 //             {
-//                 revert(i_9_346, i_9_346)
+//                 revert(i_9_296, i_9_296)
 //             }
-//             let abi_decode_i_17 := i_9_346
+//             let abi_decode_i_17 := i_9_296
 //             for {
 //             }
 //             lt(abi_decode_i_17, 0x2)
@@ -565,24 +564,23 @@
 //                 abi_decode_dst_15 := add(abi_decode_dst_15, _16)
 //                 abi_decode_src_16 := add(abi_decode_src_16, _16)
 //             }
-//             mstore(dst_7, abi_decode_array_13_124)
+//             mstore(dst_7, abi_decode_array_13_300)
 //             dst_7 := add(dst_7, _16)
-//             src_8 := abi_decode__289
+//             src_8 := abi_decode__239
 //         }
 //     }
 //     function abi_decode_t_array$_t_uint256_$dyn_memory_ptr(offset_27, end_28) -> array_29
 //     {
 //         if iszero(slt(add(offset_27, 0x1f), end_28))
 //         {
-//             revert(0, 0)
+//             revert(array_29, array_29)
 //         }
 //         let length_30 := calldataload(offset_27)
-//         let array_29_131 := allocateMemory(array_allocation_size_t_array$_t_address_$dyn_memory(length_30))
-//         array_29 := array_29_131
-//         let dst_31 := array_29_131
-//         mstore(array_29_131, length_30)
+//         array_29 := allocateMemory(array_allocation_size_t_array$_t_address_$dyn_memory(length_30))
+//         let dst_31 := array_29
+//         mstore(array_29, length_30)
 //         let _52 := 0x20
-//         dst_31 := add(array_29_131, _52)
+//         dst_31 := add(array_29, _52)
 //         let src_32 := add(offset_27, _52)
 //         if gt(add(add(offset_27, mul(length_30, _52)), _52), end_28)
 //         {
@@ -603,10 +601,9 @@
 //     }
 //     function allocateMemory(size) -> memPtr
 //     {
-//         let memPtr_157 := mload(64)
-//         memPtr := memPtr_157
-//         let newFreePtr := add(memPtr_157, size)
-//         if or(gt(newFreePtr, 0xffffffffffffffff), lt(newFreePtr, memPtr_157))
+//         memPtr := mload(64)
+//         let newFreePtr := add(memPtr, size)
+//         if or(gt(newFreePtr, 0xffffffffffffffff), lt(newFreePtr, memPtr))
 //         {
 //             revert(0, 0)
 //         }
diff --git a/test/libyul/yulOptimizerTests/fullSuite/aztec.yul b/test/libyul/yulOptimizerTests/fullSuite/aztec.yul
index fc09b5d9..e05a3c92 100644
--- a/test/libyul/yulOptimizerTests/fullSuite/aztec.yul
+++ b/test/libyul/yulOptimizerTests/fullSuite/aztec.yul
@@ -264,10 +264,10 @@
 //     }
 //     {
 //         let validateJo__34 := 0x20
-//         let validateJo__373 := add(validateJo__10, mul(validateJo_i, 0xc0))
-//         let validateJo_noteIndex := add(validateJo__373, 0x24)
+//         let validateJo__351 := add(validateJo__10, mul(validateJo_i, 0xc0))
+//         let validateJo_noteIndex := add(validateJo__351, 0x24)
 //         let validateJo_k := validateJo_i_290
-//         let validateJo_a_292 := calldataload(add(validateJo__373, 0x44))
+//         let validateJo_a_292 := calldataload(add(validateJo__351, 0x44))
 //         let validateJo_a := validateJo_a_292
 //         let validateJo_c := validateJo_challenge
 //         let validateJo__39 := add(validateJo_i, 0x01)
@@ -296,8 +296,8 @@
 //             validateJo_kn := addmod(validateJo_kn, validateJo_k, validateJo_gen_order)
 //         }
 //         let validateJo__52 := 0x40
-//         calldatacopy(0xe0, add(validateJo__373, 164), validateJo__52)
-//         calldatacopy(validateJo__34, add(validateJo__373, 100), validateJo__52)
+//         calldatacopy(0xe0, add(validateJo__351, 164), validateJo__52)
+//         calldatacopy(validateJo__34, add(validateJo__351, 100), validateJo__52)
 //         let validateJo__61 := 0x120
 //         mstore(validateJo__61, sub(validateJo_gen_order, validateJo_c))
 //         let validateJo__62 := 0x60
@@ -310,8 +310,7 @@
 //         let validateJo__80 := 0x160
 //         let validateJo_result_304 := and(validateJo_result_303, call(gas(), 7, validateJo_i_290, validateJo__6, validateJo__62, validateJo__80, validateJo__52))
 //         let validateJo_result_305 := and(validateJo_result_304, call(gas(), 6, validateJo_i_290, validateJo__61, validateJo__6, validateJo__80, validateJo__52))
-//         let validateJo_result_306 := and(validateJo_result_305, call(gas(), 6, validateJo_i_290, validateJo__80, validateJo__6, validateJo_b, validateJo__52))
-//         validateJo_result := validateJo_result_306
+//         validateJo_result := and(validateJo_result_305, call(gas(), 6, validateJo_i_290, validateJo__80, validateJo__6, validateJo_b, validateJo__52))
 //         if eq(validateJo_i, validateJo_m)
 //         {
 //             mstore(0x260, mload(validateJo__34))
@@ -323,7 +322,7 @@
 //         {
 //             mstore(validateJo__62, validateJo_c)
 //             let validateJo__120 := 0x220
-//             let validateJo_result_307 := and(validateJo_result_306, call(gas(), 7, validateJo_i_290, validateJo__34, validateJo__62, validateJo__120, validateJo__52))
+//             let validateJo_result_307 := and(validateJo_result, call(gas(), 7, validateJo_i_290, validateJo__34, validateJo__62, validateJo__120, validateJo__52))
 //             let validateJo_result_308 := and(validateJo_result_307, call(gas(), 6, validateJo_i_290, validateJo__120, validateJo__6, 0x260, validateJo__52))
 //             validateJo_result := and(validateJo_result_308, call(gas(), 6, validateJo_i_290, validateJo__65, validateJo__6, 0x1e0, validateJo__52))
 //         }
diff --git a/test/libyul/yulOptimizerTests/fullSuite/medium.yul b/test/libyul/yulOptimizerTests/fullSuite/medium.yul
index b10c6c69..1d07cd03 100644
--- a/test/libyul/yulOptimizerTests/fullSuite/medium.yul
+++ b/test/libyul/yulOptimizerTests/fullSuite/medium.yul
@@ -21,8 +21,8 @@
 // {
 //     let allocate__19 := 0x40
 //     mstore(allocate__19, add(mload(allocate__19), 0x20))
-//     let allocate_p_24_41 := mload(allocate__19)
-//     mstore(allocate__19, add(allocate_p_24_41, allocate__19))
-//     mstore(add(allocate_p_24_41, 96), 2)
+//     let allocate_p_35_39 := mload(allocate__19)
+//     mstore(allocate__19, add(allocate_p_35_39, allocate__19))
+//     mstore(add(allocate_p_35_39, 96), 2)
 //     mstore(allocate__19, 0x20)
 // }
diff --git a/test/libyul/yulOptimizerTests/fullSuite/ssaReverse.yul b/test/libyul/yulOptimizerTests/fullSuite/ssaReverse.yul
new file mode 100644
index 00000000..204b444e
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/fullSuite/ssaReverse.yul
@@ -0,0 +1,62 @@
+{
+    // This is an abi decode function with the SSA transform applied once.
+    // This test is supposed to verify that the SSA transform is correctly reversed by the full suite.
+    function abi_decode_t_bytes_calldata_ptr(offset_12, end_13) -> arrayPos_14, length_15
+    {
+        if iszero(slt(add(offset_12, 0x1f), end_13))
+        {
+            revert(0, 0)
+        }
+        let length_15_1 := calldataload(offset_12)
+        length_15 := length_15_1
+        if gt(length_15_1, 0xffffffffffffffff)
+        {
+            revert(0, 0)
+        }
+        let arrayPos_14_2 := add(offset_12, 0x20)
+        arrayPos_14 := arrayPos_14_2
+        if gt(add(arrayPos_14_2, mul(length_15_1, 0x1)), end_13)
+        {
+            revert(0, 0)
+        }
+    }
+
+    // prevent inlining
+    let a,b := abi_decode_t_bytes_calldata_ptr(mload(0),mload(1))
+    a,b := abi_decode_t_bytes_calldata_ptr(a,b)
+    a,b := abi_decode_t_bytes_calldata_ptr(a,b)
+    a,b := abi_decode_t_bytes_calldata_ptr(a,b)
+    a,b := abi_decode_t_bytes_calldata_ptr(a,b)
+    a,b := abi_decode_t_bytes_calldata_ptr(a,b)
+    a,b := abi_decode_t_bytes_calldata_ptr(a,b)
+    mstore(a,b)
+}
+// ----
+// fullSuite
+// {
+//     let a, b := abi_decode_t_bytes_calldata_ptr(mload(0), mload(1))
+//     a, b := abi_decode_t_bytes_calldata_ptr(a, b)
+//     a, b := abi_decode_t_bytes_calldata_ptr(a, b)
+//     a, b := abi_decode_t_bytes_calldata_ptr(a, b)
+//     a, b := abi_decode_t_bytes_calldata_ptr(a, b)
+//     a, b := abi_decode_t_bytes_calldata_ptr(a, b)
+//     a, b := abi_decode_t_bytes_calldata_ptr(a, b)
+//     mstore(a, b)
+//     function abi_decode_t_bytes_calldata_ptr(offset_12, end_13) -> arrayPos_14, length_15
+//     {
+//         if iszero(slt(add(offset_12, 0x1f), end_13))
+//         {
+//             revert(0, 0)
+//         }
+//         length_15 := calldataload(offset_12)
+//         if gt(length_15, 0xffffffffffffffff)
+//         {
+//             revert(0, 0)
+//         }
+//         arrayPos_14 := add(offset_12, 0x20)
+//         if gt(add(add(offset_12, length_15), 0x20), end_13)
+//         {
+//             revert(0, 0)
+//         }
+//     }
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul b/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul
new file mode 100644
index 00000000..23c433d1
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul
@@ -0,0 +1,18 @@
+{
+    let a := mload(0)
+    a := mload(1)
+    a := mload(2)
+    a := mload(3)
+    a := mload(4)
+    mstore(a, 0)
+}
+// ----
+// ssaAndBack
+// {
+//     pop(mload(0))
+//     pop(mload(1))
+//     pop(mload(2))
+//     pop(mload(3))
+//     let a_5 := mload(4)
+//     mstore(a_5, 0)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/ssaReverse.yul b/test/libyul/yulOptimizerTests/ssaAndBack/ssaReverse.yul
new file mode 100644
index 00000000..d2ba6471
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/ssaReverse.yul
@@ -0,0 +1,45 @@
+{
+     function abi_decode_t_bytes_calldata_ptr(offset_12, end_13) -> arrayPos_14, length_15
+     {
+         if iszero(slt(add(offset_12, 0x1f), end_13))
+         {
+             revert(0, 0)
+         }
+         length_15 := calldataload(offset_12)
+         if gt(length_15, 0xffffffffffffffff)
+         {
+             revert(0, 0)
+         }
+         arrayPos_14 := add(offset_12, 0x20)
+         if gt(add(add(offset_12, length_15), 0x20), end_13)
+         {
+             revert(0, 0)
+         }
+     }
+     // prevent removal of the function
+     let a,b := abi_decode_t_bytes_calldata_ptr(mload(0),mload(1))
+     mstore(a,b)
+}
+// ----
+// ssaAndBack
+// {
+//     function abi_decode_t_bytes_calldata_ptr(offset_12, end_13) -> arrayPos_14, length_15
+//     {
+//         if iszero(slt(add(offset_12, 0x1f), end_13))
+//         {
+//             revert(arrayPos_14, arrayPos_14)
+//         }
+//         length_15 := calldataload(offset_12)
+//         if gt(length_15, 0xffffffffffffffff)
+//         {
+//             revert(arrayPos_14, arrayPos_14)
+//         }
+//         arrayPos_14 := add(offset_12, 0x20)
+//         if gt(add(add(offset_12, length_15), 0x20), end_13)
+//         {
+//             revert(0, 0)
+//         }
+//     }
+//     let a, b := abi_decode_t_bytes_calldata_ptr(mload(0), mload(1))
+//     mstore(a, b)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaReverser/abi_example.yul b/test/libyul/yulOptimizerTests/ssaReverser/abi_example.yul
new file mode 100644
index 00000000..923a42ba
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaReverser/abi_example.yul
@@ -0,0 +1,44 @@
+{
+    function abi_decode_t_bytes_calldata_ptr(offset_12, end_13) -> arrayPos_14, length_15
+    {
+        if iszero(slt(add(offset_12, 0x1f), end_13))
+        {
+            revert(0, 0)
+        }
+        let length_15_1 := calldataload(offset_12)
+        length_15 := length_15_1
+        if gt(length_15_1, 0xffffffffffffffff)
+        {
+            revert(0, 0)
+        }
+        let arrayPos_14_2 := add(offset_12, 0x20)
+        arrayPos_14 := arrayPos_14_2
+        if gt(add(arrayPos_14_2, mul(length_15_1, 0x1)), end_13)
+        {
+            revert(0, 0)
+        }
+    }
+}
+// ----
+// ssaReverser
+// {
+//     function abi_decode_t_bytes_calldata_ptr(offset_12, end_13) -> arrayPos_14, length_15
+//     {
+//         if iszero(slt(add(offset_12, 0x1f), end_13))
+//         {
+//             revert(0, 0)
+//         }
+//         length_15 := calldataload(offset_12)
+//         let length_15_1 := length_15
+//         if gt(length_15_1, 0xffffffffffffffff)
+//         {
+//             revert(0, 0)
+//         }
+//         arrayPos_14 := add(offset_12, 0x20)
+//         let arrayPos_14_2 := arrayPos_14
+//         if gt(add(arrayPos_14_2, mul(length_15_1, 0x1)), end_13)
+//         {
+//             revert(0, 0)
+//         }
+//     }
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaReverser/simple.yul b/test/libyul/yulOptimizerTests/ssaReverser/simple.yul
new file mode 100644
index 00000000..eba1f5f1
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaReverser/simple.yul
@@ -0,0 +1,14 @@
+{
+	let a := mload(1)
+    let a_1 := mload(0)
+    a := a_1
+    mstore(a_1, 0)
+}
+// ----
+// ssaReverser
+// {
+//     let a := mload(1)
+//     a := mload(0)
+//     let a_1 := a
+//     mstore(a_1, 0)
+// }
diff --git a/test/tools/yulopti.cpp b/test/tools/yulopti.cpp
index efd1ba05..07ebfc7d 100644
--- a/test/tools/yulopti.cpp
+++ b/test/tools/yulopti.cpp
@@ -46,6 +46,7 @@
 #include <libyul/optimiser/UnusedPruner.h>
 #include <libyul/optimiser/ExpressionJoiner.h>
 #include <libyul/optimiser/RedundantAssignEliminator.h>
+#include <libyul/optimiser/SSAReverser.h>
 #include <libyul/optimiser/SSATransform.h>
 #include <libyul/optimiser/StructuralSimplifier.h>
 #include <libyul/optimiser/VarDeclInitializer.h>
@@ -129,7 +130,7 @@ public:
 			cout << "(q)quit/(f)flatten/(c)se/initialize var(d)ecls/(x)plit/(j)oin/(g)rouper/(h)oister/" << endl;
 			cout << "  (e)xpr inline/(i)nline/(s)implify/(u)nusedprune/ss(a) transform/" << endl;
 			cout << "  (r)edundant assign elim./re(m)aterializer/f(o)r-loop-pre-rewriter/" << endl;
-			cout << "  s(t)ructural simplifier/equi(v)alent function combiner? " << endl;
+			cout << "  s(t)ructural simplifier/equi(v)alent function combiner/ssa re(V)erser? " << endl;
 			cout.flush();
 			int option = readStandardInputChar();
 			cout << ' ' << char(option) << endl;
@@ -188,6 +189,9 @@ public:
 			case 'v':
 				EquivalentFunctionCombiner::run(*m_ast);
 				break;
+			case 'V':
+				SSAReverser{}(*m_ast);
+				break;
 			default:
 				cout << "Unknown option." << endl;
 			}