Undo second SSA transformation and add more tests.

15 files changed, 366 insertions, 75 deletions

diff --git a/libyul/optimiser/SSAReverser.cpp b/libyul/optimiser/SSAReverser.cpp
index 313a677a..2cfa3d58 100644
--- a/libyul/optimiser/SSAReverser.cpp
+++ b/libyul/optimiser/SSAReverser.cpp
@@ -29,42 +29,74 @@ void SSAReverser::operator()(Block& _block)
 		_block.statements,
 		[&](Statement& _stmt1, Statement& _stmt2) -> boost::optional<vector<Statement>>
 		{
+			auto* varDecl = boost::get<VariableDeclaration>(&_stmt1);
+
+			if (!varDecl || varDecl->variables.size() != 1 || !varDecl->value)
+				return {};
+
 			// 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
-			)
+			if (auto* assignment = boost::get<Assignment>(&_stmt2))
+			{
+				auto* identifier = boost::get<Identifier>(assignment->value.get());
+				if (
+					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) };
+				}
+			}
+			// Replaces
+			//   let a_1 := E
+			//   let a := a_1
+			// with
+			//   let a := E
+			//   let a_1 := a
+			else if (auto* varDecl2 = boost::get<VariableDeclaration>(&_stmt2))
 			{
-				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) };
+				auto* identifier = boost::get<Identifier>(varDecl2->value.get());
+				if (
+					varDecl2->variables.size() == 1 &&
+					identifier &&
+					identifier->name == varDecl->variables.front().name
+				)
+				{
+					vector<Statement> result;
+					auto varIdentifier2 = std::make_unique<Expression>(Identifier{
+						varDecl2->variables.front().location,
+						varDecl2->variables.front().name
+					});
+					result.emplace_back(VariableDeclaration{
+						std::move(varDecl2->location),
+						std::move(varDecl2->variables),
+						std::move(varDecl->value)
+					});
+					result.emplace_back(VariableDeclaration{
+						std::move(varDecl->location),
+						std::move(varDecl->variables),
+						std::move(varIdentifier2)
+					});
+					return { std::move(result) };
+				}
 			}
+
 			return {};
 		}
 	);
diff --git a/libyul/optimiser/SSAReverser.h b/libyul/optimiser/SSAReverser.h
index a4a11074..34b61647 100644
--- a/libyul/optimiser/SSAReverser.h
+++ b/libyul/optimiser/SSAReverser.h
@@ -33,11 +33,24 @@ namespace yul
  * 		let a_1 := E
  * 		a := a_1
  *
- * 	To undo this transformation, the SSAReverser changes this back to
+ * To undo this kind of transformation, the SSAReverser changes this back to
  *
  * 		a := E
  * 		let a_1 := a
  *
+ * Secondly, the SSA transform will rewrite
+ *
+ * 		let a := E
+ * to
+ *
+ * 		let a_1 := E
+ * 		let a := a_1
+ *
+ * To undo this kind of transformation, the SSAReverser changes this back to
+ *
+ * 		let 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.
  *
diff --git a/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul b/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul
index 05a35673..d38025ae 100644
--- a/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul
+++ b/test/libyul/yulOptimizerTests/fullSuite/abi_example1.yul
@@ -531,8 +531,7 @@
 //         {
 //             revert(0, 0)
 //         }
-//         let i_9_296 := 0
-//         let i_9 := i_9_296
+//         let i_9 := 0
 //         for {
 //         }
 //         lt(i_9, length_6)
@@ -542,17 +541,17 @@
 //         {
 //             if iszero(slt(add(src_8, 0x1f), end_4))
 //             {
-//                 revert(i_9_296, i_9_296)
+//                 revert(0, 0)
 //             }
-//             let abi_decode_array_13_300 := allocateMemory(array_allocation_size_t_array$_t_uint256_$2_memory(0x2))
-//             let abi_decode_dst_15 := abi_decode_array_13_300
+//             let abi_decode_dst_15 := allocateMemory(array_allocation_size_t_array$_t_uint256_$2_memory(0x2))
+//             let abi_decode_dst_15_1155 := abi_decode_dst_15
 //             let abi_decode_src_16 := src_8
 //             let abi_decode__239 := add(src_8, 0x40)
 //             if gt(abi_decode__239, end_4)
 //             {
-//                 revert(i_9_296, i_9_296)
+//                 revert(0, 0)
 //             }
-//             let abi_decode_i_17 := i_9_296
+//             let abi_decode_i_17 := 0
 //             for {
 //             }
 //             lt(abi_decode_i_17, 0x2)
@@ -564,7 +563,7 @@
 //                 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_300)
+//             mstore(dst_7, abi_decode_dst_15_1155)
 //             dst_7 := add(dst_7, _16)
 //             src_8 := abi_decode__239
 //         }
diff --git a/test/libyul/yulOptimizerTests/fullSuite/aztec.yul b/test/libyul/yulOptimizerTests/fullSuite/aztec.yul
index e05a3c92..868c6b01 100644
--- a/test/libyul/yulOptimizerTests/fullSuite/aztec.yul
+++ b/test/libyul/yulOptimizerTests/fullSuite/aztec.yul
@@ -245,17 +245,16 @@
 //         mstore(0x00, 404)
 //         revert(0x00, 0x20)
 //     }
-//     let validateJo_kn_287 := calldataload(add(calldatasize(), 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff40))
-//     let validateJo_kn := validateJo_kn_287
+//     let validateJo_kn := calldataload(add(calldatasize(), 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff40))
 //     let validateJo__24 := 0x2a0
 //     mstore(validateJo__24, caller())
-//     mstore(0x2c0, validateJo_kn_287)
+//     mstore(0x2c0, validateJo_kn)
 //     mstore(0x2e0, validateJo_m)
-//     validateJo_kn := mulmod(sub(validateJo_gen_order, validateJo_kn_287), validateJo_challenge, validateJo_gen_order)
+//     validateJo_kn := mulmod(sub(validateJo_gen_order, validateJo_kn), validateJo_challenge, validateJo_gen_order)
 //     hashCommitments(validateJo_notes, validateJo_n)
 //     let validateJo_b := add(0x300, mul(validateJo_n, validateJo__6))
-//     let validateJo_i_290 := 0
-//     let validateJo_i := validateJo_i_290
+//     let validateJo_i := 0
+//     let validateJo_i_1306 := validateJo_i
 //     for {
 //     }
 //     lt(validateJo_i, validateJo_n)
@@ -266,9 +265,8 @@
 //         let validateJo__34 := 0x20
 //         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__351, 0x44))
-//         let validateJo_a := validateJo_a_292
+//         let validateJo_k := validateJo_i_1306
+//         let validateJo_a := calldataload(add(validateJo__351, 0x44))
 //         let validateJo_c := validateJo_challenge
 //         let validateJo__39 := add(validateJo_i, 0x01)
 //         switch eq(validateJo__39, validateJo_n)
@@ -282,15 +280,15 @@
 //         case 0 {
 //             validateJo_k := calldataload(validateJo_noteIndex)
 //         }
-//         validateCommitment(validateJo_noteIndex, validateJo_k, validateJo_a_292)
+//         validateCommitment(validateJo_noteIndex, validateJo_k, validateJo_a)
 //         switch gt(validateJo__39, validateJo_m)
 //         case 1 {
 //             validateJo_kn := addmod(validateJo_kn, sub(validateJo_gen_order, validateJo_k), validateJo_gen_order)
-//             let validateJo_x := mod(mload(validateJo_i_290), validateJo_gen_order)
+//             let validateJo_x := mod(mload(validateJo_i_1306), validateJo_gen_order)
 //             validateJo_k := mulmod(validateJo_k, validateJo_x, validateJo_gen_order)
-//             validateJo_a := mulmod(validateJo_a_292, validateJo_x, validateJo_gen_order)
+//             validateJo_a := mulmod(validateJo_a, validateJo_x, validateJo_gen_order)
 //             validateJo_c := mulmod(validateJo_challenge, validateJo_x, validateJo_gen_order)
-//             mstore(validateJo_i_290, keccak256(validateJo_i_290, validateJo__34))
+//             mstore(validateJo_i_1306, keccak256(validateJo_i_1306, validateJo__34))
 //         }
 //         case 0 {
 //             validateJo_kn := addmod(validateJo_kn, validateJo_k, validateJo_gen_order)
@@ -304,13 +302,12 @@
 //         mstore(validateJo__62, validateJo_k)
 //         mstore(0xc0, validateJo_a)
 //         let validateJo__65 := 0x1a0
-//         let validateJo_result_302 := call(gas(), 7, validateJo_i_290, 0xe0, validateJo__62, validateJo__65, validateJo__52)
-//         let validateJo_result := validateJo_result_302
-//         let validateJo_result_303 := and(validateJo_result_302, call(gas(), 7, validateJo_i_290, validateJo__34, validateJo__62, validateJo__61, validateJo__52))
+//         let validateJo_result := call(gas(), 7, validateJo_i_1306, 0xe0, validateJo__62, validateJo__65, validateJo__52)
+//         let validateJo_result_303 := and(validateJo_result, call(gas(), 7, validateJo_i_1306, validateJo__34, validateJo__62, validateJo__61, validateJo__52))
 //         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))
-//         validateJo_result := and(validateJo_result_305, call(gas(), 6, validateJo_i_290, validateJo__80, validateJo__6, validateJo_b, validateJo__52))
+//         let validateJo_result_304 := and(validateJo_result_303, call(gas(), 7, validateJo_i_1306, validateJo__6, validateJo__62, validateJo__80, validateJo__52))
+//         let validateJo_result_305 := and(validateJo_result_304, call(gas(), 6, validateJo_i_1306, validateJo__61, validateJo__6, validateJo__80, validateJo__52))
+//         validateJo_result := and(validateJo_result_305, call(gas(), 6, validateJo_i_1306, validateJo__80, validateJo__6, validateJo_b, validateJo__52))
 //         if eq(validateJo_i, validateJo_m)
 //         {
 //             mstore(0x260, mload(validateJo__34))
@@ -322,14 +319,14 @@
 //         {
 //             mstore(validateJo__62, validateJo_c)
 //             let validateJo__120 := 0x220
-//             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))
+//             let validateJo_result_307 := and(validateJo_result, call(gas(), 7, validateJo_i_1306, validateJo__34, validateJo__62, validateJo__120, validateJo__52))
+//             let validateJo_result_308 := and(validateJo_result_307, call(gas(), 6, validateJo_i_1306, validateJo__120, validateJo__6, 0x260, validateJo__52))
+//             validateJo_result := and(validateJo_result_308, call(gas(), 6, validateJo_i_1306, validateJo__65, validateJo__6, 0x1e0, validateJo__52))
 //         }
 //         if iszero(validateJo_result)
 //         {
-//             mstore(validateJo_i_290, 400)
-//             revert(validateJo_i_290, validateJo__34)
+//             mstore(validateJo_i_1306, 400)
+//             revert(validateJo_i_1306, validateJo__34)
 //         }
 //         validateJo_b := add(validateJo_b, validateJo__52)
 //     }
@@ -339,13 +336,13 @@
 //     }
 //     if iszero(eq(mod(keccak256(validateJo__24, add(validateJo_b, 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffd60)), validateJo_gen_order), validateJo_challenge))
 //     {
-//         mstore(validateJo_i_290, 404)
-//         revert(validateJo_i_290, 0x20)
+//         mstore(validateJo_i_1306, 404)
+//         revert(validateJo_i_1306, 0x20)
 //     }
-//     mstore(validateJo_i_290, 0x01)
-//     return(validateJo_i_290, 0x20)
-//     mstore(validateJo_i_290, 404)
-//     revert(validateJo_i_290, 0x20)
+//     mstore(validateJo_i_1306, 0x01)
+//     return(validateJo_i_1306, 0x20)
+//     mstore(validateJo_i_1306, 404)
+//     revert(validateJo_i_1306, 0x20)
 //     function validatePairing(t2)
 //     {
 //         let t2_x_1 := calldataload(t2)
diff --git a/test/libyul/yulOptimizerTests/fullSuite/ssaReverseComplex.yul b/test/libyul/yulOptimizerTests/fullSuite/ssaReverseComplex.yul
new file mode 100644
index 00000000..2e178f31
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/fullSuite/ssaReverseComplex.yul
@@ -0,0 +1,23 @@
+{
+    let a := mload(0)
+    let b := mload(1)
+    if mload(2) {
+        a := mload(b)
+        b := mload(a)
+        a := mload(b)
+        b := mload(a)
+    }
+    mstore(a, b)
+}
+// ----
+// fullSuite
+// {
+//     let a := mload(0)
+//     let b := mload(1)
+//     if mload(2)
+//     {
+//         a := mload(mload(mload(b)))
+//         b := mload(a)
+//     }
+//     mstore(a, b)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/for_loop.yul b/test/libyul/yulOptimizerTests/ssaAndBack/for_loop.yul
new file mode 100644
index 00000000..18498e61
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/for_loop.yul
@@ -0,0 +1,36 @@
+{
+    for {
+      let a := mload(0)
+      let b := mload(1)
+    }
+    lt(mload(a),mload(b))
+    {
+       a := mload(b)
+    }
+    {
+        b := mload(a)
+        a := mload(b)
+        a := mload(b)
+        a := mload(b)
+        b := mload(a)
+    }
+}
+// ----
+// ssaAndBack
+// {
+//     for {
+//         let a := mload(0)
+//         let b := mload(1)
+//     }
+//     lt(mload(a), mload(b))
+//     {
+//         a := mload(b)
+//     }
+//     {
+//         let b_3 := mload(a)
+//         pop(mload(b_3))
+//         pop(mload(b_3))
+//         let a_6 := mload(b_3)
+//         b := mload(a_6)
+//     }
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign.yul b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign.yul
new file mode 100644
index 00000000..23c433d1
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign.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/multi_assign_if.yul b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_if.yul
new file mode 100644
index 00000000..fd5981ef
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_if.yul
@@ -0,0 +1,22 @@
+{
+    let a := mload(0)
+    if mload(1)
+    {
+        a := mload(1)
+        a := mload(2)
+        a := mload(3)
+    }
+    mstore(a, 0)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     if mload(1)
+//     {
+//         pop(mload(1))
+//         pop(mload(2))
+//         a := mload(3)
+//     }
+//     mstore(a, 0)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_multi_var_if.yul b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_multi_var_if.yul
new file mode 100644
index 00000000..b0b3efb5
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_multi_var_if.yul
@@ -0,0 +1,25 @@
+{
+    let a := mload(0)
+    let b := mload(1)
+    if mload(2) {
+        a := mload(b)
+        b := mload(a)
+        a := mload(b)
+        b := mload(a)
+    }
+    mstore(a, b)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     let b := mload(1)
+//     if mload(2)
+//     {
+//         let a_3 := mload(b)
+//         let b_4 := mload(a_3)
+//         a := mload(b_4)
+//         b := mload(a)
+//     }
+//     mstore(a, b)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_multi_var_switch.yul b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_multi_var_switch.yul
new file mode 100644
index 00000000..50f56b87
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_multi_var_switch.yul
@@ -0,0 +1,38 @@
+{
+    let a := mload(0)
+    let b := mload(1)
+    switch mload(2)
+    case 0 {
+        a := mload(b)
+        b := mload(a)
+        a := mload(b)
+        b := mload(a)
+    }
+    default {
+        b := mload(a)
+        a := mload(b)
+        b := mload(a)
+        a := mload(b)
+    }
+    mstore(a, b)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     let b := mload(1)
+//     switch mload(2)
+//     case 0 {
+//         let a_3 := mload(b)
+//         let b_4 := mload(a_3)
+//         a := mload(b_4)
+//         b := mload(a)
+//     }
+//     default {
+//         let b_7 := mload(a)
+//         let a_8 := mload(b_7)
+//         b := mload(a_8)
+//         a := mload(b)
+//     }
+//     mstore(a, b)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_switch.yul b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_switch.yul
new file mode 100644
index 00000000..1efbbde7
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/multi_assign_switch.yul
@@ -0,0 +1,32 @@
+{
+    let a := mload(0)
+    switch mload(1)
+    case 0 {
+        a := mload(1)
+        a := mload(2)
+        a := mload(3)
+    }
+    default {
+        a := mload(4)
+        a := mload(5)
+        a := mload(6)
+    }
+    mstore(a, 0)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     switch mload(1)
+//     case 0 {
+//         pop(mload(1))
+//         pop(mload(2))
+//         a := mload(3)
+//     }
+//     default {
+//         pop(mload(4))
+//         pop(mload(5))
+//         a := mload(6)
+//     }
+//     mstore(a, 0)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul b/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul
index 23c433d1..912940c5 100644
--- a/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/simple.yul
@@ -1,18 +1,12 @@
 {
     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)
+//     let a_2 := mload(1)
+//     mstore(a_2, 0)
 // }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/single_assign_if.yul b/test/libyul/yulOptimizerTests/ssaAndBack/single_assign_if.yul
new file mode 100644
index 00000000..5185245c
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/single_assign_if.yul
@@ -0,0 +1,18 @@
+{
+    let a := mload(0)
+    if mload(1)
+    {
+        a := mload(1)
+    }
+    mstore(a, 0)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     if mload(1)
+//     {
+//         a := mload(1)
+//     }
+//     mstore(a, 0)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/single_assign_switch.yul b/test/libyul/yulOptimizerTests/ssaAndBack/single_assign_switch.yul
new file mode 100644
index 00000000..e0e53b3f
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/single_assign_switch.yul
@@ -0,0 +1,24 @@
+{
+    let a := mload(0)
+    switch mload(1)
+    case 0 {
+        a := mload(1)
+    }
+    default {
+        a := mload(2)
+    }
+    mstore(a, 0)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     switch mload(1)
+//     case 0 {
+//         a := mload(1)
+//     }
+//     default {
+//         a := mload(2)
+//     }
+//     mstore(a, 0)
+// }
diff --git a/test/libyul/yulOptimizerTests/ssaAndBack/two_vars.yul b/test/libyul/yulOptimizerTests/ssaAndBack/two_vars.yul
new file mode 100644
index 00000000..85325fb9
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaAndBack/two_vars.yul
@@ -0,0 +1,20 @@
+{
+    let a := mload(0)
+    let b := mload(a)
+    a := mload(b)
+    b := mload(a)
+    a := mload(b)
+    b := mload(a)
+    mstore(a, b)
+}
+// ----
+// ssaAndBack
+// {
+//     let a := mload(0)
+//     let b := mload(a)
+//     let a_3 := mload(b)
+//     let b_4 := mload(a_3)
+//     let a_5 := mload(b_4)
+//     let b_6 := mload(a_5)
+//     mstore(a_5, b_6)
+// }