Remove useless SWAP1 in front of commutative operations

Changelog.md

@@ -5,6 +5,7 @@ Features:
  * Commandline interface: Error when missing or inaccessible file detected. Suppress it with the ``--ignore-missing`` flag.
  * General: Support accessing dynamic return data in post-byzantium EVMs.
  * Interfaces: Allow overriding external functions in interfaces with public in an implementing contract.
+ * Optimizer: Remove useless ``SWAP1`` instruction preceding a commutative instruction (such as ``ADD``, ``MUL``, etc).
  * Optimizer: Optimize across ``mload`` if ``msize()`` is not used.
  * Syntax Checker: Issue warning for empty structs (or error as experimental 0.5.0 feature).
 

libevmasm/PeepholeOptimiser.cpp

@@ -154,6 +154,25 @@ struct DoublePush: SimplePeepholeOptimizerMethod<DoublePush, 2>
 	}
 };
 
+struct CommutativeSwap: SimplePeepholeOptimizerMethod<CommutativeSwap, 2>
+{
+	static bool applySimple(AssemblyItem const& _swap, AssemblyItem const& _op, std::back_insert_iterator<AssemblyItems> _out)
+	{
+		// Remove SWAP1 if following instruction is commutative
+		if (
+			_swap.type() == Operation &&
+			_swap.instruction() == Instruction::SWAP1 &&
+			SemanticInformation::isCommutativeOperation(_op)
+		)
+		{
+			*_out = _op;
+			return true;
+		}
+		else
+			return false;
+	}
+};
+
 struct JumpToNext: SimplePeepholeOptimizerMethod<JumpToNext, 3>
 {
 	static size_t applySimple(
@@ -260,7 +279,7 @@ bool PeepholeOptimiser::optimise()
 {
 	OptimiserState state {m_items, 0, std::back_inserter(m_optimisedItems)};
 	while (state.i < m_items.size())
-		applyMethods(state, PushPop(), OpPop(), DoublePush(), DoubleSwap(), JumpToNext(), UnreachableCode(), TagConjunctions(), Identity());
+		applyMethods(state, PushPop(), OpPop(), DoublePush(), DoubleSwap(), CommutativeSwap(), JumpToNext(), UnreachableCode(), TagConjunctions(), Identity());
 	if (m_optimisedItems.size() < m_items.size() || (
 		m_optimisedItems.size() == m_items.size() && (
 			eth::bytesRequired(m_optimisedItems, 3) < eth::bytesRequired(m_items, 3) ||

test/libevmasm/Optimiser.cpp

@@ -858,6 +858,57 @@ BOOST_AUTO_TEST_CASE(peephole_pop_calldatasize)
 	BOOST_CHECK(items.empty());
 }
 
+BOOST_AUTO_TEST_CASE(peephole_commutative_swap1)
+{
+	AssemblyItems items{
+		u256(1),
+		u256(2),
+		Instruction::SWAP1,
+		Instruction::ADD,
+		u256(4),
+		u256(5)
+	};
+	AssemblyItems expectation{
+		u256(1),
+		u256(2),
+		Instruction::ADD,
+		u256(4),
+		u256(5)
+	};
+	PeepholeOptimiser peepOpt(items);
+	BOOST_REQUIRE(peepOpt.optimise());
+	BOOST_CHECK_EQUAL_COLLECTIONS(
+		items.begin(), items.end(),
+		expectation.begin(), expectation.end()
+	);
+}
+
+BOOST_AUTO_TEST_CASE(peephole_noncommutative_swap1)
+{
+	AssemblyItems items{
+		u256(1),
+		u256(2),
+		Instruction::SWAP1,
+		Instruction::SUB,
+		u256(4),
+		u256(5)
+	};
+	AssemblyItems expectation{
+		u256(1),
+		u256(2),
+		Instruction::SWAP1,
+		Instruction::SUB,
+		u256(4),
+		u256(5)
+	};
+	PeepholeOptimiser peepOpt(items);
+	BOOST_REQUIRE(!peepOpt.optimise());
+	BOOST_CHECK_EQUAL_COLLECTIONS(
+		items.begin(), items.end(),
+		expectation.begin(), expectation.end()
+	);
+}
+
 BOOST_AUTO_TEST_CASE(jumpdest_removal)
 {
 	AssemblyItems items{