Optimize RETURN x 0 to STOP.

libevmasm/CommonSubexpressionEliminator.cpp

@@ -79,26 +79,26 @@ void CommonSubexpressionEliminator::feedItem(AssemblyItem const& _item, bool _co
 
 void CommonSubexpressionEliminator::optimizeBreakingItem()
 {
-	if (!m_breakingItem || *m_breakingItem != AssemblyItem(Instruction::JUMPI))
+	if (!m_breakingItem)
 		return;
 
+	ExpressionClasses& classes = m_state.expressionClasses();
 	SourceLocation const& location = m_breakingItem->getLocation();
+	if (*m_breakingItem == AssemblyItem(Instruction::JUMPI))
+	{
 		AssemblyItem::JumpType jumpType = m_breakingItem->getJumpType();
 
 		Id condition = m_state.stackElement(m_state.stackHeight() - 1, location);
-	Id zero = m_state.expressionClasses().find(u256(0));
+		if (classes.knownNonZero(condition))
-	if (m_state.expressionClasses().knownToBeDifferent(condition, zero))
 		{
 			feedItem(AssemblyItem(Instruction::SWAP1, location), true);
 			feedItem(AssemblyItem(Instruction::POP, location), true);
 
 			AssemblyItem item(Instruction::JUMP, location);
 			item.setJumpType(jumpType);
-		m_breakingItem = m_state.expressionClasses().storeItem(item);
+			m_breakingItem = classes.storeItem(item);
-		return;
 		}
-	Id negatedCondition = m_state.expressionClasses().find(Instruction::ISZERO, {condition});
+		else if (classes.knownZero(condition))
-	if (m_state.expressionClasses().knownToBeDifferent(negatedCondition, zero))
 		{
 			AssemblyItem it(Instruction::POP, location);
 			feedItem(it, true);
@@ -106,6 +106,18 @@ void CommonSubexpressionEliminator::optimizeBreakingItem()
 			m_breakingItem = nullptr;
 		}
 	}
+	else if (*m_breakingItem == AssemblyItem(Instruction::RETURN))
+	{
+		Id size = m_state.stackElement(m_state.stackHeight() - 1, location);
+		if (classes.knownZero(size))
+		{
+			feedItem(AssemblyItem(Instruction::POP, location), true);
+			feedItem(AssemblyItem(Instruction::POP, location), true);
+			AssemblyItem item(Instruction::STOP, location);
+			m_breakingItem = classes.storeItem(item);
+		}
+	}
+}
 
 CSECodeGenerator::CSECodeGenerator(
 	ExpressionClasses& _expressionClasses,

libsolidity/Compiler.cpp

@@ -298,6 +298,9 @@ void Compiler::appendReturnValuePacker(TypePointers const& _typeParameters)
 		stackDepth -= type->getSizeOnStack();
 	}
 	// note that the stack is not cleaned up here
+	if (dataOffset == 0)
+		m_context << eth::Instruction::STOP;
+	else
 		m_context << u256(dataOffset) << u256(0) << eth::Instruction::RETURN;
 }
 

test/libsolidity/SolidityOptimizer.cpp

@@ -944,6 +944,14 @@ BOOST_AUTO_TEST_CASE(cse_access_previous_sequence)
 	// 0, SLOAD, 1, ADD, SSTORE, 0 SLOAD
 }
 
+BOOST_AUTO_TEST_CASE(cse_optimise_return)
+{
+	checkCSE(
+		AssemblyItems{u256(0), u256(7), Instruction::RETURN},
+		AssemblyItems{Instruction::STOP}
+	);
+}
+
 BOOST_AUTO_TEST_CASE(control_flow_graph_remove_unused)
 {
 	// remove parts of the code that are unused