Fix incorrect mstore removal.

Author: rodiazet

libyul/optimiser/UnusedStoreEliminator.cpp

@@ -44,6 +44,33 @@
 using namespace solidity;
 using namespace solidity::yul;
 
+namespace
+{
+	bool isInSSAClosedSet(Expression const& _expression, std::map<YulName, Expression const*>  const& _values)
+	{
+		if (auto const* functionCall = std::get_if<FunctionCall>(&_expression))
+		{
+			for (auto const& argument: functionCall->arguments)
+				if (!isInSSAClosedSet(argument, _values))
+					return false;
+
+			return true;
+		}
+		else if (auto const* identifier = std::get_if<Identifier>(&_expression))
+		{
+			auto const it = _values.find(identifier->name);
+			if (it == _values.end())
+				return false;
+			return isInSSAClosedSet(*it->second, _values);
+		}
+		else
+		{
+			solAssert(std::holds_alternative<Literal>(_expression), "Impossible expression type");
+			return true;
+		}
+	}
+}
+
 void UnusedStoreEliminator::run(OptimiserStepContext& _context, Block& _ast)
 {
 	std::map<FunctionHandle, SideEffects> functionSideEffects = SideEffectsPropagator::sideEffects(
@@ -54,8 +81,9 @@ void UnusedStoreEliminator::run(OptimiserStepContext& _context, Block& _ast)
 	SSAValueTracker ssaValues;
 	ssaValues(_ast);
 	std::map<YulName, AssignedValue> values;
-	for (auto const& [name, expression]: ssaValues.values())
-		values[name] = AssignedValue{expression, {}};
+	for (auto const& [variableName, expression]: ssaValues.values())
+		if (expression && isInSSAClosedSet(*expression, ssaValues.values()))
+			values[variableName] = AssignedValue{expression, {}};
 
 	bool const ignoreMemory = MSizeFinder::containsMSize(_context.dialect, _ast);
 	UnusedStoreEliminator rse{