Merge pull request #4057 from ethereum/cfg

Control flow graph for uninitialized storage return detection.

31 files changed, 1419 insertions, 0 deletions

diff --git a/Changelog.md b/Changelog.md
index dfa24a25..6f6f672c 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -4,6 +4,8 @@ Features:
  * Remove deprecated ``constant`` as function state modifier from documentation and tests (but still leave it as a valid feature).
  * Build System: Update internal dependency of jsoncpp to 1.8.4, which introduces more strictness and reduces memory usage.
  * Code Generator: Use native shift instructions on target Constantinople.
+ * Control Flow Graph: Add Control Flow Graph as analysis structure.
+ * Control Flow Graph: Warn about returning uninitialized storage pointers.
  * Gas Estimator: Only explore paths with higher gas costs. This reduces accuracy but greatly improves the speed of gas estimation.
  * Optimizer: Remove unnecessary masking of the result of known short instructions (``ADDRESS``, ``CALLER``, ``ORIGIN`` and ``COINBASE``).
  * Parser: Display nicer error messages by showing the actual tokens and not internal names.
diff --git a/libsolidity/analysis/ControlFlowAnalyzer.cpp b/libsolidity/analysis/ControlFlowAnalyzer.cpp
new file mode 100644
index 00000000..6edf7986
--- /dev/null
+++ b/libsolidity/analysis/ControlFlowAnalyzer.cpp
@@ -0,0 +1,156 @@
+/*
+	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 <libsolidity/analysis/ControlFlowAnalyzer.h>
+
+using namespace std;
+using namespace dev::solidity;
+
+bool ControlFlowAnalyzer::analyze(ASTNode const& _astRoot)
+{
+	_astRoot.accept(*this);
+	return Error::containsOnlyWarnings(m_errorReporter.errors());
+}
+
+bool ControlFlowAnalyzer::visit(FunctionDefinition const& _function)
+{
+	auto const& functionFlow = m_cfg.functionFlow(_function);
+	checkUnassignedStorageReturnValues(_function, functionFlow.entry, functionFlow.exit);
+	return false;
+}
+
+set<VariableDeclaration const*> ControlFlowAnalyzer::variablesAssignedInNode(CFGNode const *node)
+{
+	set<VariableDeclaration const*> result;
+	for (auto expression: node->block.expressions)
+	{
+		if (auto const* assignment = dynamic_cast<Assignment const*>(expression))
+		{
+			stack<Expression const*> expressions;
+			expressions.push(&assignment->leftHandSide());
+			while (!expressions.empty())
+			{
+				Expression const* expression = expressions.top();
+				expressions.pop();
+
+				if (auto const *tuple = dynamic_cast<TupleExpression const*>(expression))
+					for (auto const& component: tuple->components())
+						expressions.push(component.get());
+				else if (auto const* identifier = dynamic_cast<Identifier const*>(expression))
+					if (auto const* variableDeclaration = dynamic_cast<VariableDeclaration const*>(
+						identifier->annotation().referencedDeclaration
+					))
+						result.insert(variableDeclaration);
+			}
+		}
+	}
+	return result;
+}
+
+void ControlFlowAnalyzer::checkUnassignedStorageReturnValues(
+	FunctionDefinition const& _function,
+	CFGNode const* _functionEntry,
+	CFGNode const* _functionExit
+) const
+{
+	if (_function.returnParameterList()->parameters().empty())
+		return;
+
+	map<CFGNode const*, set<VariableDeclaration const*>> unassigned;
+
+	{
+		auto& unassignedAtFunctionEntry = unassigned[_functionEntry];
+		for (auto const& returnParameter: _function.returnParameterList()->parameters())
+			if (returnParameter->type()->dataStoredIn(DataLocation::Storage))
+				unassignedAtFunctionEntry.insert(returnParameter.get());
+	}
+
+	stack<CFGNode const*> nodesToTraverse;
+	nodesToTraverse.push(_functionEntry);
+
+	// walk all paths from entry with maximal set of unassigned return values
+	while (!nodesToTraverse.empty())
+	{
+		auto node = nodesToTraverse.top();
+		nodesToTraverse.pop();
+
+		auto& unassignedAtNode = unassigned[node];
+
+		if (node->block.returnStatement != nullptr)
+			if (node->block.returnStatement->expression())
+				unassignedAtNode.clear();
+		if (!unassignedAtNode.empty())
+		{
+			// kill all return values to which a value is assigned
+			for (auto const* variableDeclaration: variablesAssignedInNode(node))
+				unassignedAtNode.erase(variableDeclaration);
+
+			// kill all return values referenced in inline assembly
+			// a reference is enough, checking whether there actually was an assignment might be overkill
+			for (auto assembly: node->block.inlineAssemblyStatements)
+				for (auto const& ref: assembly->annotation().externalReferences)
+					if (auto variableDeclaration = dynamic_cast<VariableDeclaration const*>(ref.second.declaration))
+						unassignedAtNode.erase(variableDeclaration);
+		}
+
+		for (auto const& exit: node->exits)
+		{
+			auto& unassignedAtExit = unassigned[exit];
+			auto oldSize = unassignedAtExit.size();
+			unassignedAtExit.insert(unassignedAtNode.begin(), unassignedAtNode.end());
+			// (re)traverse an exit, if we are on a path with new unassigned return values to consider
+			// this will terminate, since there is only a finite number of unassigned return values
+			if (unassignedAtExit.size() > oldSize)
+				nodesToTraverse.push(exit);
+		}
+	}
+
+	if (!unassigned[_functionExit].empty())
+	{
+		vector<VariableDeclaration const*> unassignedOrdered(
+			unassigned[_functionExit].begin(),
+			unassigned[_functionExit].end()
+		);
+		sort(
+			unassignedOrdered.begin(),
+			unassignedOrdered.end(),
+			[](VariableDeclaration const* lhs, VariableDeclaration const* rhs) -> bool {
+				return lhs->id() < rhs->id();
+			}
+		);
+		for (auto const* returnVal: unassignedOrdered)
+		{
+			SecondarySourceLocation ssl;
+			for (CFGNode* lastNodeBeforeExit: _functionExit->entries)
+				if (unassigned[lastNodeBeforeExit].count(returnVal))
+				{
+					if (!!lastNodeBeforeExit->block.returnStatement)
+						ssl.append("Problematic return:", lastNodeBeforeExit->block.returnStatement->location());
+					else
+						ssl.append("Problematic end of function:", _function.location());
+				}
+
+			m_errorReporter.warning(
+				returnVal->location(),
+				"This variable is of storage pointer type and might be returned without assignment. "
+				"This can cause storage corruption. Assign the variable (potentially from itself) "
+				"to remove this warning.",
+				ssl
+			);
+		}
+	}
+}
diff --git a/libsolidity/analysis/ControlFlowAnalyzer.h b/libsolidity/analysis/ControlFlowAnalyzer.h
new file mode 100644
index 00000000..43e13fb6
--- /dev/null
+++ b/libsolidity/analysis/ControlFlowAnalyzer.h
@@ -0,0 +1,52 @@
+/*
+	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 <libsolidity/analysis/ControlFlowGraph.h>
+
+#include <set>
+
+namespace dev
+{
+namespace solidity
+{
+
+class ControlFlowAnalyzer: private ASTConstVisitor
+{
+public:
+	explicit ControlFlowAnalyzer(CFG const& _cfg, ErrorReporter& _errorReporter):
+		m_cfg(_cfg), m_errorReporter(_errorReporter) {}
+
+	bool analyze(ASTNode const& _astRoot);
+
+	virtual bool visit(FunctionDefinition const& _function) override;
+
+private:
+	static std::set<VariableDeclaration const*> variablesAssignedInNode(CFGNode const *node);
+	void checkUnassignedStorageReturnValues(
+		FunctionDefinition const& _function,
+		CFGNode const* _functionEntry,
+		CFGNode const* _functionExit
+	) const;
+
+	CFG const& m_cfg;
+	ErrorReporter& m_errorReporter;
+};
+
+}
+}
diff --git a/libsolidity/analysis/ControlFlowBuilder.cpp b/libsolidity/analysis/ControlFlowBuilder.cpp
new file mode 100644
index 00000000..35d7687c
--- /dev/null
+++ b/libsolidity/analysis/ControlFlowBuilder.cpp
@@ -0,0 +1,370 @@
+/*
+	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 <libsolidity/analysis/ControlFlowBuilder.h>
+
+using namespace dev;
+using namespace solidity;
+using namespace std;
+
+ControlFlowBuilder::ControlFlowBuilder(CFG::NodeContainer& _nodeContainer, FunctionFlow const& _functionFlow):
+	m_nodeContainer(_nodeContainer), m_currentFunctionFlow(_functionFlow), m_currentNode(_functionFlow.entry)
+{
+}
+
+unique_ptr<FunctionFlow> ControlFlowBuilder::createFunctionFlow(
+	CFG::NodeContainer& _nodeContainer,
+	FunctionDefinition const& _function
+)
+{
+	auto functionFlow = unique_ptr<FunctionFlow>(new FunctionFlow());
+	functionFlow->entry = _nodeContainer.newNode();
+	functionFlow->exit = _nodeContainer.newNode();
+	functionFlow->revert = _nodeContainer.newNode();
+	ControlFlowBuilder builder(_nodeContainer, *functionFlow);
+	builder.appendControlFlow(_function);
+	connect(builder.m_currentNode, functionFlow->exit);
+	return functionFlow;
+}
+
+
+unique_ptr<ModifierFlow> ControlFlowBuilder::createModifierFlow(
+	CFG::NodeContainer& _nodeContainer,
+	ModifierDefinition const& _modifier
+)
+{
+	auto modifierFlow = unique_ptr<ModifierFlow>(new ModifierFlow());
+	modifierFlow->entry = _nodeContainer.newNode();
+	modifierFlow->exit = _nodeContainer.newNode();
+	modifierFlow->revert = _nodeContainer.newNode();
+	modifierFlow->placeholderEntry = _nodeContainer.newNode();
+	modifierFlow->placeholderExit = _nodeContainer.newNode();
+	ControlFlowBuilder builder(_nodeContainer, *modifierFlow);
+	builder.appendControlFlow(_modifier);
+	connect(builder.m_currentNode, modifierFlow->exit);
+	return modifierFlow;
+}
+
+bool ControlFlowBuilder::visit(BinaryOperation const& _operation)
+{
+	solAssert(!!m_currentNode, "");
+
+	switch(_operation.getOperator())
+	{
+		case Token::Or:
+		case Token::And:
+		{
+			appendControlFlow(_operation.leftExpression());
+
+			auto nodes = splitFlow<2>();
+			nodes[0] = createFlow(nodes[0], _operation.rightExpression());
+			mergeFlow(nodes, nodes[1]);
+
+			return false;
+		}
+		default:
+			break;
+	}
+	return ASTConstVisitor::visit(_operation);
+}
+
+bool ControlFlowBuilder::visit(Conditional const& _conditional)
+{
+	solAssert(!!m_currentNode, "");
+
+	_conditional.condition().accept(*this);
+
+	auto nodes = splitFlow<2>();
+
+	nodes[0] = createFlow(nodes[0], _conditional.trueExpression());
+	nodes[1] = createFlow(nodes[1], _conditional.falseExpression());
+
+	mergeFlow(nodes);
+
+	return false;
+}
+
+bool ControlFlowBuilder::visit(IfStatement const& _ifStatement)
+{
+	solAssert(!!m_currentNode, "");
+
+	_ifStatement.condition().accept(*this);
+
+	auto nodes = splitFlow<2>();
+	nodes[0] = createFlow(nodes[0], _ifStatement.trueStatement());
+
+	if (_ifStatement.falseStatement())
+	{
+		nodes[1] = createFlow(nodes[1], *_ifStatement.falseStatement());
+		mergeFlow(nodes);
+	}
+	else
+		mergeFlow(nodes, nodes[1]);
+
+	return false;
+}
+
+bool ControlFlowBuilder::visit(ForStatement const& _forStatement)
+{
+	solAssert(!!m_currentNode, "");
+
+	if (_forStatement.initializationExpression())
+		_forStatement.initializationExpression()->accept(*this);
+
+	auto condition = createLabelHere();
+
+	if (_forStatement.condition())
+		appendControlFlow(*_forStatement.condition());
+
+	auto loopExpression = newLabel();
+	auto nodes = splitFlow<2>();
+	auto afterFor = nodes[1];
+	m_currentNode = nodes[0];
+
+	{
+		BreakContinueScope scope(*this, afterFor, loopExpression);
+		appendControlFlow(_forStatement.body());
+	}
+
+	placeAndConnectLabel(loopExpression);
+
+	if (auto expression = _forStatement.loopExpression())
+		appendControlFlow(*expression);
+
+	connect(m_currentNode, condition);
+	m_currentNode = afterFor;
+
+	return false;
+}
+
+bool ControlFlowBuilder::visit(WhileStatement const& _whileStatement)
+{
+	solAssert(!!m_currentNode, "");
+
+	if (_whileStatement.isDoWhile())
+	{
+		auto afterWhile = newLabel();
+		auto whileBody = createLabelHere();
+
+		{
+			// Note that "continue" in this case currently indeed jumps to whileBody
+			// and not to the condition. This is inconsistent with JavaScript and C and
+			// therefore a bug. This will be fixed in the future (planned for 0.5.0)
+			// and the Control Flow Graph will have to be adjusted accordingly.
+			BreakContinueScope scope(*this, afterWhile, whileBody);
+			appendControlFlow(_whileStatement.body());
+		}
+		appendControlFlow(_whileStatement.condition());
+
+		connect(m_currentNode, whileBody);
+		placeAndConnectLabel(afterWhile);
+	}
+	else
+	{
+		auto whileCondition = createLabelHere();
+
+		appendControlFlow(_whileStatement.condition());
+
+		auto nodes = splitFlow<2>();
+
+		auto whileBody = nodes[0];
+		auto afterWhile = nodes[1];
+
+		m_currentNode = whileBody;
+		{
+			BreakContinueScope scope(*this, afterWhile, whileCondition);
+			appendControlFlow(_whileStatement.body());
+		}
+
+		connect(m_currentNode, whileCondition);
+
+		m_currentNode = afterWhile;
+	}
+
+
+	return false;
+}
+
+bool ControlFlowBuilder::visit(Break const&)
+{
+	solAssert(!!m_currentNode, "");
+	solAssert(!!m_breakJump, "");
+	connect(m_currentNode, m_breakJump);
+	m_currentNode = newLabel();
+	return false;
+}
+
+bool ControlFlowBuilder::visit(Continue const&)
+{
+	solAssert(!!m_currentNode, "");
+	solAssert(!!m_continueJump, "");
+	connect(m_currentNode, m_continueJump);
+	m_currentNode = newLabel();
+	return false;
+}
+
+bool ControlFlowBuilder::visit(Throw const&)
+{
+	solAssert(!!m_currentNode, "");
+	solAssert(!!m_currentFunctionFlow.revert, "");
+	connect(m_currentNode, m_currentFunctionFlow.revert);
+	m_currentNode = newLabel();
+	return false;
+}
+
+bool ControlFlowBuilder::visit(Block const&)
+{
+	solAssert(!!m_currentNode, "");
+	createLabelHere();
+	return true;
+}
+
+void ControlFlowBuilder::endVisit(Block const&)
+{
+	solAssert(!!m_currentNode, "");
+	createLabelHere();
+}
+
+bool ControlFlowBuilder::visit(Return const& _return)
+{
+	solAssert(!!m_currentNode, "");
+	solAssert(!!m_currentFunctionFlow.exit, "");
+	solAssert(!m_currentNode->block.returnStatement, "");
+	m_currentNode->block.returnStatement = &_return;
+	connect(m_currentNode, m_currentFunctionFlow.exit);
+	m_currentNode = newLabel();
+	return true;
+}
+
+
+bool ControlFlowBuilder::visit(PlaceholderStatement const&)
+{
+	solAssert(!!m_currentNode, "");
+	auto modifierFlow = dynamic_cast<ModifierFlow const*>(&m_currentFunctionFlow);
+	solAssert(!!modifierFlow, "");
+
+	connect(m_currentNode, modifierFlow->placeholderEntry);
+
+	m_currentNode = newLabel();
+
+	connect(modifierFlow->placeholderExit, m_currentNode);
+	return false;
+}
+
+bool ControlFlowBuilder::visitNode(ASTNode const& node)
+{
+	solAssert(!!m_currentNode, "");
+	if (auto const* expression = dynamic_cast<Expression const*>(&node))
+		m_currentNode->block.expressions.emplace_back(expression);
+	else if (auto const* variableDeclaration = dynamic_cast<VariableDeclaration const*>(&node))
+		m_currentNode->block.variableDeclarations.emplace_back(variableDeclaration);
+	else if (auto const* assembly = dynamic_cast<InlineAssembly const*>(&node))
+		m_currentNode->block.inlineAssemblyStatements.emplace_back(assembly);
+
+	return true;
+}
+
+bool ControlFlowBuilder::visit(FunctionCall const& _functionCall)
+{
+	solAssert(!!m_currentNode, "");
+	solAssert(!!_functionCall.expression().annotation().type, "");
+
+	if (auto functionType = dynamic_pointer_cast<FunctionType const>(_functionCall.expression().annotation().type))
+		switch (functionType->kind())
+		{
+			case FunctionType::Kind::Revert:
+				solAssert(!!m_currentFunctionFlow.revert, "");
+				_functionCall.expression().accept(*this);
+				ASTNode::listAccept(_functionCall.arguments(), *this);
+				connect(m_currentNode, m_currentFunctionFlow.revert);
+				m_currentNode = newLabel();
+				return false;
+			case FunctionType::Kind::Require:
+			case FunctionType::Kind::Assert:
+			{
+				solAssert(!!m_currentFunctionFlow.revert, "");
+				_functionCall.expression().accept(*this);
+				ASTNode::listAccept(_functionCall.arguments(), *this);
+				connect(m_currentNode, m_currentFunctionFlow.revert);
+				auto nextNode = newLabel();
+				connect(m_currentNode, nextNode);
+				m_currentNode = nextNode;
+				return false;
+			}
+			default:
+				break;
+		}
+	return ASTConstVisitor::visit(_functionCall);
+}
+
+void ControlFlowBuilder::appendControlFlow(ASTNode const& _node)
+{
+	_node.accept(*this);
+}
+
+CFGNode* ControlFlowBuilder::createFlow(CFGNode* _entry, ASTNode const& _node)
+{
+	auto oldCurrentNode = m_currentNode;
+	m_currentNode = _entry;
+	appendControlFlow(_node);
+	auto endNode = m_currentNode;
+	m_currentNode = oldCurrentNode;
+	return endNode;
+}
+
+void ControlFlowBuilder::connect(CFGNode* _from, CFGNode* _to)
+{
+	solAssert(_from, "");
+	solAssert(_to, "");
+	_from->exits.push_back(_to);
+	_to->entries.push_back(_from);
+}
+
+CFGNode* ControlFlowBuilder::newLabel()
+{
+	return m_nodeContainer.newNode();
+}
+
+CFGNode* ControlFlowBuilder::createLabelHere()
+{
+	auto label = m_nodeContainer.newNode();
+	connect(m_currentNode, label);
+	m_currentNode = label;
+	return label;
+}
+
+void ControlFlowBuilder::placeAndConnectLabel(CFGNode* _node)
+{
+	connect(m_currentNode, _node);
+	m_currentNode = _node;
+}
+
+ControlFlowBuilder::BreakContinueScope::BreakContinueScope(
+	ControlFlowBuilder& _parser,
+	CFGNode* _breakJump,
+	CFGNode* _continueJump
+): m_parser(_parser), m_origBreakJump(_parser.m_breakJump), m_origContinueJump(_parser.m_continueJump)
+{
+	m_parser.m_breakJump = _breakJump;
+	m_parser.m_continueJump = _continueJump;
+}
+
+ControlFlowBuilder::BreakContinueScope::~BreakContinueScope()
+{
+	m_parser.m_breakJump = m_origBreakJump;
+	m_parser.m_continueJump = m_origContinueJump;
+}
diff --git a/libsolidity/analysis/ControlFlowBuilder.h b/libsolidity/analysis/ControlFlowBuilder.h
new file mode 100644
index 00000000..e9d96e5f
--- /dev/null
+++ b/libsolidity/analysis/ControlFlowBuilder.h
@@ -0,0 +1,143 @@
+/*
+	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 <libsolidity/analysis/ControlFlowGraph.h>
+#include <libsolidity/ast/AST.h>
+#include <libsolidity/ast/ASTVisitor.h>
+
+#include <array>
+#include <memory>
+
+namespace dev {
+namespace solidity {
+
+/** Helper class that builds the control flow of a function or modifier.
+ * Modifiers are not yet applied to the functions. This is done in a second
+ * step in the CFG class.
+ */
+class ControlFlowBuilder: private ASTConstVisitor
+{
+public:
+	static std::unique_ptr<FunctionFlow> createFunctionFlow(
+		CFG::NodeContainer& _nodeContainer,
+		FunctionDefinition const& _function
+	);
+	static std::unique_ptr<ModifierFlow> createModifierFlow(
+		CFG::NodeContainer& _nodeContainer,
+		ModifierDefinition const& _modifier
+	);
+
+private:
+	explicit ControlFlowBuilder(CFG::NodeContainer& _nodeContainer, FunctionFlow const& _functionFlow);
+
+	virtual bool visit(BinaryOperation const& _operation) override;
+	virtual bool visit(Conditional const& _conditional) override;
+	virtual bool visit(IfStatement const& _ifStatement) override;
+	virtual bool visit(ForStatement const& _forStatement) override;
+	virtual bool visit(WhileStatement const& _whileStatement) override;
+	virtual bool visit(Break const&) override;
+	virtual bool visit(Continue const&) override;
+	virtual bool visit(Throw const&) override;
+	virtual bool visit(Block const&) override;
+	virtual void endVisit(Block const&) override;
+	virtual bool visit(Return const& _return) override;
+	virtual bool visit(PlaceholderStatement const&) override;
+	virtual bool visit(FunctionCall const& _functionCall) override;
+
+
+	/// Appends the control flow of @a _node to the current control flow.
+	void appendControlFlow(ASTNode const& _node);
+
+	/// Starts at @a _entry and parses the control flow of @a _node.
+	/// @returns The node at which the parsed control flow ends.
+	/// m_currentNode is not affected (it is saved and restored).
+	CFGNode* createFlow(CFGNode* _entry, ASTNode const& _node);
+
+	/// Creates an arc from @a _from to @a _to.
+	static void connect(CFGNode* _from, CFGNode* _to);
+
+
+protected:
+	virtual bool visitNode(ASTNode const& node) override;
+
+private:
+
+	/// Splits the control flow starting at the current node into n paths.
+	/// m_currentNode is set to nullptr and has to be set manually or
+	/// using mergeFlow later.
+	template<size_t n>
+	std::array<CFGNode*, n> splitFlow()
+	{
+		std::array<CFGNode*, n> result;
+		for (auto& node: result)
+		{
+			node = m_nodeContainer.newNode();
+			connect(m_currentNode, node);
+		}
+		m_currentNode = nullptr;
+		return result;
+	}
+
+	/// Merges the control flow of @a _nodes to @a _endNode.
+	/// If @a _endNode is nullptr, a new node is creates and used as end node.
+	/// Sets the merge destination as current node.
+	/// Note: @a _endNode may be one of the nodes in @a _nodes.
+	template<size_t n>
+	void mergeFlow(std::array<CFGNode*, n> const& _nodes, CFGNode* _endNode = nullptr)
+	{
+		CFGNode* mergeDestination = (_endNode == nullptr) ? m_nodeContainer.newNode() : _endNode;
+		for (auto& node: _nodes)
+			if (node != mergeDestination)
+				connect(node, mergeDestination);
+		m_currentNode = mergeDestination;
+	}
+
+	CFGNode* newLabel();
+	CFGNode* createLabelHere();
+	void placeAndConnectLabel(CFGNode *_node);
+
+	CFG::NodeContainer& m_nodeContainer;
+
+	/// The control flow of the function that is currently parsed.
+	/// Note: this can also be a ModifierFlow
+	FunctionFlow const& m_currentFunctionFlow;
+
+	CFGNode* m_currentNode = nullptr;
+
+	/// The current jump destination of break Statements.
+	CFGNode* m_breakJump = nullptr;
+	/// The current jump destination of continue Statements.
+	CFGNode* m_continueJump = nullptr;
+
+	/// Helper class that replaces the break and continue jump destinations for the
+	/// current scope and restores the originals at the end of the scope.
+	class BreakContinueScope
+	{
+	public:
+		BreakContinueScope(ControlFlowBuilder& _parser, CFGNode* _breakJump, CFGNode* _continueJump);
+		~BreakContinueScope();
+	private:
+		ControlFlowBuilder& m_parser;
+		CFGNode* m_origBreakJump;
+		CFGNode* m_origContinueJump;
+	};
+};
+
+}
+}
diff --git a/libsolidity/analysis/ControlFlowGraph.cpp b/libsolidity/analysis/ControlFlowGraph.cpp
new file mode 100644
index 00000000..9b3da0eb
--- /dev/null
+++ b/libsolidity/analysis/ControlFlowGraph.cpp
@@ -0,0 +1,136 @@
+/*
+	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 <libsolidity/analysis/ControlFlowGraph.h>
+#include <libsolidity/analysis/ControlFlowBuilder.h>
+
+#include <boost/range/adaptor/reversed.hpp>
+
+#include <algorithm>
+
+using namespace std;
+using namespace dev::solidity;
+
+bool CFG::constructFlow(ASTNode const& _astRoot)
+{
+	_astRoot.accept(*this);
+	applyModifiers();
+	return Error::containsOnlyWarnings(m_errorReporter.errors());
+}
+
+
+bool CFG::visit(ModifierDefinition const& _modifier)
+{
+	m_modifierControlFlow[&_modifier] = ControlFlowBuilder::createModifierFlow(m_nodeContainer, _modifier);
+	return false;
+}
+
+bool CFG::visit(FunctionDefinition const& _function)
+{
+	m_functionControlFlow[&_function] = ControlFlowBuilder::createFunctionFlow(m_nodeContainer, _function);
+	return false;
+}
+
+FunctionFlow const& CFG::functionFlow(FunctionDefinition const& _function) const
+{
+	solAssert(m_functionControlFlow.count(&_function), "");
+	return *m_functionControlFlow.find(&_function)->second;
+}
+
+CFGNode* CFG::NodeContainer::newNode()
+{
+	m_nodes.emplace_back(new CFGNode());
+	return m_nodes.back().get();
+}
+
+void CFG::applyModifiers()
+{
+	for (auto const& function: m_functionControlFlow)
+	{
+		for (auto const& modifierInvocation: boost::adaptors::reverse(function.first->modifiers()))
+		{
+			if (auto modifierDefinition = dynamic_cast<ModifierDefinition const*>(
+				modifierInvocation->name()->annotation().referencedDeclaration
+			))
+			{
+				solAssert(m_modifierControlFlow.count(modifierDefinition), "");
+				applyModifierFlowToFunctionFlow(*m_modifierControlFlow[modifierDefinition], function.second.get());
+			}
+		}
+	}
+}
+
+void CFG::applyModifierFlowToFunctionFlow(
+	ModifierFlow const& _modifierFlow,
+	FunctionFlow* _functionFlow
+)
+{
+	solAssert(!!_functionFlow, "");
+
+	map<CFGNode*, CFGNode*> copySrcToCopyDst;
+
+	// inherit the revert node of the function
+	copySrcToCopyDst[_modifierFlow.revert] = _functionFlow->revert;
+
+	// replace the placeholder nodes by the function entry and exit
+	copySrcToCopyDst[_modifierFlow.placeholderEntry] = _functionFlow->entry;
+	copySrcToCopyDst[_modifierFlow.placeholderExit] = _functionFlow->exit;
+
+	stack<CFGNode*> nodesToCopy;
+	nodesToCopy.push(_modifierFlow.entry);
+
+	// map the modifier entry to a new node that will become the new function entry
+	copySrcToCopyDst[_modifierFlow.entry] = m_nodeContainer.newNode();
+
+	while (!nodesToCopy.empty())
+	{
+		CFGNode* copySrcNode = nodesToCopy.top();
+		nodesToCopy.pop();
+
+		solAssert(copySrcToCopyDst.count(copySrcNode), "");
+
+		CFGNode* copyDstNode = copySrcToCopyDst[copySrcNode];
+
+		copyDstNode->block = copySrcNode->block;
+		for (auto const& entry: copySrcNode->entries)
+		{
+			if (!copySrcToCopyDst.count(entry))
+			{
+				copySrcToCopyDst[entry] = m_nodeContainer.newNode();
+				nodesToCopy.push(entry);
+			}
+			copyDstNode->entries.emplace_back(copySrcToCopyDst[entry]);
+		}
+		for (auto const& exit: copySrcNode->exits)
+		{
+			if (!copySrcToCopyDst.count(exit))
+			{
+				copySrcToCopyDst[exit] = m_nodeContainer.newNode();
+				nodesToCopy.push(exit);
+			}
+			copyDstNode->exits.emplace_back(copySrcToCopyDst[exit]);
+		}
+	}
+
+	// if the modifier control flow never reached its exit node,
+	// we need to create a new (disconnected) exit node now
+	if (!copySrcToCopyDst.count(_modifierFlow.exit))
+		copySrcToCopyDst[_modifierFlow.exit] = m_nodeContainer.newNode();
+
+	_functionFlow->entry = copySrcToCopyDst[_modifierFlow.entry];
+	_functionFlow->exit = copySrcToCopyDst[_modifierFlow.exit];
+}
\ No newline at end of file
diff --git a/libsolidity/analysis/ControlFlowGraph.h b/libsolidity/analysis/ControlFlowGraph.h
new file mode 100644
index 00000000..c646e4f1
--- /dev/null
+++ b/libsolidity/analysis/ControlFlowGraph.h
@@ -0,0 +1,148 @@
+/*
+	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 <libsolidity/ast/AST.h>
+#include <libsolidity/ast/ASTVisitor.h>
+#include <libsolidity/interface/ErrorReporter.h>
+
+#include <map>
+#include <memory>
+#include <stack>
+#include <vector>
+
+namespace dev
+{
+namespace solidity
+{
+
+/** Basic Control Flow Block.
+ * Basic block of control flow. Consists of a set of (unordered) AST nodes
+ * for which control flow is always linear. A basic control flow block
+ * encompasses at most one scope. Reverts are considered to break the control
+ * flow.
+ * @todo Handle function calls correctly. So far function calls are not considered
+ * to change the control flow.
+ */
+struct ControlFlowBlock
+{
+	/// All variable declarations inside this control flow block.
+	std::vector<VariableDeclaration const*> variableDeclarations;
+	/// All expressions inside this control flow block (this includes all subexpressions!).
+	std::vector<Expression const*> expressions;
+	/// All inline assembly statements inside in this control flow block.
+	std::vector<InlineAssembly const*> inlineAssemblyStatements;
+	/// If control flow returns in this node, the return statement is stored in returnStatement,
+	/// otherwise returnStatement is nullptr.
+	Return const* returnStatement = nullptr;
+};
+
+/** Node of the Control Flow Graph.
+  * The control flow is a directed graph connecting control flow blocks.
+  * An arc between two nodes indicates that the control flow can possibly
+  * move from its start node to its end node during execution.
+  */
+struct CFGNode
+{
+	/// Entry nodes. All CFG nodes from which control flow may move into this node.
+	std::vector<CFGNode*> entries;
+	/// Exit nodes. All CFG nodes to which control flow may continue after this node.
+	std::vector<CFGNode*> exits;
+
+	/// Control flow in the node.
+	ControlFlowBlock block;
+};
+
+/** Describes the control flow of a function. */
+struct FunctionFlow
+{
+	virtual ~FunctionFlow() {}
+	/// Entry node. Control flow of the function starts here.
+	/// This node is empty and does not have any entries.
+	CFGNode* entry = nullptr;
+	/// Exit node. All non-reverting control flow of the function ends here.
+	/// This node is empty and does not have any exits, but may have multiple entries
+	/// (e.g. all return statements of the function).
+	CFGNode* exit = nullptr;
+	/// Revert node. Control flow of the function in case of revert.
+	/// This node is empty does not have any exits, but may have multiple entries
+	/// (e.g. all assert, require, revert and throw statements).
+	CFGNode* revert = nullptr;
+};
+
+/** Describes the control flow of a modifier.
+ * Every placeholder breaks the control flow. The node preceding the
+ * placeholder is assigned placeholderEntry as exit and the node
+ * following the placeholder is assigned placeholderExit as entry.
+ */
+struct ModifierFlow: FunctionFlow
+{
+	/// Control flow leading towards a placeholder exit in placeholderEntry.
+	CFGNode* placeholderEntry = nullptr;
+	/// Control flow coming from a placeholder enter from placeholderExit.
+	CFGNode* placeholderExit = nullptr;
+};
+
+class CFG: private ASTConstVisitor
+{
+public:
+	explicit CFG(ErrorReporter& _errorReporter): m_errorReporter(_errorReporter) {}
+
+	bool constructFlow(ASTNode const& _astRoot);
+
+	virtual bool visit(ModifierDefinition const& _modifier) override;
+	virtual bool visit(FunctionDefinition const& _function) override;
+
+	FunctionFlow const& functionFlow(FunctionDefinition const& _function) const;
+
+	class NodeContainer
+	{
+	public:
+		CFGNode* newNode();
+	private:
+		std::vector<std::unique_ptr<CFGNode>> m_nodes;
+	};
+private:
+	/// Initially the control flow for all functions *ignoring* modifiers and for
+	/// all modifiers is constructed. Afterwards the control flow of functions
+	/// is adjusted by applying all modifiers.
+	void applyModifiers();
+
+	/// Creates a copy of the modifier flow @a _modifierFlow, while replacing the
+	/// placeholder entry and exit with the function entry and exit, as well as
+	/// replacing the modifier revert node with the function's revert node.
+	/// The resulting control flow is the new function flow with the modifier applied.
+	/// @a _functionFlow is updated in-place.
+	void applyModifierFlowToFunctionFlow(
+		ModifierFlow const& _modifierFlow,
+		FunctionFlow* _functionFlow
+	);
+
+	ErrorReporter& m_errorReporter;
+
+	/// Node container.
+	/// All nodes allocated during the construction of the control flow graph
+	/// are owned by the CFG class and stored in this container.
+	NodeContainer m_nodeContainer;
+
+	std::map<FunctionDefinition const*, std::unique_ptr<FunctionFlow>> m_functionControlFlow;
+	std::map<ModifierDefinition const*, std::unique_ptr<ModifierFlow>> m_modifierControlFlow;
+};
+
+}
+}
diff --git a/libsolidity/interface/CompilerStack.cpp b/libsolidity/interface/CompilerStack.cpp
index 4ff14aa2..47dc30cf 100644
--- a/libsolidity/interface/CompilerStack.cpp
+++ b/libsolidity/interface/CompilerStack.cpp
@@ -29,6 +29,8 @@
 #include <libsolidity/ast/AST.h>
 #include <libsolidity/parsing/Scanner.h>
 #include <libsolidity/parsing/Parser.h>
+#include <libsolidity/analysis/ControlFlowAnalyzer.h>
+#include <libsolidity/analysis/ControlFlowGraph.h>
 #include <libsolidity/analysis/GlobalContext.h>
 #include <libsolidity/analysis/NameAndTypeResolver.h>
 #include <libsolidity/analysis/TypeChecker.h>
@@ -224,6 +226,22 @@ bool CompilerStack::analyze()
 
 		if (noErrors)
 		{
+			CFG cfg(m_errorReporter);
+			for (Source const* source: m_sourceOrder)
+				if (!cfg.constructFlow(*source->ast))
+					noErrors = false;
+
+			if (noErrors)
+			{
+				ControlFlowAnalyzer controlFlowAnalyzer(cfg, m_errorReporter);
+				for (Source const* source: m_sourceOrder)
+					if (!controlFlowAnalyzer.analyze(*source->ast))
+						noErrors = false;
+			}
+		}
+
+		if (noErrors)
+		{
 			StaticAnalyzer staticAnalyzer(m_errorReporter);
 			for (Source const* source: m_sourceOrder)
 				if (!staticAnalyzer.analyze(*source->ast))
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/assembly_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/assembly_fine.sol
new file mode 100644
index 00000000..65902cc8
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/assembly_fine.sol
@@ -0,0 +1,26 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal returns (S storage c) {
+        assembly {
+            sstore(c_slot, sload(s_slot))
+        }
+    }
+    function g(bool flag) internal returns (S storage c) {
+        // control flow in assembly will not be analyzed for now,
+        // so this will not issue a warning
+        assembly {
+            if flag {
+                sstore(c_slot, sload(s_slot))
+            }
+        }
+    }
+    function h() internal returns (S storage c) {
+        // any reference from assembly will be sufficient for now,
+        // so this will not issue a warning
+        assembly {
+            sstore(s_slot, sload(c_slot))
+        }
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/assembly_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/assembly_warn.sol
new file mode 100644
index 00000000..09c13847
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/assembly_warn.sol
@@ -0,0 +1,10 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal pure returns (S storage) {
+        assembly {
+        }
+    }
+}
+// ----
+// Warning: (87-88): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/default_location.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/default_location.sol
new file mode 100644
index 00000000..9a42192d
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/default_location.sol
@@ -0,0 +1,19 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S c) {
+        c = s;
+    }
+    function g() internal view returns (S) {
+        return s;
+    }
+    function h() internal pure returns (S) {
+    }
+    function i(bool flag) internal view returns (S c) {
+        if (flag) c = s;
+    }
+    function j(bool flag) internal view returns (S) {
+        if (flag) return s;
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/dowhile_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/dowhile_fine.sol
new file mode 100644
index 00000000..6520672c
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/dowhile_fine.sol
@@ -0,0 +1,36 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        do {} while((c = s).f);
+    }
+    function g() internal view returns (S storage c) {
+        do { c = s; } while(false);
+    }
+    function h() internal view returns (S storage c) {
+        c = s;
+        do {} while(false);
+    }
+    function i() internal view returns (S storage c) {
+        do {} while(false);
+        c = s;
+    }
+    function j() internal view returns (S storage c) {
+        do {
+            c = s;
+            break;
+        } while(false);
+    }
+    function k() internal view returns (S storage c) {
+        do {
+            if (s.f) {
+                continue;
+                break;
+            }
+            else {
+                c = s;
+            }
+        } while(false);
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/dowhile_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/dowhile_warn.sol
new file mode 100644
index 00000000..f1a92e9c
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/dowhile_warn.sol
@@ -0,0 +1,35 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        do {
+            break;
+            c = s;
+        } while(false);
+    }
+    function g() internal view returns (S storage c) {
+        do {
+            if (s.f) {
+                continue;
+                c = s;
+            }
+            else {
+            }
+        } while(false);
+    }
+    function h() internal view returns (S storage c) {
+        do {
+            if (s.f) {
+                break;
+                continue;
+            }
+            else {
+                c = s;
+            }
+        } while(false);
+    }
+}
+// ----
+// Warning: (87-98): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (223-234): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (440-451): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/emptyReturn_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/emptyReturn_fine.sol
new file mode 100644
index 00000000..3a0a30ea
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/emptyReturn_fine.sol
@@ -0,0 +1,6 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c, S storage d) { c = s; d = s; return; }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/emptyReturn_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/emptyReturn_warn.sol
new file mode 100644
index 00000000..0a5b2fbf
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/emptyReturn_warn.sol
@@ -0,0 +1,15 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal pure returns (S storage) { return; }
+    function g() internal view returns (S storage c, S storage) { c = s; return; }
+    function h() internal view returns (S storage, S storage d) { d = s; return; }
+    function i() internal pure returns (S storage, S storage) { return; }
+    function j() internal view returns (S storage, S storage) { return (s,s); }
+}
+// ----
+// Warning: (87-88): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (163-164): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (233-234): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (316-317): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (327-328): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/for_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/for_fine.sol
new file mode 100644
index 00000000..aa82cb9a
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/for_fine.sol
@@ -0,0 +1,13 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        for(c = s;;) {
+        }
+    }
+    function g() internal view returns (S storage c) {
+        for(; (c = s).f;) {
+        }
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/for_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/for_warn.sol
new file mode 100644
index 00000000..ba9a2440
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/for_warn.sol
@@ -0,0 +1,16 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        for(;; c = s) {
+        }
+    }
+    function g() internal view returns (S storage c) {
+        for(;;) {
+            c = s;
+        }
+    }
+}
+// ----
+// Warning: (87-98): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (182-193): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/if_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/if_fine.sol
new file mode 100644
index 00000000..b809e95d
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/if_fine.sol
@@ -0,0 +1,29 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f(bool flag) internal view returns (S storage c) {
+        if (flag) c = s;
+        else c = s;
+    }
+    function g(bool flag) internal view returns (S storage c) {
+        if (flag) c = s;
+        else { c = s; }
+    }
+    function h(bool flag) internal view returns (S storage c) {
+        if (flag) c = s;
+        else
+        {
+            if (!flag) c = s;
+            else c = s;
+        }
+    }
+    function i() internal view returns (S storage c) {
+        if ((c = s).f) {
+        }
+    }
+    function j() internal view returns (S storage c) {
+        if ((c = s).f && !(c = s).f) {
+        }
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/if_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/if_warn.sol
new file mode 100644
index 00000000..c257c252
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/if_warn.sol
@@ -0,0 +1,18 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f(bool flag) internal view returns (S storage c) {
+        if (flag) c = s;
+    }
+    function g(bool flag) internal returns (S storage c) {
+        if (flag) c = s;
+        else
+        {
+            if (!flag) c = s;
+            else s.f = true;
+        }
+    }
+}
+// ----
+// Warning: (96-107): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (186-197): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/modifier_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/modifier_fine.sol
new file mode 100644
index 00000000..ee37f6d6
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/modifier_fine.sol
@@ -0,0 +1,20 @@
+contract C {
+    modifier revertIfNoReturn() {
+        _;
+        revert();
+    }
+    modifier ifFlag(bool flag) {
+        if (flag)
+            _;
+    }
+    struct S { uint a; }
+    S s;
+    function f(bool flag) revertIfNoReturn() internal view returns(S storage) {
+        if (flag) return s;
+    }
+    function g(bool flag) revertIfNoReturn() ifFlag(flag) internal view returns(S storage) {
+        return s;
+    }
+
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/modifier_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/modifier_warn.sol
new file mode 100644
index 00000000..50c6dd99
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/modifier_warn.sol
@@ -0,0 +1,22 @@
+contract C {
+    modifier revertIfNoReturn() {
+        _;
+        revert();
+    }
+    modifier ifFlag(bool flag) {
+        if (flag)
+            _;
+    }
+    struct S { uint a; }
+    S s;
+    function f(bool flag) ifFlag(flag) internal view returns(S storage) {
+        return s;
+    }
+
+    function g(bool flag) ifFlag(flag) revertIfNoReturn() internal view returns(S storage) {
+        return s;
+    }
+}
+// ----
+// Warning: (249-250): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (367-368): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/revert_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/revert_fine.sol
new file mode 100644
index 00000000..022f2d1c
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/revert_fine.sol
@@ -0,0 +1,12 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal pure returns (S storage) {
+        revert();
+    }
+    function g(bool flag) internal view returns (S storage c) {
+        if (flag) c = s;
+        else revert();
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/short_circuit_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/short_circuit_fine.sol
new file mode 100644
index 00000000..699849c0
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/short_circuit_fine.sol
@@ -0,0 +1,11 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        (c = s).f && false;
+    }
+    function g() internal view returns (S storage c) {
+        (c = s).f || true;
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/short_circuit_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/short_circuit_warn.sol
new file mode 100644
index 00000000..9f660f11
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/short_circuit_warn.sol
@@ -0,0 +1,18 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        false && (c = s).f;
+    }
+    function g() internal view returns (S storage c) {
+        true || (c = s).f;
+    }
+    function h() internal view returns (S storage c) {
+        // expect warning, although this is always fine
+        true && (false || (c = s).f);
+    }
+}
+// ----
+// Warning: (87-98): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (176-187): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (264-275): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/smoke.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/smoke.sol
new file mode 100644
index 00000000..beeadbe4
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/smoke.sol
@@ -0,0 +1,10 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal pure {}
+    function g() internal view returns (S storage) { return s; }
+    function h() internal view returns (S storage c) { return s; }
+    function i() internal view returns (S storage c) { c = s; }
+    function j() internal view returns (S storage c) { (c) = s; }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/ternary_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/ternary_fine.sol
new file mode 100644
index 00000000..ee3869bd
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/ternary_fine.sol
@@ -0,0 +1,14 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f(bool flag) internal view returns (S storage c) {
+        flag ? c = s : c = s;
+    }
+    function g(bool flag) internal view returns (S storage c) {
+        flag ? c = s : (c = s);
+    }
+    function h(bool flag) internal view returns (S storage c) {
+        flag ? (c = s).f : (c = s).f;
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/ternary_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/ternary_warn.sol
new file mode 100644
index 00000000..57561fbb
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/ternary_warn.sol
@@ -0,0 +1,13 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f(bool flag) internal view returns (S storage c) {
+        flag ? (c = s).f : false;
+    }
+    function g(bool flag) internal view returns (S storage c) {
+        flag ? false : (c = s).f;
+    }
+}
+// ----
+// Warning: (96-107): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
+// Warning: (200-211): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/throw_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/throw_fine.sol
new file mode 100644
index 00000000..4cecc27c
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/throw_fine.sol
@@ -0,0 +1,9 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal pure returns (S storage) {
+        throw;
+    }
+}
+// ----
+// Warning: (108-113): "throw" is deprecated in favour of "revert()", "require()" and "assert()".
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/tuple_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/tuple_fine.sol
new file mode 100644
index 00000000..0b171560
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/tuple_fine.sol
@@ -0,0 +1,12 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage, uint) {
+        return (s,2);
+    }
+    function g() internal view returns (S storage c) {
+        uint a;
+        (c, a) = f();
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/while_fine.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/while_fine.sol
new file mode 100644
index 00000000..71543422
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/while_fine.sol
@@ -0,0 +1,19 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        while((c = s).f) {
+        }
+    }
+    function g() internal view returns (S storage c) {
+        c = s;
+        while(false) {
+        }
+    }
+    function h() internal view returns (S storage c) {
+        while(false) {
+        }
+        c = s;
+    }
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/controlFlow/storageReturn/while_warn.sol b/test/libsolidity/syntaxTests/controlFlow/storageReturn/while_warn.sol
new file mode 100644
index 00000000..26db892f
--- /dev/null
+++ b/test/libsolidity/syntaxTests/controlFlow/storageReturn/while_warn.sol
@@ -0,0 +1,11 @@
+contract C {
+    struct S { bool f; }
+    S s;
+    function f() internal view returns (S storage c) {
+        while(false) {
+            c = s;
+        }
+    }
+}
+// ----
+// Warning: (87-98): This variable is of storage pointer type and might be returned without assignment. This can cause storage corruption. Assign the variable (potentially from itself) to remove this warning.