Known state: store tags on stack as unions.

CommonSubexpressionEliminator.cpp

@@ -153,7 +153,9 @@ AssemblyItems CSECodeGenerator::generateCode(
 		assertThrow(!m_classPositions[targetItem.second].empty(), OptimizerException, "");
 		if (m_classPositions[targetItem.second].count(targetItem.first))
 			continue;
-		SourceLocation const& location = m_expressionClasses.representative(targetItem.second).item->getLocation();
+		SourceLocation location;
+		if (m_expressionClasses.representative(targetItem.second).item)
+			location = m_expressionClasses.representative(targetItem.second).item->getLocation();
 		int position = classElementPosition(targetItem.second);
 		if (position < targetItem.first)
 			// it is already at its target, we need another copy
@@ -197,7 +199,9 @@ void CSECodeGenerator::addDependencies(Id _c)
 		addDependencies(argument);
 		m_neededBy.insert(make_pair(argument, _c));
 	}
-	if (expr.item->type() == Operation && (
+	if (
+		expr.item &&
+		expr.item->type() == Operation && (
 		expr.item->instruction() == Instruction::SLOAD ||
 		expr.item->instruction() == Instruction::MLOAD ||
 		expr.item->instruction() == Instruction::SHA3
@@ -288,6 +292,7 @@ void CSECodeGenerator::generateClassElement(Id _c, bool _allowSequenced)
 		OptimizerException,
 		"Sequence constrained operation requested out of sequence."
 	);
+	assertThrow(expr.item, OptimizerException, "Non-generated expression without item.");
 	vector<Id> const& arguments = expr.arguments;
 	for (Id arg: boost::adaptors::reverse(arguments))
 		generateClassElement(arg);

ControlFlowGraph.cpp

@@ -24,6 +24,7 @@
 #include <libevmasm/ControlFlowGraph.h>
 #include <map>
 #include <memory>
+#include <algorithm>
 #include <libevmasm/Exceptions.h>
 #include <libevmasm/AssemblyItem.h>
 #include <libevmasm/SemanticInformation.h>
@@ -217,7 +218,6 @@ void ControlFlowGraph::gatherKnowledge()
 	// @todo actually we know that memory is filled with zeros at the beginning,
 	// we could make use of that.
 	KnownStatePointer emptyState = make_shared<KnownState>();
-	ExpressionClasses& expr = emptyState->expressionClasses();
 	bool unknownJumpEncountered = false;
 
 	vector<pair<BlockId, KnownStatePointer>> workQueue({make_pair(BlockId::initial(), emptyState->copy())});
@@ -238,8 +238,6 @@ void ControlFlowGraph::gatherKnowledge()
 		}
 
 		block.startState = state->copy();
-		//@todo we might know the return address for the first pass, but not anymore for the second,
-		// -> store knowledge about tags as a union.
 
 		// Feed all items except for the final jump yet because it will erase the target tag.
 		unsigned pc = block.begin;
@@ -254,22 +252,29 @@ void ControlFlowGraph::gatherKnowledge()
 			assertThrow(block.begin <= pc && pc == block.end - 1, OptimizerException, "");
 			//@todo in the case of JUMPI, add knowledge about the condition to the state
 			// (for both values of the condition)
-			BlockId nextBlock = expressionClassToBlockId(
-				state->stackElement(state->stackHeight(), SourceLocation()),
-				expr
+			set<u256> tags = state->tagsInExpression(
+				state->stackElement(state->stackHeight(), SourceLocation())
 			);
 			state->feedItem(m_items.at(pc++));
-			if (nextBlock)
-				workQueue.push_back(make_pair(nextBlock, state->copy()));
-			else if (!unknownJumpEncountered)
+
+			if (tags.empty() || std::any_of(tags.begin(), tags.end(), [&](u256 const& _tag)
 			{
-				// We do not know where this jump goes, so we have to reset the states of all
-				// JUMPDESTs.
-				unknownJumpEncountered = true;
-				for (auto const& it: m_blocks)
-					if (it.second.begin < it.second.end && m_items[it.second.begin].type() == Tag)
-						workQueue.push_back(make_pair(it.first, emptyState->copy()));
+				return !m_blocks.count(BlockId(_tag));
+			}))
+			{
+				if (!unknownJumpEncountered)
+				{
+					// We do not know the target of this jump, so we have to reset the states of all
+					// JUMPDESTs.
+					unknownJumpEncountered = true;
+					for (auto const& it: m_blocks)
+						if (it.second.begin < it.second.end && m_items[it.second.begin].type() == Tag)
+							workQueue.push_back(make_pair(it.first, emptyState->copy()));
+				}
 			}
+			else
+				for (auto tag: tags)
+					workQueue.push_back(make_pair(BlockId(tag), state->copy()));
 		}
 		else if (block.begin <= pc && pc < block.end)
 			state->feedItem(m_items.at(pc++));
@@ -329,7 +334,11 @@ BasicBlocks ControlFlowGraph::rebuildCode()
 			if (previousHandedOver && !pushes[blockId] && m_items[block.begin].type() == Tag)
 				++block.begin;
 			if (block.begin < block.end)
+			{
 				blocks.push_back(block);
+				blocks.back().startState->clearTagUnions();
+				blocks.back().endState->clearTagUnions();
+			}
 			previousHandedOver = (block.endType == BasicBlock::EndType::HANDOVER);
 		}
 	}
@@ -337,18 +346,6 @@ BasicBlocks ControlFlowGraph::rebuildCode()
 	return blocks;
 }
 
-BlockId ControlFlowGraph::expressionClassToBlockId(
-	ExpressionClasses::Id _id,
-	ExpressionClasses& _exprClasses
-)
-{
-	ExpressionClasses::Expression expr = _exprClasses.representative(_id);
-	if (expr.item && expr.item->type() == PushTag)
-		return BlockId(expr.item->data());
-	else
-		return BlockId::invalid();
-}
-
 BlockId ControlFlowGraph::generateNewId()
 {
 	BlockId id = BlockId(++m_lastUsedId);

ControlFlowGraph.h

@@ -108,10 +108,6 @@ private:
 	void setPrevLinks();
 	BasicBlocks rebuildCode();
 
-	/// @returns the corresponding BlockId if _id is a pushed jump tag,
-	/// and an invalid BlockId otherwise.
-	BlockId expressionClassToBlockId(ExpressionClasses::Id _id, ExpressionClasses& _exprClasses);
-
 	BlockId generateNewId();
 
 	unsigned m_lastUsedId = 0;

ExpressionClasses.cpp

@@ -82,6 +82,16 @@ ExpressionClasses::Id ExpressionClasses::find(
 	return exp.id;
 }
 
+ExpressionClasses::Id ExpressionClasses::newClass(SourceLocation const& _location)
+{
+	Expression exp;
+	exp.id = m_representatives.size();
+	exp.item = storeItem(AssemblyItem(UndefinedItem, (u256(1) << 255) + exp.id, _location));
+	m_representatives.push_back(exp);
+	m_expressions.insert(exp);
+	return exp.id;
+}
+
 bool ExpressionClasses::knownToBeDifferent(ExpressionClasses::Id _a, ExpressionClasses::Id _b)
 {
 	// Try to simplify "_a - _b" and return true iff the value is a non-zero constant.

ExpressionClasses.h

@@ -52,7 +52,8 @@ public:
 		Id id;
 		AssemblyItem const* item = nullptr;
 		Ids arguments;
-		unsigned sequenceNumber; ///< Storage modification sequence, only used for SLOAD/SSTORE instructions.
+		/// Storage modification sequence, only used for storage and memory operations.
+		unsigned sequenceNumber = 0;
 		/// Behaves as if this was a tuple of (item->type(), item->data(), arguments, sequenceNumber).
 		bool operator<(Expression const& _other) const;
 	};
@@ -73,6 +74,9 @@ public:
 	/// @returns the number of classes.
 	Id size() const { return m_representatives.size(); }
 
+	/// @returns the id of a new class which is different to all other classes.
+	Id newClass(SourceLocation const& _location);
+
 	/// @returns true if the values of the given classes are known to be different (on every input).
 	/// @note that this function might still return false for some different inputs.
 	bool knownToBeDifferent(Id _a, Id _b);

KnownState.cpp

@@ -162,29 +162,41 @@ KnownState::StoreOperation KnownState::feedItem(AssemblyItem const& _item, bool
 
 /// Helper function for KnownState::reduceToCommonKnowledge, removes everything from
 /// _this which is not in or not equal to the value in _other.
-template <class _Mapping, class _KeyType> void intersect(
-	_Mapping& _this,
-	_Mapping const& _other,
-	function<_KeyType(_KeyType)> const& _keyTrans = [](_KeyType _k) { return _k; }
-)
+template <class _Mapping> void intersect(_Mapping& _this, _Mapping const& _other)
 {
 	for (auto it = _this.begin(); it != _this.end();)
-		if (_other.count(_keyTrans(it->first)) && _other.at(_keyTrans(it->first)) == it->second)
+		if (_other.count(it->first) && _other.at(it->first) == it->second)
 			++it;
 		else
 			it = _this.erase(it);
 }
 
-template <class _Mapping> void intersect(_Mapping& _this, _Mapping const& _other)
-{
-	intersect<_Mapping, ExpressionClasses::Id>(_this, _other, [](ExpressionClasses::Id _k) { return _k; });
-}
-
 void KnownState::reduceToCommonKnowledge(KnownState const& _other)
 {
 	int stackDiff = m_stackHeight - _other.m_stackHeight;
-	function<int(int)> stackKeyTransform = [=](int _key) -> int { return _key - stackDiff; };
-	intersect(m_stackElements, _other.m_stackElements, stackKeyTransform);
+	for (auto it = m_stackElements.begin(); it != m_stackElements.end();)
+		if (_other.m_stackElements.count(it->first - stackDiff))
+		{
+			Id other = _other.m_stackElements.at(it->first - stackDiff);
+			if (it->second == other)
+				++it;
+			else
+			{
+				set<u256> theseTags = tagsInExpression(it->second);
+				set<u256> otherTags = tagsInExpression(other);
+				if (!theseTags.empty() && !otherTags.empty())
+				{
+					theseTags.insert(otherTags.begin(), otherTags.end());
+					it->second = tagUnion(theseTags);
+					++it;
+				}
+				else
+					it = m_stackElements.erase(it);
+			}
+		}
+		else
+			it = m_stackElements.erase(it);
+
 	// Use the smaller stack height. Essential to terminate in case of loops.
 	if (m_stackHeight > _other.m_stackHeight)
 	{
@@ -201,10 +213,15 @@ void KnownState::reduceToCommonKnowledge(KnownState const& _other)
 
 bool KnownState::operator==(const KnownState& _other) const
 {
-	return m_storageContent == _other.m_storageContent &&
-		m_memoryContent == _other.m_memoryContent &&
-		m_stackHeight == _other.m_stackHeight &&
-		m_stackElements == _other.m_stackElements;
+	if (m_storageContent != _other.m_storageContent || m_memoryContent != _other.m_memoryContent)
+		return false;
+	int stackDiff = m_stackHeight - _other.m_stackHeight;
+	auto thisIt = m_stackElements.cbegin();
+	auto otherIt = _other.m_stackElements.cbegin();
+	for (; thisIt != m_stackElements.cend() && otherIt != _other.m_stackElements.cend(); ++thisIt, ++otherIt)
+		if (thisIt->first - stackDiff != otherIt->first || thisIt->second != otherIt->second)
+			return false;
+	return (thisIt == m_stackElements.cend() && otherIt == _other.m_stackElements.cend());
 }
 
 ExpressionClasses::Id KnownState::stackElement(int _stackHeight, SourceLocation const& _location)
@@ -212,18 +229,17 @@ ExpressionClasses::Id KnownState::stackElement(int _stackHeight, SourceLocation
 	if (m_stackElements.count(_stackHeight))
 		return m_stackElements.at(_stackHeight);
 	// Stack element not found (not assigned yet), create new unknown equivalence class.
-	//@todo check that we do not infer incorrect equivalences when the stack is cleared partially
-	//in between.
-	return m_stackElements[_stackHeight] = initialStackElement(_stackHeight, _location);
+	return m_stackElements[_stackHeight] =
+			m_expressionClasses->find(AssemblyItem(UndefinedItem, _stackHeight, _location));
 }
 
-ExpressionClasses::Id KnownState::initialStackElement(
-	int _stackHeight,
-	SourceLocation const& _location
-)
+void KnownState::clearTagUnions()
 {
-	// This is a special assembly item that refers to elements pre-existing on the initial stack.
-	return m_expressionClasses->find(AssemblyItem(UndefinedItem, u256(_stackHeight), _location));
+	for (auto it = m_stackElements.begin(); it != m_stackElements.end();)
+		if (m_tagUnions.left.count(it->second))
+			it = m_stackElements.erase(it);
+		else
+			++it;
 }
 
 void KnownState::setStackElement(int _stackHeight, Id _class)
@@ -352,3 +368,27 @@ KnownState::Id KnownState::applySha3(
 	return m_knownSha3Hashes[arguments] = v;
 }
 
+set<u256> KnownState::tagsInExpression(KnownState::Id _expressionId)
+{
+	if (m_tagUnions.left.count(_expressionId))
+		return m_tagUnions.left.at(_expressionId);
+	// Might be a tag, then return the set of itself.
+	ExpressionClasses::Expression expr = m_expressionClasses->representative(_expressionId);
+	if (expr.item && expr.item->type() == PushTag)
+		return set<u256>({expr.item->data()});
+	else
+		return set<u256>();
+}
+
+KnownState::Id KnownState::tagUnion(set<u256> _tags)
+{
+	if (m_tagUnions.right.count(_tags))
+		return m_tagUnions.right.at(_tags);
+	else
+	{
+		Id id = m_expressionClasses->newClass(SourceLocation());
+		m_tagUnions.right.insert(make_pair(_tags, id));
+		return id;
+	}
+}
+

KnownState.h

@@ -29,6 +29,7 @@
 #include <tuple>
 #include <memory>
 #include <ostream>
+#include <boost/bimap.hpp>
 #include <libdevcore/CommonIO.h>
 #include <libdevcore/Exceptions.h>
 #include <libevmasm/ExpressionClasses.h>
@@ -107,15 +108,16 @@ public:
 	/// @returns true if the knowledge about the state of both objects is (known to be) equal.
 	bool operator==(KnownState const& _other) const;
 
-	///@todo the sequence numbers in two copies of this class should never be the same.
-	/// might be doable using two-dimensional sequence numbers, where the first value is incremented
-	/// for each copy
-
 	/// Retrieves the current equivalence class fo the given stack element (or generates a new
 	/// one if it does not exist yet).
 	Id stackElement(int _stackHeight, SourceLocation const& _location);
-	/// @returns the equivalence class id of the special initial stack element at the given height.
-	Id initialStackElement(int _stackHeight, SourceLocation const& _location);
+
+	/// @returns its set of tags if the given expression class is a known tag union; returns a set
+	/// containing the tag if it is a PushTag expression and the empty set otherwise.
+	std::set<u256> tagsInExpression(Id _expressionId);
+	/// During analysis, different tags on the stack are partially treated as the same class.
+	/// This removes such classes not to confuse later analyzers.
+	void clearTagUnions();
 
 	int stackHeight() const { return m_stackHeight; }
 	std::map<int, Id> const& stackElements() const { return m_stackElements; }
@@ -142,6 +144,9 @@ private:
 	/// Finds or creates a new expression that applies the sha3 hash function to the contents in memory.
 	Id applySha3(Id _start, Id _length, SourceLocation const& _location);
 
+	/// @returns a new or already used Id representing the given set of tags.
+	Id tagUnion(std::set<u256> _tags);
+
 	/// Current stack height, can be negative.
 	int m_stackHeight = 0;
 	/// Current stack layout, mapping stack height -> equivalence class
@@ -157,6 +162,8 @@ private:
 	std::map<std::vector<Id>, Id> m_knownSha3Hashes;
 	/// Structure containing the classes of equivalent expressions.
 	std::shared_ptr<ExpressionClasses> m_expressionClasses;
+	/// Container for unions of tags stored on the stack.
+	boost::bimap<Id, std::set<u256>> m_tagUnions;
 };
 
 }