Store alternative stack locations during code generation.

libevmasm/CommonSubexpressionEliminator.cpp

@@ -121,10 +121,7 @@ AssemblyItems CSECodeGenerator::generateCode(
 	m_stack = _initialStack;
 	m_targetStack = _targetStackContents;
 	for (auto const& item: m_stack)
-		if (!m_classPositions.count(item.second))
-			m_classPositions[item.second] = item.first;
-
-	// @todo: provide information about the positions of copies of class elements
+		m_classPositions[item.second].insert(item.first);
 
 	// generate the dependency graph starting from final storage and memory writes and target stack contents
 	for (auto const& p: m_storeOperations)
@@ -152,11 +149,12 @@ AssemblyItems CSECodeGenerator::generateCode(
 	{
 		if (m_stack.count(targetItem.first) && m_stack.at(targetItem.first) == targetItem.second)
 			continue; // already there
-		int position = generateClassElement(targetItem.second);
-		assertThrow(position != c_invalidPosition, OptimizerException, "");
-		if (position == targetItem.first)
+		generateClassElement(targetItem.second);
+		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();
+		int position = classElementPosition(targetItem.second);
 		if (position < targetItem.first)
 			// it is already at its target, we need another copy
 			appendDup(position, location);
@@ -266,19 +264,23 @@ void CSECodeGenerator::addDependencies(Id _c)
 	}
 }
 
-int CSECodeGenerator::generateClassElement(Id _c, bool _allowSequenced)
+void CSECodeGenerator::generateClassElement(Id _c, bool _allowSequenced)
 {
+	for (auto it: m_classPositions)
+		for (auto p: it.second)
+			if (p > m_stackHeight)
+				assertThrow(false, OptimizerException, "");
 	// do some cleanup
 	removeStackTopIfPossible();
 
 	if (m_classPositions.count(_c))
 	{
 		assertThrow(
-			m_classPositions[_c] != c_invalidPosition,
+			!m_classPositions[_c].empty(),
 			OptimizerException,
 			"Element already removed but still needed."
 		);
-		return m_classPositions[_c];
+		return;
 	}
 	ExpressionClasses::Expression const& expr = m_expressionClasses.representative(_c);
 	assertThrow(
@@ -351,16 +353,16 @@ int CSECodeGenerator::generateClassElement(Id _c, bool _allowSequenced)
 			m_generatedItems.back() == AssemblyItem(Instruction::SWAP1))
 		// this will not append a swap but remove the one that is already there
 		appendOrRemoveSwap(m_stackHeight - 1, location);
-	for (auto arg: arguments)
-		if (m_classPositions[arg] != c_invalidPosition && canBeRemoved(arg, _c))
-			m_classPositions[arg] = c_invalidPosition;
 	for (size_t i = 0; i < arguments.size(); ++i)
+	{
+		m_classPositions[m_stack[m_stackHeight - i]].erase(m_stackHeight - i);
 		m_stack.erase(m_stackHeight - i);
+	}
 	appendItem(*expr.item);
 	if (expr.item->type() != Operation || instructionInfo(expr.item->instruction()).ret == 1)
 	{
 		m_stack[m_stackHeight] = _c;
-		return m_classPositions[_c] = m_stackHeight;
+		m_classPositions[_c].insert(m_stackHeight);
 	}
 	else
 	{
@@ -369,18 +371,18 @@ int CSECodeGenerator::generateClassElement(Id _c, bool _allowSequenced)
 			OptimizerException,
 			"Invalid number of return values."
 		);
-		return m_classPositions[_c] = c_invalidPosition;
+		m_classPositions[_c]; // ensure it is created to mark the expression as generated
 	}
 }
 
 int CSECodeGenerator::classElementPosition(Id _id) const
 {
 	assertThrow(
-		m_classPositions.count(_id) && m_classPositions.at(_id) != c_invalidPosition,
+		m_classPositions.count(_id) && !m_classPositions.at(_id).empty(),
 		OptimizerException,
 		"Element requested but is not present."
 	);
-	return m_classPositions.at(_id);
+	return *max_element(m_classPositions.at(_id).begin(), m_classPositions.at(_id).end());
 }
 
 bool CSECodeGenerator::canBeRemoved(Id _element, Id _result, int _fromPosition)
@@ -389,20 +391,19 @@ bool CSECodeGenerator::canBeRemoved(Id _element, Id _result, int _fromPosition)
 	if (_fromPosition == c_invalidPosition)
 		_fromPosition = classElementPosition(_element);
 
-	bool isCopy = _fromPosition != classElementPosition(_element);
+	bool haveCopy = m_classPositions.at(_element).size() > 1;
 	if (m_finalClasses.count(_element))
 		// It is part of the target stack. It can be removed if it is a copy that is not in the target position.
-		return isCopy && (!m_targetStack.count(_fromPosition) || m_targetStack[_fromPosition] != _element);
-	else if (isCopy)
-		// It is only a copy, can be removed.
-		return true;
-
-	// Can be removed unless it is needed by a class that has not been computed yet.
-	// Note that m_classPositions also includes classes that were deleted in the meantime.
-	auto range = m_neededBy.equal_range(_element);
-	for (auto it = range.first; it != range.second; ++it)
-		if (it->second != _result && !m_classPositions.count(it->second))
-			return false;
+		return haveCopy && (!m_targetStack.count(_fromPosition) || m_targetStack[_fromPosition] != _element);
+	else if (!haveCopy)
+	{
+		// Can be removed unless it is needed by a class that has not been computed yet.
+		// Note that m_classPositions also includes classes that were deleted in the meantime.
+		auto range = m_neededBy.equal_range(_element);
+		for (auto it = range.first; it != range.second; ++it)
+			if (it->second != _result && !m_classPositions.count(it->second))
+				return false;
+	}
 	return true;
 }
 
@@ -414,9 +415,9 @@ bool CSECodeGenerator::removeStackTopIfPossible()
 	Id top = m_stack[m_stackHeight];
 	if (!canBeRemoved(top, Id(-1), m_stackHeight))
 		return false;
-	m_generatedItems.push_back(AssemblyItem(Instruction::POP));
+	m_classPositions[m_stack[m_stackHeight]].erase(m_stackHeight);
 	m_stack.erase(m_stackHeight);
-	m_stackHeight--;
+	appendItem(AssemblyItem(Instruction::POP));
 	return true;
 }
 
@@ -428,6 +429,7 @@ void CSECodeGenerator::appendDup(int _fromPosition, SourceLocation const& _locat
 	assertThrow(1 <= instructionNum, OptimizerException, "Invalid stack access.");
 	appendItem(AssemblyItem(dupInstruction(instructionNum), _location));
 	m_stack[m_stackHeight] = m_stack[_fromPosition];
+	m_classPositions[m_stack[m_stackHeight]].insert(m_stackHeight);
 }
 
 void CSECodeGenerator::appendOrRemoveSwap(int _fromPosition, SourceLocation const& _location)
@@ -439,13 +441,15 @@ void CSECodeGenerator::appendOrRemoveSwap(int _fromPosition, SourceLocation cons
 	assertThrow(instructionNum <= 16, StackTooDeepException, "Stack too deep.");
 	assertThrow(1 <= instructionNum, OptimizerException, "Invalid stack access.");
 	appendItem(AssemblyItem(swapInstruction(instructionNum), _location));
-	// The value of a class can be present in multiple locations on the stack. We only update the
-	// "canonical" one that is tracked by m_classPositions
-	if (m_classPositions[m_stack[m_stackHeight]] == m_stackHeight)
-		m_classPositions[m_stack[m_stackHeight]] = _fromPosition;
-	if (m_classPositions[m_stack[_fromPosition]] == _fromPosition)
-		m_classPositions[m_stack[_fromPosition]] = m_stackHeight;
-	swap(m_stack[m_stackHeight], m_stack[_fromPosition]);
+
+	if (m_stack[m_stackHeight] != m_stack[_fromPosition])
+	{
+		m_classPositions[m_stack[m_stackHeight]].erase(m_stackHeight);
+		m_classPositions[m_stack[m_stackHeight]].insert(_fromPosition);
+		m_classPositions[m_stack[_fromPosition]].erase(_fromPosition);
+		m_classPositions[m_stack[_fromPosition]].insert(m_stackHeight);
+		swap(m_stack[m_stackHeight], m_stack[_fromPosition]);
+	}
 	if (m_generatedItems.size() >= 2 &&
 		SemanticInformation::isSwapInstruction(m_generatedItems.back()) &&
 		*(m_generatedItems.end() - 2) == m_generatedItems.back())

libevmasm/CommonSubexpressionEliminator.h

@@ -119,10 +119,8 @@ private:
 	void addDependencies(Id _c);
 
 	/// Produce code that generates the given element if it is not yet present.
-	/// @returns the stack position of the element or c_invalidPosition if it does not actually
-	/// generate a value on the stack.
 	/// @param _allowSequenced indicates that sequence-constrained operations are allowed
-	int generateClassElement(Id _c, bool _allowSequenced = false);
+	void generateClassElement(Id _c, bool _allowSequenced = false);
 	/// @returns the position of the representative of the given id on the stack.
 	/// @note throws an exception if it is not on the stack.
 	int classElementPosition(Id _id) const;
@@ -151,8 +149,8 @@ private:
 	std::multimap<Id, Id> m_neededBy;
 	/// Current content of the stack.
 	std::map<int, Id> m_stack;
-	/// Current positions of equivalence classes, equal to c_invalidPosition if already deleted.
-	std::map<Id, int> m_classPositions;
+	/// Current positions of equivalence classes, equal to the empty set if already deleted.
+	std::map<Id, std::set<int>> m_classPositions;
 
 	/// The actual eqivalence class items and how to compute them.
 	ExpressionClasses& m_expressionClasses;