Tune Rematerializer

libyul/optimiser/DataFlowAnalyzer.cpp

@@ -145,10 +145,14 @@ void DataFlowAnalyzer::operator()(FunctionDefinition& _fun)
 	// Save all information. We might rather reinstantiate this class,
 	// but this could be difficult if it is subclassed.
 	map<YulString, Expression const*> value;
+	map<YulString, size_t> variableLoopDepth;
+	size_t loopDepth{0};
 	InvertibleRelation<YulString> references;
 	InvertibleMap<YulString, YulString> storage;
 	InvertibleMap<YulString, YulString> memory;
-	m_value.swap(value);
+	swap(m_value, value);
+	swap(m_variableLoopDepth, variableLoopDepth);
+	swap(m_loopDepth, loopDepth);
 	swap(m_references, references);
 	swap(m_storage, storage);
 	swap(m_memory, memory);
@@ -168,7 +172,9 @@ void DataFlowAnalyzer::operator()(FunctionDefinition& _fun)
 	// statement.
 
 	popScope();
-	m_value.swap(value);
+	swap(m_value, value);
+	swap(m_variableLoopDepth, variableLoopDepth);
+	swap(m_loopDepth, loopDepth);
 	swap(m_references, references);
 	swap(m_storage, storage);
 	swap(m_memory, memory);
@@ -180,6 +186,8 @@ void DataFlowAnalyzer::operator()(ForLoop& _for)
 	// we would have to deal with more complicated scoping rules.
 	assertThrow(_for.pre.statements.empty(), OptimizerException, "");
 
+	++m_loopDepth;
+
 	AssignmentsSinceContinue assignmentsSinceCont;
 	assignmentsSinceCont(_for.body);
 
@@ -202,6 +210,8 @@ void DataFlowAnalyzer::operator()(ForLoop& _for)
 	clearKnowledgeIfInvalidated(*_for.condition);
 	clearKnowledgeIfInvalidated(_for.post);
 	clearKnowledgeIfInvalidated(_for.body);
+
+	--m_loopDepth;
 }
 
 void DataFlowAnalyzer::operator()(Block& _block)
@@ -222,7 +232,7 @@ void DataFlowAnalyzer::handleAssignment(set<YulString> const& _variables, Expres
 		movableChecker.visit(*_value);
 	else
 		for (auto const& var: _variables)
-			m_value[var] = &m_zero;
+			assignValue(var, &m_zero);
 
 	if (_value && _variables.size() == 1)
 	{
@@ -230,7 +240,7 @@ void DataFlowAnalyzer::handleAssignment(set<YulString> const& _variables, Expres
 		// Expression has to be movable and cannot contain a reference
 		// to the variable that will be assigned to.
 		if (movableChecker.movable() && !movableChecker.referencedVariables().count(name))
-			m_value[name] = _value;
+			assignValue(name, _value);
 	}
 
 	auto const& referencedVariables = movableChecker.referencedVariables();
@@ -296,11 +306,20 @@ void DataFlowAnalyzer::clearValues(set<YulString> _variables)
 
 	// Clear the value and update the reference relation.
 	for (auto const& name: _variables)
+	{
 		m_value.erase(name);
+		m_variableLoopDepth.erase(name);
+	}
 	for (auto const& name: _variables)
 		m_references.eraseKey(name);
 }
 
+void DataFlowAnalyzer::assignValue(YulString _variable, Expression const* _value)
+{
+	m_value[_variable] = _value;
+	m_variableLoopDepth[_variable] = m_loopDepth;
+}
+
 void DataFlowAnalyzer::clearKnowledgeIfInvalidated(Block const& _block)
 {
 	SideEffectsCollector sideEffects(m_dialect, _block, &m_functionSideEffects);

libyul/optimiser/DataFlowAnalyzer.h

@@ -110,6 +110,8 @@ protected:
 	/// for example at points where control flow is merged.
 	void clearValues(std::set<YulString> _names);
 
+	void assignValue(YulString _variable, Expression const* _value);
+
 	/// Clears knowledge about storage or memory if they may be modified inside the block.
 	void clearKnowledgeIfInvalidated(Block const& _block);
 
@@ -144,6 +146,8 @@ protected:
 
 	/// Current values of variables, always movable.
 	std::map<YulString, Expression const*> m_value;
+	/// The loop nesting depth of the definition of variables (those used in m_value).
+	std::map<YulString, size_t> m_variableLoopDepth;
 	/// m_references.forward[a].contains(b) <=> the current expression assigned to a references b
 	/// m_references.backward[b].contains(a) <=> the current expression assigned to a references b
 	InvertibleRelation<YulString> m_references;
@@ -153,6 +157,9 @@ protected:
 
 	KnowledgeBase m_knowledgeBase;
 
+	/// Current nesting depth of loops.
+	size_t m_loopDepth{0};
+
 	struct Scope
 	{
 		explicit Scope(bool _isFunction): isFunction(_isFunction) {}

libyul/optimiser/Rematerialiser.cpp

@@ -78,7 +78,12 @@ void Rematerialiser::visit(Expression& _e)
 			auto const& value = *m_value.at(name);
 			size_t refs = m_referenceCounts[name];
 			size_t cost = CodeCost::codeCost(m_dialect, value);
-			if (refs <= 1 || cost == 0 || (refs <= 5 && cost <= 1) || m_varsToAlwaysRematerialize.count(name))
+			if (
+				(refs <= 1 && m_variableLoopDepth.at(name) == m_loopDepth) ||
+				cost == 0 ||
+				(refs <= 5 && cost <= 1) ||
+				m_varsToAlwaysRematerialize.count(name)
+			)
 			{
 				assertThrow(m_referenceCounts[name] > 0, OptimizerException, "");
 				for (auto const& ref: m_references.forward[name])

libyul/optimiser/Rematerialiser.h

@@ -27,9 +27,10 @@ namespace solidity::yul
 {
 
 /**
- * Optimisation stage that replaces variables by their most recently assigned expressions,
+ * Optimisation stage that replaces variable references by those expressions
+ * that are most recently assigned to the referenced variables,
  * but only if the expression is movable and one of the following holds:
- *  - the variable is referenced exactly once
+ *  - the variable is referenced exactly once (and definition-to-reference does not cross a loop boundary)
  *  - the value is extremely cheap ("cost" of zero like ``caller()``)
  *  - the variable is referenced at most 5 times and the value is rather cheap
  *    ("cost" of at most 1 like a constant up to 0xff)
@@ -68,6 +69,8 @@ protected:
 		std::set<YulString> _varsToAlwaysRematerialize = {}
 	);
 
+	using DataFlowAnalyzer::operator();
+
 	using ASTModifier::visit;
 	void visit(Expression& _e) override;