Merge pull request #5641 from ethereum/supportUnassigned

[Yul] Support unassigned variables in the SSA value tracker and the data flow analyzer.

libyul/optimiser/DataFlowAnalyzer.cpp

@@ -136,17 +136,22 @@ void DataFlowAnalyzer::operator()(Block& _block)
 
 void DataFlowAnalyzer::handleAssignment(set<YulString> const& _variables, Expression* _value)
 {
+	static Expression const zero{Literal{{}, LiteralKind::Number, YulString{"0"}, {}}};
 	clearValues(_variables);
 
 	MovableChecker movableChecker;
 	if (_value)
 		movableChecker.visit(*_value);
-	if (_variables.size() == 1)
+	else
+		for (auto const& var: _variables)
+			m_value[var] = &zero;
+
+	if (_value && _variables.size() == 1)
 	{
 		YulString name = *_variables.begin();
 		// Expression has to be movable and cannot contain a reference
 		// to the variable that will be assigned to.
-		if (_value && movableChecker.movable() && !movableChecker.referencedVariables().count(name))
+		if (movableChecker.movable() && !movableChecker.referencedVariables().count(name))
 			m_value[name] = _value;
 	}
 

libyul/optimiser/DataFlowAnalyzer.h

@@ -36,6 +36,8 @@ namespace yul
  * Tracks assignments and is used as base class for both Rematerialiser and
  * Common Subexpression Eliminator.
  *
+ * A special zero constant expression is used for the default value of variables.
+ *
  * Prerequisite: Disambiguator
  */
 class DataFlowAnalyzer: public ASTModifier

libyul/optimiser/SSAValueTracker.cpp

@@ -35,11 +35,12 @@ void SSAValueTracker::operator()(Assignment const& _assignment)
 
 void SSAValueTracker::operator()(VariableDeclaration const& _varDecl)
 {
-	if (_varDecl.variables.size() == 1)
-		setValue(_varDecl.variables.front().name, _varDecl.value.get());
-	else if (!_varDecl.value)
+	static Expression const zero{Literal{{}, LiteralKind::Number, YulString{"0"}, {}}};
+	if (!_varDecl.value)
 		for (auto const& var: _varDecl.variables)
-			setValue(var.name, nullptr);
+			setValue(var.name, &zero);
+	else if (_varDecl.variables.size() == 1)
+		setValue(_varDecl.variables.front().name, _varDecl.value.get());
 }
 
 void SSAValueTracker::setValue(YulString _name, Expression const* _value)

libyul/optimiser/SSAValueTracker.h

@@ -33,7 +33,7 @@ namespace yul
  * Class that walks the AST and stores the initial value of each variable
  * that is never assigned to.
  *
- * Default value is represented as nullptr.
+ * A special zero constant expression is used for the default value of variables.
  *
  * Prerequisite: Disambiguator
  */